CONTENTS

• Cell-based coagulation model
• Traditional tests of enzymatic coagulation
  • INR prolongation with normal PTT
  • PTT prolongation with normal INR
  • Prolongation of both INR & PTT
  • Thrombin Time & fibrinogen “level” (Clauss assay)
• Thromboelastography (TEG)
  • Strengths and weaknesses of TEG
  • TEG 5000 interpretation
  • TEG 6S interpretation
  • TEG with platelet mapping
• PFA (Platelet function analyzer)
• ACT (activated clotting time)
• Apixaban level ➡️

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cell-based coagulation model

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All models are wrong, but some are useful. –George Box

Coagulation is an enormously complex process involving dozens of factors and numerous cell types – all occurring within the dynamic context of flowing blood. A perfect model for this process doesn't exist. Traditionally, our understanding of coagulation has been based on a severely reductionist approach that focuses solely on clotting factors (figure below).

This two-pathway coagulation model is inexorably linked to traditional coagulation tests (INR and PTT). However, reality is far more complex than this. For starters, coagulation factors participate in dynamic auto-amplifying loops (see green arrows below). Thus, rather than being a linear process, coagulation involves cyclical amplification.

The next level of complexity involves understanding how coagulation factors interact with cellular elements. Traditional coagulation assays begin by separating the cellular components of blood via centrifugation and discarding the cells. However, coagulation factors interact with cells in critical ways. Therefore, the most accurate model of coagulation will account for these interactions (i.e., a cell-based coagulation model). (30711233) This model still requires understanding coagulation factors, but takes this understanding to the next level by mapping the activation of coagulation factors onto cells. For example, cellular activation may alter its phospholipid expression, facilitating the binding of coagulation factors. Current cell-based coagulation models divide coagulation into roughly three phases:

#1 initiation phase

• Coagulation is initiated by the expression of tissue factor (TF) on the cell. The cells involved at this phase are often connective tissue cells that are not generally exposed to the bloodstream (e.g., fibroblasts, vascular smooth muscle cells). During hemorrhage, blood comes into contact with these cells, initiating coagulation.  Immunostimulated endothelial cells may also express tissue factor.
• Tissue factor promotes the activation of factors VIIa, Xa, Va, and IIa. This generates thrombin via the tissue factor (extrinsic) pathway.
• Any activated factor Xa that dissociates from the cell membrane will be rapidly inactivated by tissue factor pathway inhibitor (TFPI) or antithrombin. Thus, thrombin generation is localized to the surface of the tissue factor-bearing cell.

#2 amplification phase

• Some thrombin (factor IIa) binds to nearby platelets, activating them. Platelet activation has numerous consequences:
  • Activation stimulates the release of platelet granules, which contain numerous procoagulant substances.
  • Platelet activation triggers changes in the platelet membrane phospholipids, producing a procoagulant membrane surface on the platelets.
• Thrombin on the platelet surface starts to generate some activated coagulation factors:
  • Thrombin cleaves XI to XIa
  • Thrombin cleaves V to Va
  • Thrombin cleaves the vWF-VIII complex, yielding activated VIIIa and free vWF. The free vWF (von Willebrand factor) mediates platelet adhesion and aggregation.

#3 propagation phase

• Additional platelets are recruited due to platelet granule release and vWF activity.
• XIa activates the intrinsic pathway, leading to further activation of thrombin. This leads to a cyclical auto-amplifying loop involving factors XI, IX, X, V, and II (the intrinsic clotting pathway). This results in the generation of lots of thrombin (known as a “thrombin burst”).
• Large amounts of thrombin (IIa) lead to fibrin generation and clot formation.

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traditional tests of enzymatic coagulation

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The coagulation cascades are shown above, along with standard tests that investigate specific portions of the cascade. Please note that these cascades are useful for diagnostic purposes, especially for identifying patients with single-factor deficiency (e.g., hemophilia). However, these labs don't correlate well with clinical bleeding in patients with complex coagulopathies (e.g., cirrhosis, DIC).

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prolongation of INR with normal PTT

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physiology and general comments

• Isolated INR prolongation often indicates factor VII deficiency, as this is the factor involved in INR but not PTT (figure above). However, mild deficiencies in common pathway proteins (factor X, V, II, or fibrinogen) can also cause predominant abnormalities in the INR.
• INR prolongation correlates with clinical clotting in patients on warfarin. However, INR prolongation does not correlate well with bleeding among patients with liver disease or DIC (who often have numerous coagulation abnormalities, including deficiencies of endogenous anticoagulants, such as proteins C and S).
• INR represents a method of standardizing PT (prothrombin time) values across different laboratories. The clinical significance of INR and PT prolongation is identical. INR is preferred over PT because it is more reproducible and easier to interpret.

differential diagnosis of INR elevation with normal PTT

• If INR corrects one day after IV vitamin K:
  • Vitamin K deficiency (not uncommon in the ICU, especially among chronically critically ill patients).
  • Warfarin.
• If INR doesn't correct one day after IV vitamin K:
  • Liver disease (cirrhosis or acute liver failure).
  • Factor Xa inhibitors (e.g., rivaroxaban, apixaban).
  • DIC.
  • Severe lupus anticoagulant.
  • Factor VII inhibitor (extremely rare).
  • Mild, nonspecific elevation (ICU patients often have elevations in the ~1.2-1.6 range, of no clinical significance). (DeLoughery 2019)

intravenous vitamin K challenge

• 10 mg of IV vitamin K may be used as a therapeutic and diagnostic approach to help understand the cause of INR prolongation.
• If vitamin K administration results in INR reduction, this reveals and treats vitamin K deficiency or warfarin effect.
• If vitamin K administration doesn't normalize the INR (or incompletely normalizes the INR), this excludes the presence of isolated vitamin K deficiency.
• Vitamin K must be given intravenously (rather than orally) to exclude the possibility of malabsorption. (More discussion about the administration of intravenous vitamin K here.)

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prolonged PTT with normal INR

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physiology and general comments

• Isolated PTT abnormalities often reflect deficiencies of factors XII, XI, IX, or VIII (as these are unique to the intrinsic activation pathway; see figure above).
• Deficiency of factors XII or XI may elevate PTT without causing a substantial clinical coagulopathy. These factors are involved in laboratory measurement of PTT, but are less critical for in vivo clotting.
• Elevated levels of factor VIII may shorten the PTT. This is most often seen in inflammatory states, but also in pregnancy, uremia, and in patients on cyclosporine. (DeLoughery 2019) Elevated levels of VIII may lead to heparin or argatroban pseudoresistance (discussed further here).

mixing studies

• Only ~50% of most coagulation factors are required to generate normal-range coagulation labs. Therefore, if PTT elevation is simply due to a factor deficiency, mixing the patient's plasma with normal plasma in a 1:1 ratio should yield a mixture with normal PTT values.
• If the PTT elevation is caused by a factor inhibitor (e.g., neutralizing antibodies or lupus anticoagulant), then mixing with normal plasma in a 1:1 ratio will not result in a mixture with normal PTT values.
• Mixing studies may be used to parse out the causes of PTT elevation as follows.

differential diagnosis of isolated PTT elevation

• PTT corrects with mixing: Deficiency of factors XII, XI, IX, or VIII.
  • Hemophilia A (factor VIII deficiency)
  • Severe von Willebrand disease with factor VIII deficiency
  • Hemophilia B (factor IX deficiency)
  • Factor XI deficiency (minimally symptomatic)
• PTT fails to correct with mixing:
  • Unfractionated heparin (including heparin contamination). Note that low-molecular-weight heparin often doesn't affect the PTT, so standard coagulation assays may not detect it.
  • Lupus anticoagulant (will correct if exogenous phospholipid is added).
  • Acquired inhibitor of factor VIII, IX, or XI.
• (PTT elevation may also result from artifact; in one series 14% of PTT elevations were artefactual)(32685885)

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prolongation of both INR and PTT

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physiology and general comments

• Prolongation of both INR and PTT suggests a deficiency of factors shared by both pathways (factors X, V, II, and fibrinogen).
• Global aberrations in the coagulation system will tend to affect both INR and PTT (e.g., DIC).

differential diagnosis of INR & PTT elevation

• More common:
  • Severe warfarin effect or vitamin K deficiency.
  • Severe liver dysfunction.
  • Medication effect:
    • Direct thrombin inhibitor (e.g., argatroban, dabigatran).
    • High level of unfractionated heparin.
  • DIC.
  • Extremely low fibrinogen (below ~80 mg/dL). Note that normal PT and INR don't exclude hypofibrinogenemia.
• Less common:
  • Lupus anticoagulant, severe.
  • Rare deficiency or inhibitor involving factors V, X, or II.
  • Elevated fibrinogen degradation products (following thrombolysis).
  • Massive hemorrhage with dilution of coagulation factors.
  • Factor X deficiency is associated with systemic amyloidosis. (32685885)

approach to elevated INR & PTT of unclear etiology

• Review the medication list (look for any thrombin inhibitors or heparin).
• Check fibrinogen and D-dimer levels (to evaluate for hypofibrinogenemia or DIC).
• Check liver function tests.
• If vitamin K deficiency or warfarin is a possibility, a diagnostic/therapeutic challenge with IV vitamin K could be considered.

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thrombin time & fibrinogen level

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physiology

• Both tests involve adding activated thrombin to the patient's plasma. The assays evaluate the ability of the thrombin to catalyze the polymerization of fibrinogen.
• Thrombin time uses undiluted plasma.
  • This renders the thrombin time more sensitive to inhibitors (e.g., heparin).
  • Thrombin time will also be prolonged by hypofibrinogenemia or dysfunctional fibrinogen.
• The Clauss assay for fibrinogen uses diluted plasma.
  • This dilutes out inhibitors (e.g., heparin), allowing the Clauss assay to focus more on fibrinogen function.
  • Although the Clauss assay is typically referred to as a “fibrinogen level,” it actually measures fibrinogen function.

differential diagnosis of low fibrinogen function (using the Clauss fibrinogen assay)

• DIC.
• Liver disease.
• Dilution following massive transfusion.
• Hyperfibrinolysis:
  • Following administration of a thrombolytic agent.
  • Malignancy or cirrhosis (i.e., Accelerated Intravascular Coagulation and Fibrinolysis).
• Inherited fibrinogen abnormalities (e.g., hypofibrinogenemia, dysfibrinogenemia).

differential diagnosis of a prolonged thrombin time

• (1) Any cause of low fibrinogen (see list immediately above 👆).
• (2) Inhibitor substances (these will not affect the Clauss fibrinogen assay very much, although high levels of unfractionated heparin may cause some reduction in the Clauss fibrinogen level.)
  • Medications inhibiting thrombin:
    • Direct thrombin inhibitor (e.g., argatroban, dabigatran).
    • High level of unfractionated heparin.
  • Elevated fibrinogen degradation products (following thrombolysis).

clinical utilization of thrombin time versus Clauss fibrinogen assay

• Thrombin time and fibrinogen provide relatively similar information.
• Thrombin time is less widely available and has a slower turnaround time. Therefore, fibrinogen is more widely utilized clinically. Fibrinogen is also a more actionable laboratory study, with defined transfusion targets.

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strengths and weaknesses of TEG

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strengths of the TEG

• TEG provides a more integrative evaluation of coagulation, allowing assessment of interactions among coagulation factors, endogenous anticoagulants, and cellular elements.
  • TEG allows evaluation of platelet function, whereas traditional coagulation laboratories merely monitor the platelet count.
  • TEG can account for the balance of coagulation by measuring both clotting factors and endogenous anticoagulant proteins.
• TEG allows evaluation of thrombolysis, which isn't measurable using conventional coagulation assays.
• TEG allows rapid testing of the effects of heparin on coagulation (this isn't possible using conventional studies available at most hospitals).
• TEG allows clinicians to avoid focusing on INR (leading to excessive use of fresh-frozen plasma).  Avoidance of INR-driven blood product usage might be the single most significant advantage of using TEG.
• TEG allows clinicians to evaluate platelet function, rather than platelet number. When compared with platelet counts, using TEG as a trigger for platelet transfusion may also reduce platelet transfusion. (33089934)
• TEG may achieve faster turnaround times than conventional coagulation laboratories. Many hospitals have online portals that allow clinicians to view the TEG tracing in real time.

weaknesses of TEG

• Standard TEG typically doesn't measure the effects of most antiplatelet drugs (e.g., aspirin, clopidogrel). Detection of these medications requires a specific platelet mapping study.
  • The low-shear conditions under which TEG occurs result in so much thrombin generation that most platelet-activation pathways are unnecessary. Thus, TEG is insensitive to antiplatelet medications other than glycoprotein IIb/IIIa inhibitors. (33089939)
  • TEG can sometimes be affected by GPIIb-IIIa inhibitors (e.g., abciximab, eptifibatide). (31878831)
• TEG isn't sensitive for detecting DOACs (e.g., rivaroxaban, apixaban, dabigatran).
• TEG is insensitive to many other coagulation abnormalities:
  • Von Willebrand disease.
  • Hypothermia (the assay is performed at 37 °C).
  • Hypocalcemia.
  • Deficiencies in antithrombin-III, protein C, protein S, or factor V Leiden.
• TEG is not able to pinpoint the diagnosis of uncommon coagulation abnormalities (e.g., differentiate factor VII deficiency versus factor VIII deficiency).

situations where TEG is most useful

• TEG is most helpful in complex coagulopathies where multiple coagulation abnormalities are occurring simultaneously:
  • Cirrhosis.
  • DIC, including trauma-induced coagulopathy.
  • Complex intraoperative coagulopathies (e.g., cardiothoracic surgery, liver transplant surgery).
  • Extracorporeal membrane oxygenation (ECMO).
• TEG is less valuable for investigating a simple coagulation question:
  • Coagulation status of an uncomplicated patient on coumadin (INR works just fine).
  • Coagulation status of an uncomplicated patient on heparin (PTT or anti-Xa level is fine).

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TEG 5000 interpretation

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TEG-5000 yields a multitude of parameters. These parameters are often misinterpreted. Most notably, the alpha-angle is frequently regarded as a measurement of fibrinogen function. This is a myth: the alpha-angle is actually a reflection of both the fibrinogen and platelet functions. (26197367) To actually assess fibrinogen function, a TEG 6S assay is required (see below).  

If we can ignore the redundant and non-evidence-based parameters, interpreting the TEG-5000 is more straightforward. There are really just a few key pieces of information:

reaction time (R-time)

• This measures the time lag before clotting begins following the addition of kaolin (a contact activator that triggers the intrinsic pathway).
• R-time is a measurement of enzymatic coagulation via the intrinsic pathway (most analogous to PTT).
• Prolonged R-time in a bleeding patient indicates hypocoagulability. This may trigger the administration of fresh-frozen plasma or prothrombin complex concentrates (PCC).
• Normal R-time reveals intact enzymatic coagulation. Such patients won't benefit from fresh frozen plasma (FFP). Finding a normal R-time can be an enormously beneficial finding in patients with cirrhosis or DIC who have an elevated INR and/or PTT, who might otherwise tend to be transfused with fresh frozen plasma.
• Reduced R-time may suggest hypercoagulability, with an increased risk of thromboembolic complications. (31263903)

heparin effect

• If the R-time is prolonged (revealing enzymatic hypocoagulation), the lab will reflexively perform a TEG with heparinase (which eliminates the effect of heparin).
• If the R-time of the second TEG (the “heparinase TEG”) is >25% lower than the initial TEG (the “native TEG”), this indicates the presence of a heparin effect.
• Exogenous or endogenous heparins may cause a heparin effect:
  • Exogenous heparins are therapeutically administered to the patient (including unfractionated heparin or low-molecular-weight heparin). (28267938)
  • Endogenous heparins are generated by the patient as a result of degradation of the endothelial glycocalyx (which contains heparins). Presence of endogenous heparins (“autoheparinization”) is very worrisome, as it often reflects severe inflammation or endothelial damage.

maximum amplitude (MA)

• This is the maximal clot strength. It reflects the combined function of fibrinogen and platelets.
• Low MA in a bleeding patient may suggest benefit from platelet and/or fibrinogen transfusion. The choice of platelets versus fibrinogen may depend on the clinical context:
  • Traditional laboratory testing (e.g., complete blood count and/or fibrinogen levels) may help determine which is more deficient. Likewise, a more specialized TEG assay (e.g., TEG functional fibrinogen) may help sort this out.
  • When in doubt, the use of fibrinogen may be preferable, as fibrinogen is consumed more slowly than platelets and may pose fewer risks than platelet transfusion. Critically ill patients (especially those with cirrhosis) may tend to consume platelets, rendering platelet transfusion ineffective. Furthermore, even in the presence of thrombocytopenia, fibrinogen administration may still improve the MA. (19224779) Some studies have found that using fibrinogen as a front-line therapy to maintain adequate maximum amplitude was associated with reduced blood loss. (28267938)

lysis in 30 minutes (LY30)

• LY30 reflects the degree of clot breakdown over thirty minutes. Unfortunately, TEG-5000 is relatively insensitive to hyperfibrinolysis. (33089934) Thus, a normal LY30 doesn't exclude hyperfibrinolysis. Most studies and clinicians define an abnormal LY30 as >3%. (31263903)
• Elevated LY30 suggests hyperfibrinolysis. In the context of clinical hemorrhage, this suggests benefit from a fibrinolytic inhibitor (e.g., tranexamic acid).
• No clot lysis at all (i.e., LY30 = 0) may suggest a situation where fibrinolysis is inhibited (a.k.a., “fibrinolytic shutdown”), which might increase the risk of thrombosis. However, this is commonly observed among critically ill patients, so it is a nonspecific finding.

consensus algorithm of the Society of Cardiovascular Anesthesiologists

• Below is a consensus algorithm for using TEG-5000 to guide blood product use in the management of hemorrhage during cardiothoracic surgery. (31613811) Some points are notable regarding this algorithm:
  • (1) The algorithm recognizes that alpha-angle is not a suitable target to drive fibrinogen transfusion (as discussed above). Ideally, the administration of fibrinogen should be based upon the combination of a reduced MA and a reduction in the functional fibrinogen (FF).
  • (2) TEG parameters don't need to be normalized. Instead, only substantially abnormal values may mandate action (e.g., MA <40).

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TEG 6S interpretation

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TEG 6S normal values

If there is any possibility that the patient is under the influence of endogenous or exogenous heparin, a global hemostasis with heparin neutralization cartridge should be sent. This may generally be preferable for critically ill patients, because it is unpredictable which patients shed their endothelial glycocalyx, leading to auto-heparinization.  

🏆 Global hemostasis with Heparin Neutralization (HN) cartridge

• Citrated Kaolin (“native TEG”):
  • CK R-time 4.6-9.1 minutes (40016048)
  • CK MA 52-69 mm (40016048)
  • CK LY-30 <2.6% (40170216)
• Kaolin with Heparinase (“heparinase TEG”):
  • CKH R-time 4.3-8.3 minutes (40016048)
  • CKH LY-30 <3.2% (40016048)
• RapidTEG with Heparinase:
  • CRT MA 53-69 mm (40016048)
• Citrated Functional Fibrinogen with Heparinase:
  • CFF MA 15-43 mm (40016048)
  • Fibrinogen level (mg/dL) is roughly equal to [13(CCFH-MA) – 35]. (40408275)
  • (The older Global Hemostasis cartridge doesn't include heparinase in the functional fibrinogen assay. Consequently, if heparin is present, it may lead to aberrant fibrinogen results. (41177689)

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interpretation of TEG 6S & use to guide hemorrhage management

The following algorithm is based primarily on an expert opinion review regarding the management of bleeding during cardiothoracic surgery. (40016048) It may need to be modified when approaching other forms of bleeding (e.g., gastrointestinal hemorrhage is often less threatening and may require less aggressive product support). This algorithm doesn't apply to patients who aren't actively bleeding.

[1/5] Residual heparin effect

• Diagnosis:
  • CK R-time >1.25-2 times higher than CKH R-time indicates heparin effect. (40016048)
  • TEG is highly sensitive to heparin, so slight differences in R-time may not reflect clinically significant heparin levels. Therapeutically anticoagulated patients often have unmeasurably prolonged R-time.
  • Protamine has a significant side-effect profile, and heparin will generally wear off on its own. Consequently, the decision to treat must be made on a patient-by-patient basis.
• Action:
  • Administration of protamine (e.g., 50 mg). (40016048)
  • (Further discussion of protamine pharmacology & dose adjustment based on heparin exposure: 💉)

[2/5] Clotting factor deficiency

• Diagnosis:
  • CKH-R-time is significantly prolonged (normal range: 4.3-8.3 minutes).
• Action:
  • CKH R-time 10-12 minutes: 12-15 U/kg PCC or ~2 U FFP. (40016048)
  • CKH R-time 13-15 minutes: 15-20 IU/kg PCC or ~3 U FFP. (40016048)
  • CKH R-time >15 minutes: 20-25 U/kg PCC or ~4 U FFP. (40016048)

[3/5] Fibrinogen deficiency

• Diagnosis:
  • Functional Fibrinogen MA is low (normal CFF-MA is 15-43 mm).
  • The CFF-MA cutoff of 15 mm is designed to detect a fibrinogen <150 mg/dL. (40016048) In some contexts, it may be desirable to target a higher fibrinogen level (e.g., >200 mg/dL). When targeting a fibrinogen level >200 mg/dL, the ideal CCF-MA target may be >19-20 mm. (40408275) Indeed, some authors have recommended targeting a CFF-MA value of >20 mm even if only targeting a fibrinogen level >150 mg/dL. (33141779)
• Action:
  • CFF-MA 15-20: May consider giving fibrinogen (discussion above). (33141779)
  • CFF-MA 10-14 mm: 4-6 grams fibrinogen concentrate 💉 or 5-10 pool units of cryoprecipitate 💉. (40016048, 40170216)
  • CFF-MA <10 mm: Consider higher dosing. (40016048)

[4/5] Platelet deficiency

• Diagnosis:
  • CFF-MA is normal (>15-20 mm) and CRT-MA is low (normal CRT-MA is 53-69 mm).
  • (If CFF-MA and CRT-MA are both low, then give fibrinogen as described above in step #3 and then re-assess with a repeat TEG.)
  • (The rapid TEG is utilized in this algorithm to accelerate therapy, because it will come back quickly. The CRT-MA and CK-MA are generally the same.)
• Action:
  • MA 48-50 mm: DDAVP 0.3 ug/kg.
  • MA 43-47 mm: DDAVP 0.3 ug/kg plus 1 unit of platelets.
  • MA ≦42 mm: DDAVP 0.3 ug/kg plus 2 units of platelets.

[5/5] Hyperfibrinolysis

• Diagnosis:
  • LY-30 increased (normal CK LY-30 is <2.6%). (40016048)
• Action:
  • LY-30 >2.6-3%: Aminocaproic or tranexamic acid 💉. (40016048, 40170216)

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thromboelastography with platelet mapping

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In standard TEG assays, so much thrombin is generated that platelets will become fully activated, regardless of whether aspirin or P2Y12 inhibitors (e.g., clopidogrel) are present. To detect aspirin or P2Y12 inhibitors, the assay must be modified.

Complete platelet mapping involves the comparison of four curves, as shown above:

• Citrated kaolin-activated TEG (a standard TEG). This evaluates the contributions of platelets plus fibrinogen to clot strength.
• ActF (Activation of Fibrinogen Only). This tracing is generated by the addition of reptilase and factor XIIIa, which activate fibrinogen alone. The amplitude of this curve reflects the contribution of fibrinogen to clot strength.
• AA (Arachidonic Acid) – This tracing is generated by activating fibrinogen (using reptilase and factor XIIIa) and also activating platelets via the arachidonic acid receptor. Usually, this combination should result in fibrinogen and platelet activation, yielding an MA similar to the citrated kaolin-activated TEG. However, if the arachidonic acid receptors are inhibited by aspirin, this tracing will be knocked down, pushing it closer to the ActF TEG.
• ADP (Adenosine DiPhosphate) – This tracing is generated by activating fibrinogen (using reptilase and factor XIIIa) and also activating platelets via the ADP receptor. In the presence of P2Y12 inhibitors (e.g., clopidogrel) that block the ADP receptor, this curve will be knocked down below the citrated kaolin-activated TEG.

The risk of bleeding can be assessed in a few different ways:

• % Inhibition measures the percentage reduction in MA when platelets are stimulated only via the arachidonic acid or ADP receptor. For example, 95% inhibition with arachidonic acid indicates a strong effect of aspirin on platelets (figure above, left).
  • One study of non-cardiac surgery found that, among patients exposed to clopidogrel, below ~35% inhibition with ADP indicated a low risk of perioperative bleeding. (25088505)
• The values of the MA of the AA-TEG and the ADP-TEG may be used to provide a more absolute assessment of clot strength.

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platelet function analyzer (PFA)

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basics of the test

• Whole blood flows through a narrow hole in a membrane coated with collagen and epinephrine (COL/EPI), or with collagen and ADP (COL/ADP).  Platelets adhere to the membrane, eventually forming a platelet plug. The time to platelet plug formation is the “closure time.”
  • A longer closure time indicates impaired platelet function.
• PFA is an integrative test that provides a global indicator of platelet function.
• ⚠️ Anemia or thrombocytopenia may cause prolonged closing times.  The PFA is not valid if the platelet count is <80,000/uL or the hematocrit is <30%. (15982339)

interpretation of the Platelet Function Analyzer (PFA) 100

• Normal closing times:
  • COL/EPI: 94 – 193 Seconds.
  • COL/ADP: 71 – 118 Seconds.
• Both closing times are normal:
  • No evident drug effect.
  • This excludes: severe thrombocytopenia, severe von Willebrand disease, or severe platelet dysfunction.
• Prolonged COL/EPI and normal COL/ADP:
  • Drug effect (aspirin or NSAID usually only affects COL/EPI).
  • Low hematocrit.
  • Mild thrombocytopenia.
  • Mild platelet dysfunction.
  • Mild von Willebrand's Disease (vWD).
• Normal COL/ADP and prolonged COL/EPI:
  • Rare event – consider laboratory error and consider repeating the test.
  • (The collagen/epinephrine cartridge tends to be more sensitive than the collagen/ADP cartridge.)
• Prolonged COL/EPI and prolonged COL/ADP:
  • Drug effect.
  • Very low hematocrit.
  • Severe thrombocytopenia.
  • Severe platelet dysfunction.
  • Severe von Willebrand disease. (27935090)  

factors affecting the PFA

• Platelet count and function (e.g., younger platelets are more active).
• Hematocrit:
  • Higher hematocrit levels will accelerate plug formation.
  • Anemia may prolong the closure time and, thereby, be misconstrued as indicating platelet dysfunction.
• Uremia may cause platelet dysfunction.
• Antiplatelet medications:
  • COX inhibitors (e.g., aspirin or NSAIDs) should affect only the collagen/epinephrine cartridge.
  • GpIIb/IIIa receptor inhibitors tend to affect both cartridges.
  • P2Y12 inhibitors (e.g., clopidogrel) have variable effects. Recently, a new cartridge type has been designed to detect P2Y12 inhibition (Innovance PFA P2Y™️).
• Plasma levels of von Willebrand factor (vWF).
• Genetic abnormalities:
  • PFA will detect many genetic platelet abnormalities (e.g., Bernard-Soulier syndrome, Glanzmann's thrombasthenia, and von Willebrand disease).
  • Some mild genetic platelet disorders (e.g., secretion defects) may be missed.

possible clinical roles of the PFA

• (1) Screening for von Willebrand disease or other platelet disorders.
• (2) Identification of patients with residual antiplatelet effects (especially from aspirin).
  • PFA is a global test of platelet function, so it won't always correlate with aspirin use. For example, patients with high levels of von Willebrand factor may have normal PFA results despite taking aspirin. (27935090)
  • Platelet mapping might be superior for evaluating specific drug effects, since platelet mapping tests the impact of a particular receptor activation (unlike PFA, which assesses global platelet function).
• (3) Prediction of the hemorrhage risk during surgery:
  • PFA may predict the need for platelet transfusion following surgery, especially if closure time is prolonged with both the collagen/epinephrine and collagen/ADP cartridges.(32125177; 30623627) The PFA test has also been recommended to evaluate for antiplatelet medication effect before neurosurgery, which seems to be its most common application in the ICU.(26714677)
  • If prolonged closure time is detected, pretreatment with DDAVP may reduce the need for perioperative transfusion. (27935090)

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ACT (activated clotting time)

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• ACT is measured by adding kaolin/celite/glass beads to a whole blood sample to trigger the intrinsic pathway (most analogous to PTT).
• ACT is often used in the OR, but not the ICU.
• ACT may be prolonged by heparin, hemofiltration, low fibrinogen, thrombocytopenia, and glycoprotein IIB/IIIa antagonists. (Flynn 2020)
• Benchmark values are below, but different devices may yield varying results:
  • ~105-167 seconds is a normal range.
  • ~250-300 seconds correlates with therapeutic heparin.
  • >450 seconds may be targeted during cardiac surgery.

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questions & discussion

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