Blue Top Stube Contains 32 Percent of Citrate

Coagulation-Based Tests and Their Interpretation

Andy Nguyen MD , ... Amer Wahed MD , in Management of Hemostasis and Coagulopathies for Surgical and Critically Ill Patients, 2016

1.10 Conclusions

Coagulation testings are essential for evaluating bleeding patients, including patients scheduled for surgery or during postsurgical care. The VFN test is useful in evaluating bleeding risk of a patient scheduled for surgery, especially if the patient is on aspirin or Plavix. TEG is a method that has been used for more than 60 years, but with modern modification of the technique and the availability of automated analyzers, TEG analysis is useful in evaluating the bleeding state of a patient, although values may remain normal in some patients with moderate coagulopathy. Correlation of TEG values with DIC screen is optimal.

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Clotting factors: Clinical biochemistry and their roles as plasma enzymes

William E. Winter , ... Neil Harris , in Advances in Clinical Chemistry, 2020

3 Coagulation testing in the clinical laboratory

Citrated plasma is the substrate for almost all coagulation-specific laboratory tests and is derived from whole blood drawn into a tube containing liquid 3.2% sodium citrate (109   mM) at a ratio of 9 parts whole blood and 1 part citrate. Citrate acts as a calcium chelating agent to prevent coagulation of the sample so that all the clotting factors are preserved and can be evaluated. The citrated blood specimen is centrifuged to generate platelet poor plasma (PPP) which is defined as plasma with a platelet count of <   1   ×   10¹⁰/L (1   ×   10⁴/μL).

Plasma coagulation testing in vitro proceeds along the following steps:

(a)

The specimen is warmed to 37   °C to allow optimal activity of the coagulation enzymes.

(b)

Coagulation is initiated by adding an activator such as thromboplastin (see Section 3.1), silica particles or kaolin (a soft white clay composed of a hydrous aluminum silicate).

(c)

The reaction is supplemented with cell membrane phospholipids, which are essential because the reactions occur on cell or platelet surfaces in vivo. Therefore, in vitro reactions require phospholipid supplementation.

(d)

The specimen is re-calcified by adding a pre-warmed solution of calcium chloride.

(e)

In the clinical laboratory, the end point of coagulation is usually detected by increased plasma viscosity and/or turbidity. In the point-of-care iSTAT device (Abbott Diagnostics), an electrochemical sensor is used. Specifically, in the electrochemical assay, thrombin cleavage produces an electroactive compound that is detected amperometrically, and the time of detection is measured in seconds [11].

(f)

Mechanical, photo-optical and electrochemical methods are not sensitive to the later fibrin cross-linking or FXIII stabilization step. Failure of FXIII stabilization will go undetected by these methods

3.1 PT and APTT

The prothrombin time (PT) and activated partial thromboplastin time (APTT) are in vitro approximations of the coagulation pathway and they essentially define the extrinsic, intrinsic and common pathways [6–8,12]. The extrinsic pathway is triggered by the addition of tissue factor and phospholipid, while the intrinsic pathway is initiated by negatively charged surfaces (Fig. 1A and B ). The endpoint of the reaction is gauged by an increase in the viscosity or a decrease in light transmission of the test plasma.

Fig. 1. (A) The extrinsic and common pathway of coagulation. (B) The intrinsic and common pathway of coagulation. HMWK   =   High molecular weight kininogen, PK   =   Prekallikrein.

The laboratory-based PT and APTT reactions occur in a platelet-free environment in the absence of leukocytes, red blood cells and endothelial cells. The PT tests the extrinsic and common pathways, while the APTT measures the functional activity of the intrinsic and common pathway components.

3.2 Prothrombin time

In the PT, addition of supraphysiological concentrations of tissue factor (TF) drives the extrinsic and common pathways with rapid formation of coagulated plasma, typically within 15   s or less.

In the PT assay the extrinsic pathway is initiated (Fig. 1A) by adding an extract that is rich in TF as well as phospholipid to plasma. Mixtures of TF and phospholipid are traditionally termed "thromboplastin." The added TF together with endogenous FVIIa activates FX to FXa. The TF/FVIIa is termed a "tenase" complex because of its functional ability to cleave FX. Factors Xa and Va then form a functional in vitro "prothrombinase" complex (that converts prothrombin (or FII) to thrombin (FIIa)). The reactions of the PT pathway are illustrated in Fig. 1A.

Warfarin anticoagulation [6–8,12] is best monitored with the prothrombin time since all factors of the extrinsic and common pathways are vitamin K-dependent with the exception of Factor V and fibrinogen (see Section 4.1). Conversely, the main contact factors of the intrinsic pathway (FXII, prekallikrein and FXI) are vitamin K independent proteases. Hence the APTT is less useful as a monitor of warfarin.

In order to standardize the PT for patients on warfarin, a ratio termed the International Normalized Ratio or INR can be calculated.

The INR   =   (PT_specimen/PT_mean) ^ISI where:

•

PT_specimen is the patient's current PT determination

•

PT_mean is the geometric mean ^a of the reference range population.

•

ISI   =   International Sensitivity Index of the thromboplastin reagent.

The ISI is a reagent lot-specific constant that relates the sensitivity of a commercial laboratory thromboplastin reagent lot to a reduction in a standard amount of vitamin K-dependent clotting factor activity. An ideal reagent has a low ISI of close to 1.0. A low-sensitivity reagent, in contrast, will have a higher ISI. The ISI is specific for a particular reagent and a reagent lot number in combination with the coagulation analyzer.

3.3 PTT or APTT activated partial thromboplastin time

In the APTT [6–8,12], the plasma is initially mixed with a negatively charged surface such as silica or kaolin as well as a phospholipid extract that is free of tissue factor (termed "partial thromboplastin") for an activation step that typically lasts several minutes. This is followed by adding 1 part of a calcium chloride solution that leads to clot formation via the intrinsic pathway (Fig. 1B). Partial thromboplastin is incapable of generating the FVIIa/TF complex because it lacks tissue factor. The negatively-charged surface activates FXII to form FXIIa. This step is also dependent upon the presence of high molecular weight kininogen (HMWK) as well as plasma prekallikrein. These will be discussed in detail later. Active FXIIa transduces downstream coagulation effects by cleaving FXI to generate FXIa. FXIa next activates FIX leading ultimately to the production of a FIXa/FVIIIa tenase complex that activates FX forming FXa. Note that while FIX is activated by FXIa, FVIII is activated by thrombin (FIIa).

The APTT is utilized to monitor the active concentrations of the hemophilia factors (FVIII and FIX) and should be one of the first line tests in a young male patient with unexplained deep tissue bleeding. For many years, the APTT was used as a surrogate for determining therapeutic concentrations of unfractionated heparin (UFH). This use has decreased over the past decade because of the introduction of chromogenic heparin assays. Interestingly, the prolongation of the APTT by UFH may possibly be an effect of its actions on the contact factor proteases and FIXa because low and ultra-low molecular weight heparins, that target mainly FXa and to a lesser extent FIIa, have little or no effect on the APTT.

Note that the intrinsic pathway can be conceived as consisting of a number of sequential steps:

(a)

The "Contact System" that comprises FXII, prekallikrein and the non-enzymatic factor HMWK.

(b)

FXIa

(c)

FIXa/FVIIIa tenase complex (which cleaves and activates FX)

The "Common Pathway" refers to the reactions involving FXa/FVa as well as the conversion of prothrombin to thrombin and the action of thrombin on fibrinogen.

3.4 Clotting factor activity assays

Clotting factor activity assays determine whether the patient has optimal or suboptimal concentrations of the relevant coagulation factor in their plasma. This determination is always based upon functional activity assays rather than overall abundance as judged by immunoassay [12] which will not detect functionally defective coagulation proteins. This includes under-or non-carboxylated GLA-domain containing factors (see later). Factor activities in the clinical laboratory are either expressed as a percentage of normal or as international units/mL (IU/mL) or per deciliter (dL) where 1.0   IU/mL or 100   IU/dL is equivalent to 100% of normal.

The traditional way of determining FVIII activity (one of the more common assays performed in the coagulation laboratory) is the one-stage clot-based assay which is a modified APTT. The key to this procedure is the use of FVIII-deficient plasma which contains normal activities of all other coagulation factors (aside from FVIII) in the reaction. The patient's citrated plasma is diluted 1:10, 1:20 and 1:40 using a diluent buffer. The diluted specimen is then mixed 1-to-1 with the factor-deficient substrate plasma and an APTT is initiated as described above. The FVIII in the final mixture derives entirely from the patient specimen in which the concentration can be extrapolated from the calibration curve of the clotting times. A very similar protocol is followed in determining FIX, FXI and FXII. The major difference is that the factor-deficient substrate plasma is missing FIX, FXI or FXII, respectively. For coagulation factors VII, X, V and II a PT reaction is performed, rather than an APTT.

In all cases, a calibration curve is set up using a commercial reference plasma with known, defined activities of the clotting factor in question. Activities are given either as a percentage of normal or IU/mL. These activities are plotted on the X-axis. The Y-axis denotes the APTT or PT clotting times (in seconds) and the lower the factor activity, the more prolonged the clotting times expressed as either APTT or PT. There are different ways of representing the relationship, but classically the factor activity on the X-axis is plotted logarithmically.

Chromogenic or amidolytic ^b two stage FVIII assays have been of interest to clinical laboratories for a long time [13,14]. In particular, certain patients, including about 5–40% of all males having mild or moderate hemophilia A (a genetic deficiency of FVIII) manifest differences between clot-based and chromogenic factor VIII measurements. These discrepancies are related to the particular underlying hemophilia mutations, and reflect imperfections or limitations in existing in vitro activity measurements [14]. Both assays, used simultaneously, are recommended for initial diagnosis of non-severe hemophilia A. Such a disagreement becomes more prominent when measuring the new modified recombinant FVIII activities in hemophilia patients. These discrepancies can be attributed to differences in modification of the FVIII protein that are introduced to achieve a longer half-life (such as PEGylation, IgG Fc fusion, or use of a single chain protein) and how the modified protein behaves in the assay (clot-based vs. chromogenic). This phenomenon impacts how different FVIII replacement products should be monitored. For instance, silica-based APTT reagents often underestimate B-domain deleted FVIII and PEGylated FVIII preparations. Chromogenic assays do not use a factor-deficient plasma and they do not rely on an APTT clot-based reaction. In the first stage of the chromogenic factor VIII assay, exogenous Factor IXa (in excess), IIa (thrombin), phospholipid and calcium are added to the reaction with the patient specimen, as is exogenous excess Factor X (i.e., non-activated FX). Thrombin will activate endogenous FVIII in the patient specimen thereby generating FVIIIa. The latter together with the exogenous FIXa forms a "ten-ase" complex leading to the conversion of factor X to Xa. The assay may utilize either human or bovine factor IXa, X and IIa.

In the second stage, a chromogenic substrate is added; the latter is a target for the recently activated Xa which cleaves it and releases the paranitroaniline (pNA) that can be detected optically at 405   nm. Also, in the second stage, a thrombin inhibitor is added to block any enzymatic action by thrombin on the chromogenic Xa-substrate. Some chromogenic FVIII kits also contain a heparin neutralizer to minimize the impact of heparin on the assay, because heparin could potentially lower the apparent FVIII activity by inhibiting FXa in the presence of antithrombin III.

In contrast to APTT clot-based one-stage assays, the chromogenic assay is mostly unaffected by interfering antiphospholipid antibodies which can artificially prolong the APTT and falsely lower the FVIII activity. There is also a lower inter-laboratory variability with the chromogenic assay. The one-stage assay relies on the activation of FVIII via the intrinsic pathway. Any pre-activated FVIIIa in the one-stage assay can shorten the APTT and lead to an apparent elevation of FVIII activity. Conversely, the chromogenic assay is not affected by pre-activated FVIIIa. The chromogenic assay is however not impervious to the effects of heparin or direct oral anticoagulant (DOAC) medications. These FXa inhibitors will inhibit the chromogenic reaction, again leading to an artifactual lowering of the FVIII activity.

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Hematologic Problems in the Surgical Patient

Iqbal H. Jaffer , ... Jeffrey I. Weitz , in Hematology (Seventh Edition), 2018

Coagulation Testing

The need for routine coagulation testing before surgical or invasive procedures remains controversial. Those in favor of testing point to the asymptomatic nature of some hemostatic abnormalities that may cause surgical bleeding and the occasional failure to obtain a detailed history. ^5-7 A prospective study of preoperative screening in children before tonsillectomy found history and laboratory screening to have high specificity but a low positive-predictive value for perioperative bleeding. ⁶ Another study found that perioperative blood loss in adult cardiothoracic surgery patients could be predicted with a model, which included the bleeding time, prothrombin time (PT), and platelet count. ⁸ Given the variety of potential hemostatic defects however, no simple screening system will identify all patients at increased risk for bleeding. Those against routine laboratory testing ⁹ point to retrospective studies indicating that they rarely detect unexpected bleeding disorders ^10,11 and emphasize the problems associated with evaluation of false-positive results. A literature review found insufficient evidence to conclude that an abnormal PT/international normalized ratio (INR) predicts bleeding during invasive procedures. ¹² A retrospective review of the value of preoperative determination of the platelet count, PT, and activated partial thromboplastin time (aPTT) in 828 patients undergoing major noncardiac surgery found that only 2% had abnormal results, and most were expected on the basis of history and physical examination. ¹³ Furthermore, abnormal laboratory test results and intraoperative blood loss or postoperative bleeding complications were not related. This is not surprising given the lack of studies using an evidence-based approach to determine the degree of abnormality in the PT/INR or aPTT at which invasive procedures may be unsafe. ¹⁴ A number of prospective studies have also concluded that routine laboratory screening tests in asymptomatic patients are not predictive of perioperative or postoperative bleeding. ^15-19 Thus in the absence of historical risk factors or physical examination findings suggestive of an underlying bleeding tendency, the likelihood of an unsuspected, clinically significant congenital or acquired coagulopathy is low enough that routine laboratory screening is not warranted, particularly for those undergoing low-risk procedures. ²⁰ The British Committee for Standards in Haematology recently issued guidelines reiterating this position: indiscriminate coagulation testing before surgical or invasive procedures is a poor predictor of bleeding risk and is not recommended in the absence of a positive personal or family bleeding history. ²¹

There are a variety of reasons why routine coagulation tests such as the PT/INR and aPTT may be poorly predictive of underlying coagulopathy and bleeding risk. ²¹ First, these tests were designed to measure time to clot formation in artificial in vitro assays and do not reliably depict the global hemostatic situation in vivo (see Liver Disease section). More importantly, the PT/INR and aPTT may be insensitive to mild but clinically relevant bleeding disorders such as von Willebrand disease or mild hemophilia. Conversely, they may detect conditions such as factor XII deficiency or a lupus anticoagulant that do not increase the risk for bleeding.

The platelet function analyzer (PFA-100) has been developed for rapid, quantitative in vitro global testing of platelet function. ^22,23 The clinical sensitivity (94%–95%) and specificity (88%–89%) of this instrument are virtually identical to platelet aggregometery. ²⁴ Although some have suggested that the PFA-100 could be used for screening for primary hemostatic defects, such testing has limitations. ²⁵ The PFA-100 has high sensitivity for detection of moderate-to-severe von Willebrand disease and severe platelet defects, such as Glanzmann thrombasthenia and Bernard-Soulier syndrome, but it has poor sensitivity for detection of milder platelet disorders such as storage pool disease, Hermansky-Pudlak syndrome, and primary secretion defects. ²⁶ Although the PFA-100 is most useful when a hemostatic defect is clinically likely, in such cases additional testing is usually necessary to establish a specific diagnosis. ^25-27 If clinical suspicion is high, further testing is indicated even with normal PFA-100 results. ²⁶ Thus the PFA-100 should not be used for general unselected screening. ²⁷

Although some studies have demonstrated the ability of the PFA-100 to predict recurrent ischemic events following percutaneous coronary interventions ²⁸ and coronary artery bypass graft surgery, ²⁹ such testing does not predict bleeding in patients undergoing cardiac or hip fracture surgery. ^30-32

Notwithstanding the controversy about the value of preoperative laboratory screening, it is reasonable to personalize the approach to preoperative evaluation depending on the hemostatic risk of the proposed surgery or invasive procedure (Tables 159.1 and 159.2). A hemostatic history should always be obtained, and no laboratory testing is required in patients undergoing procedures at low risk for bleeding. For procedures associated with a high risk of bleeding, screening could include a PT/INR, aPTT, and determination of the platelet count.

In practice, hematologists are rarely consulted for routine screening because surgeons have adopted approaches based on their own training and local practice patterns. Instead, consultation is sought because of a history suggestive of a bleeding disorder or because an abnormal test result is found on screening. If a referral is requested because of an abnormal screening test, the abnormality needs to be identified. However, regardless of the reason for the referral, the history is of central importance and must include a thorough review of any bleeding episodes, including results of prior hemostatic testing, as well as careful attention to the family history. The physical examination should focus on evidence of bleeding and on identifying systemic disorders such as hepatic or renal disease. If the history of bleeding is negative or minimal, appropriate laboratory testing would include a PT/INR, aPTT, and a biochemical profile to evaluate hepatic and renal function. A complete blood count and examination of the peripheral blood smear are useful to identify myeloproliferative disorders, gray platelet syndrome, or thrombocytopenia. If the history is suggestive of a hemostatic abnormality, a full evaluation is indicated and additional specific testing is usually required because von Willebrand disease; mild deficiencies of factors VIII, IX, and XI; severe factor XIII deficiency; platelet function defects; and fibrinolytic abnormalities may not be identified by global screening tests (see Table 159.2).

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Blood and Coagulation

Jerrold H. Levy , ... Linda J. Demma , in Pharmacology and Physiology for Anesthesia, 2013

Transfusion Algorithms

Most studies demonstrate that transfusion algorithms based on coagulation testing reduce the need for platelets, fresh frozen plasma, or cryoprecipitate. Indeed, any test that prevents empirical administration likely will reduce transfusions. Most transfusion algorithms suggest transfusion when bleeding is accompanied by a PT or aPTT greater than 1.5 times normal value, thrombocytopenia with a platelet count less than 50,000-100,000/µL, or fibrinogen concentration less than 100 mg/dL (or 1 g/L). Because most laboratory testing is slow, POC testing, TEG, ROTEM, or platelet function testing has been the focus of these studies. ⁷³ While thromboelastographic-based algorithms can reduce blood product use, the POC machines are not widely available. Prospective algorithms using platelet function data have also been reported. However, platelet function testing is problematic in that most tests need a relatively higher number of platelets to work and it is unclear whether they are applicable to the platelet dysfunction encountered following cardiopulmonary bypass.

Ideally, surgical patients should receive transfusions based on laboratory-guided algorithms, and the possible benefits of POC testing should be tested against this standard. Battlefield trauma can cause massive bleeding and massive transfusion coagulopathy, defined as 10 or more units of RBC transfusions in a 24-hour period. With life-threatening bleeding, multiple blood volumes might be replaced before hemostatic test results are available. Therefore transfusion protocols often use fixed doses of component therapies that include fresh frozen plasma and platelets following transfusion of a defined number of RBCs, often in an attempt to mimic the transfusion of fresh whole blood. ⁷⁴ This battlefield transfusion protocol has been demonstrated to improve survival. ^75,76 Perioperative blood transfusion and blood conservation guidelines have been defined in certain patient populations and usually include a multimodal approach to blood conservation (see Chapter 36). ⁵⁶

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Blood and Coagulation

Jerrold H. Levy , ... Ian Welsby , in Pharmacology and Physiology for Anesthesia (Second Edition), 2019

Transfusion Algorithms and Massive Transfusion

Most studies demonstrate that transfusion algorithms based on coagulation testing reduce the need for platelets, fresh frozen plasma, or cryoprecipitate. Indeed, any test that prevents empirical administration likely will reduce transfusions. Most transfusion algorithms suggest transfusion when bleeding is accompanied by a PT or aPTT greater than 1.5 times the normal value, thrombocytopenia with a platelet count less than 50,000 to 100,000/µL, or fibrinogen concentration less than 100 mg/dL (or 1 g/L). Because most laboratory testing is slow, POC viscoelastic testing has become the focus of most transfusion algorithms. ⁷² Prospective algorithms using platelet function data have also been reported. However, platelet function testing is problematic in that most tests need a relatively higher number of platelets to work and it is unclear whether they are applicable to the platelet dysfunction encountered after cardiopulmonary bypass.

Battlefield trauma can cause massive bleeding, and extensive information on massive transfusion has come from experience in the Iraq War. Massive transfusion is commonly defined as 10 or more units of RBC transfusions in a 24-hour period, but use of 4 or more red cell units within 1 hour when the ongoing need is foreseeable; or replacing 50% of the total blood volume within 3 hours might also be appropriate in the acute clinical setting. ^73,74 Blood transfusion is the main therapeutic option for treating acute hemorrhage, and in trauma patients the ideal solution is fresh whole blood, although this is not widely available. ⁶ With life-threatening bleeding, multiple blood volumes may be transfused, leading to coagulopathy if sufficient factors are not replaced. The etiology of coagulopathy during massive transfusion is complex, involving dilution of factors, hypothermia, tissue hypoperfusion/ischemia, acidosis, and potential DIC. ⁷⁵

Because coagulation test results are not often available, massive transfusion protocols often use fixed doses of component therapies that include fresh frozen plasma and platelets after transfusion of a defined number of RBCs, often in an attempt to mimic the transfusion of fresh whole blood. In addition to fixed ratios, antifibrinolytic agents are also considered if fibrinolysis is present and are an important consideration of multimodal therapy as demonstrated in surgical patients and trauma. The role of factor concentrates to manage bleeding is increasingly evolving in clinical algorithms. ^76–78

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Biochemistry of Hemostasis

N.V. BHAGAVAN , in Medical Biochemistry (Fourth Edition), 2002

Laboratory-Created Artifacts

A plasma sample was obtained from a female patient for routine coagulation testing, i.e., there was no history of bleeding. The PT was 11.9 s (normal 10.5-12.2 s). The APTT was 80 s (normal 20-32 s). Sample was sent to a reference laboratory specializing in coagulation factor testing. At the reference laboratory, the PT was 11.6 s and the APTT was 43 s. Because of this discrepancy, blood was drawn at the reference laboratory and a new plasma sample prepared. In a second set of tests performed at the reference laboratory, the PT was 10.9 s and the APTT was 94 s. What might account for the discrepancy between the results obtained on the original plasma sample in the two laboratories?

•

If the initial sample was not centrifuged appropriately to remove the platelets, platelet fragmentation and microparticle formation could have occurred during transport to the reference laboratory The membrane fragments could artifactually shorten the APTT

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Coagulation Monitoring

Linda Shore-Lesserson MD , in Essentials of Cardiac Anesthesia, 2008

Thromboelastography

The coaguloviscometers that were developed in the 1920s formed the basis of viscoelastic coagulation testing that is now known as thromboelastography. Thromboelastography in its current form was developed by Hartert in 1948 and has been used in many different clinical scenarios to diagnose coagulation abnormalities. ²⁰ Although not yet truly portable, the thromboelastograph (TEG; Haemoscope, Skokie, IL) can be performed "on site" either in the operating room or in a laboratory and provides a rapid whole blood analysis that yields information about clot formation and clot dissolution. Within minutes, information is obtained regarding the integrity of the coagulation cascade, platelet function, platelet-fibrin interactions, and fibrinolysis. The principle is as follows: whole blood (0.36 mL) is placed into a plastic cuvette into which a plastic pin is suspended; this plastic pin is attached to a torsion wire that is coupled to an amplifier and recorded; a thin layer of oil is added to the surface of the blood to prevent drying of the specimen; and the cuvette oscillates through an arc of 4 degrees, 45 minutes at 37°C. When the blood is liquid, movement of the cuvette does not affect the pin. However, as clot begins to form, the pin becomes coupled to the motion of the cuvette and the torsion wire generates a signal that is recorded. The recorded tracing can be stored by computer, and the parameters of interest are calculated using a simple software package. Alternatively, the tracing can be generated on line with a recording speed of 2 mm/min. The tracing generated has a characteristic conformation that is the signature of the TEG (Fig. 12-5).

The specific parameters measured by the TEG include the reaction time (R value), coagulation time (K value), a angle, maximal amplitude (MA), amplitude 60 minutes after the MA (A60), and clot lysis indices at 30 and 60 minutes after MA (LY30 and LY60, respectively). The reaction time, R, represents the time for initial fibrin formation and is a measure of the intrinsic coagulation pathway, the extrinsic coagulation pathway, and the final common pathway. R is measured from the start of the bioassay until fibrin begins to form and the amplitude of the tracing is 2 mm. Normal values vary depending on the types of activator used and range from 7 to 14 minutes using celite activator and are as short as 1 to 3 minutes using tissue factor activator. The K value is a measure of the speed of clot formation and is measured from the end of the R time to the time that the amplitude reaches 20 mm. Normal values (3 to 6 minutes) also vary with the type of activators used. The a angle, another index of speed of clot formation, is the angle formed between the horizontal axis of the tracing and the tangent to the tracing at 20-mm amplitude. Alpha values normally range from 45 to 55 degrees. Because both the K value and the a angle are measures of the speed of clot strengthening, each is improved by high levels of functional fibrinogen. MA (normal is 50 to 60 mm) is an index of clot strength as determined by platelet function, the cross-linkage of fibrin, and the interactions of platelets with polymerizing fibrin. The peak strength of the clot, or the shear elastic modulus "G," has a curvilinear relation with MA and is defined as G = (5000 • MA)/(96 – MA). The percent reduction in MA after 30 minutes reflects the fibrinolytic activity present and is normally not more than 7.5%.

Characteristic TEG tracings can be recognized to be indicative of particular coagulation defects. A prolonged R value indicates a deficiency in coagulation factor activity or level and is seen typically in patients with liver disease and in patients on anticoagulants such as warfarin or heparin. MA and a angle are reduced in states associated with platelet dysfunction or thrombocytopenia and are even further reduced in the presence of a fibrinogen defect. LY 30, or the lysis index at 30 minutes after MA, is increased in conjunction with fibrinolysis. These particular signature tracings are depicted in Figure 12-6.

TEG is a useful tool for diagnosing and treating perioperative coagulopathy in patients undergoing cardiac surgical procedures due to a variety of potential coagulation defects that may exist. Within 15 to 30 minutes, on-site information is available regarding the integrity of the coagulation system, the platelet function, fibrinogen function, and fibrinolysis. With the addition of heparinase, TEG can be performed during CPB and can provide valuable and timely information regarding coagulation status. ²¹ Because TEG is a viscoelastic test and evaluates whole blood hemostasis interactions, it is suggested that TEG is a more accurate predictor of postoperative hemorrhage than routine coagulation tests that analyze individual components of the hemostasis system. A number of clinical trials have confirmed that in cardiac surgical patients TEG has a greater predictive value and greater specificity than routine coagulation tests for diagnosing patients known as "bleeders." Tuman and associates studied 42 patients, of whom 9 were classified as bleeders.²¹ A routine coagulation screen consisting of ACT, PT, aPTT, and platelet count had only a 33% accuracy for predicting bleeding, whereas TEG and Sonoclot (Sienco Inc., Morrison, CO) (another viscoelastic test) had 88% and 74% accuracy, respectively. Other investigators have also found that TEG abnormalities predict postoperative bleeding and, using TEG parameters, they were also able to identify a population of patients who respond to therapy with desmopressin acetate.

In a large retrospective evaluation in more than 1000 patients, Spiess and associates found that the institution of a transfusion algorithm using TEG resulted in a significant reduction in the incidence of mediastinal exploration and in the rate of transfusion of allogeneic blood products. ²² Because of its ease of use and application at the bedside, TEG has been used in many research settings to assess drug effects on platelet function and clot strength.

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Overview of Purposes of Hemostasis Testing and Common Sources of Error

Mikhail Roshal MD, PhD , Morayma Reyes Gil MD, PhD , in Transfusion Medicine and Hemostasis (Third Edition), 2019

Monitoring Hemostasis System Under Stress and Assessing the Need for Replacement Products

Surgery and severe trauma challenge the hemostasis system in numerous ways. Because of the reductive nature of much of the current coagulation testing, no individual standard test can measure the numerous perturbations associated with these challenges. Numerous tests measuring plasma elements (PT, PTT, and fibrinogen), platelet number and function, and occasionally fibrinolysis may be needed to sufficiently assess the status of the hemostasis system under stress. More integrative tests, such as thrombin generation assays that are capable of capturing thrombin activity past the point of clot formation and thromboelastography/thromboelastometry that measure clot formation and stability in whole blood, offer attractive alternatives to the more specific tests. However, the usage of these tests is still evolving and their exact place in the arsenal of testing is yet to be determined. At present, it is not clear whether the global hemostasis tests improve patient outcomes compared with the standard screening tests.

Outside of trauma and surgery, various disease states can have numerous effects on the hemostasis system either as primary manifestation (e.g., heparin-induced thrombocytopenia) or as secondary manifestation (disseminated intravascular coagulation in sepsis). In such cases testing may be targeted to specific suspected abnormalities.

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Overview of Purposes of Hemostasis Testing and Common Sources of Error

Mikhail Roshal MD, PhD , in Transfusion Medicine and Hemostasis (Second Edition), 2013

Monitoring Hemostasis System Under Stress, and Assessing the Need for Replacement Products

Surgery and severe trauma challenge the hemostasis system in numerous ways. Because of the reductive nature of much of the current coagulation testing, no individual standard test can measure the numerous perturbations associated with these challenges. Numerous tests measuring plasma elements (PT, PTT, and fibrinogen), platelet number and function, and occasionally fibrinolysis may be needed to sufficiently assess the status of the hemostasis system under stress. More integrative tests, such as thrombin generation assays that are capable of capturing thrombin activity past the point of clot formation, and thromboelastography/thromboelastometry that measure clot formation and stability in whole blood, offer attractive alternatives to the more specific tests. However, the usage of these tests is still evolving and their exact place in the arsenal of testing is yet to be determined. At present it is not clear whether the global hemostasis tests improve patient outcomes compared to the standard screening tests.

Outside of trauma and surgery, various disease states can have numerous effects on the hemostasis system either as primary manifestation (e.g. heparin-induced thrombocytopenia [HIT]) or as secondary manifestation (disseminated intravascular coagulation [DIC] in sepsis). In such cases testing may be targeted to specific suspected abnormalities.

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TRANSFUSION: MANAGEMENT OF BLOOD AND BLOOD PRODUCTS IN TRAUMA

Lena M. Napolitano , in Current Therapy of Trauma and Surgical Critical Care, 2008

MASSIVE TRANSFUSION

Massive blood transfusion is most commonly defined as complete replacement of a patient's blood volume within a 24-hour period or more than 10 units of PRBCs in 24 hours. Newer definitions include an ongoing blood loss of more than 150 ml/min, or the replacement of 50% of the circulating blood volume in 3 hours or less. ²¹ These newer definitions have the benefit of allowing early recognition of major blood loss and of the need for effective intervention to prevent hemorrhagic shock and other complications of massive hemorrhage and transfusion.

Massive transfusion therapy for the treatment of hemorrhagic shock requires a coordinated and detailed approach with the fundamental components listed in Table 4. In the absence of a predefined massive transfusion protocol, access to the appropriate blood products (and adequate volume of these products) may be significantly delayed. Without prompt replacement of these blood products, the resultant coagulopathy may worsen and bleeding will continue. In fact, the implementation of an organized "Massive Transfusion Policy" to address exsanguinating hemorrhage in the trauma population has proven of benefit in patient outcomes and in reducing blood product utilization. Studies have demonstrated an increase in survival (16%–45%) in patients with exsanguinating hemorrhage following the implementation of such a protocol. ^{22, 23} Guidelines for the treatment of acute massive blood loss are listed in Table 5.

Blood Component Therapy: Fresh Frozen Plasma, Platelets, and Cryoprecipitate

Coagulopathy and thrombocytopenia are a common occurrence during major trauma resuscitation, and hemorrhage remains a major cause of traumatic deaths. Current coagulation factor replacement practices vary, and may be inadequate. A recent pharmacokinetic model was used to simulate the dilutional component of coagulopathy during hemorrhage and compared various fresh frozen plasma (FFP) transfusion strategies for the prevention or correction, or both, of dilutional coagulopathy. This study documented that once excessive deficiency of factors has developed and bleeding is unabated 1–1.5 units of FFP must be given for every unit of PRBC transfused. If FFP transfusion should start before plasma factor concentration drops below 50% of normal, an FFP:PRBC transfusion ratio of 1:1 would prevent further dilution. They concluded that during resuscitation of a patient who has undergone major trauma the equivalent of whole-blood transfusion is required to correct or prevent dilutional coagulopathy. ²⁴

Ultimately, additional blood product administration for the treatment of dilutional and consumptive coagulopathy and thrombocytopenia in trauma must be guided by blood coagulation testing, with regular monitoring of hemoglobin, platelet count, prothrombin time (PT), partial thromboplastin time (PTT), and fibrinogen levels (see Table 5). Component replacement therapy recommendations differ in many institutions. Some advocate 1:1:1 unit replacements of PRBCs, FFP, and platelets. Others recommend platelet concentrations (1 pack/10 kg) if platelet count falls below 50,000, recognizing that each platelet concentrate also provides about 50 ml of fresh plasma. FFP (12 ml/kg) is administered if the INR, PT, or PTT are greater than 1.5 times control levels. Cryoprecipitate (1–1.5 packs/10 kg) is administered if fibrinogen concentrations are lower than 0.8 g/l. Some suggested massive transfusion protocols include:

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Initial massive transfusion response: 10 units PRBCs, 2 units platelets, 4 units plasma

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Subsequent massive transfusion response: 6–8 units PRBCs, 2 units platelets, 4 units plasma

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Cryoprecipitate requested in patients with reduced fibrinogen concentration

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