7.5 Viscoelastic Testing, Heparin Neutralization & Chromogenic Assays
Key Takeaways
- Viscoelastic assays measure whole-blood clot kinetics: R time / CT reflects coagulation factors, K value and alpha angle reflect fibrinogen, maximum amplitude / maximum clot firmness reflects platelets, and LY30 / maximum lysis reflects fibrinolysis.
- Each viscoelastic abnormality maps to a specific product: prolonged R to plasma, low alpha or K to cryoprecipitate, low maximum amplitude to platelets, and raised LY30 to an antifibrinolytic.
- A prolonged thrombin time with a NORMAL reptilase time identifies heparin, because reptilase cleaves fibrinopeptide A directly and is not inhibited by the antithrombin-heparin complex.
- Heparin is neutralized by heparinase (enzymatic cleavage), polybrene / hexadimethrine bromide (cationic binding, built into heparin-insensitive reagents), and protamine sulfate; none of these neutralize fondaparinux or the direct oral anticoagulants.
- The chromogenic anti-Xa assay is inversely proportional to drug concentration, requires drug-specific calibration, and is falsely LOW if platelet factor 4 is released from an uncentrifuged specimen.
Viscoelastic Testing, Heparin Neutralization & Chromogenic Assays
The ASCP BOC hemostasis content area lists thromboelastography and heparin neutralization as named laboratory determinations alongside PT, APTT, and factor assays. Both sit outside the routine coagulation screen, and both are commonly under-studied. This section also covers the chromogenic anti-Xa assay that has displaced the APTT for several anticoagulants.
1. Viscoelastic Testing: Thromboelastography (TEG) and Rotational Thromboelastometry (ROTEM)
Routine PT and APTT measure only the time to the first strands of fibrin, roughly 5% of total thrombin generation, and they say nothing about platelet contribution or clot lysis. Viscoelastic assays measure the whole clot in whole blood, from initiation through maximum strength to fibrinolysis, and generate a single tracing.
Measurement Principle
A whole blood sample (citrated, then recalcified) sits in a cup with a pin suspended in it. In classic TEG the cup oscillates through a 4 degree 45 minute arc while the pin is stationary; in ROTEM the pin rotates while the cup is fixed. As fibrin strands and platelets bridge the cup and pin, the mechanical coupling increases, and the transmitted motion is plotted as a symmetric, cigar-shaped tracing whose width represents clot firmness.
Parameters and Their Clinical Translation
| TEG Term | ROTEM Term | What It Measures | Typical Adult Range (kaolin TEG) | Abnormality Points To | Targeted Product |
|---|---|---|---|---|---|
| R time (reaction time) | CT (clotting time) | Time from start to the first measurable clot (2 mm amplitude) | 5-10 minutes | Coagulation factor deficiency or heparin effect | Fresh frozen plasma (or protamine if heparin) |
| K value | CFT (clot formation time) | Time from 2 mm to 20 mm amplitude | 1-3 minutes | Fibrinogen and early platelet function | Cryoprecipitate or fibrinogen concentrate |
| Alpha angle | Alpha angle | Slope of the tracing; speed of fibrin cross-linking | 53-72 degrees | Fibrinogen level | Cryoprecipitate |
| MA (maximum amplitude) | MCF (maximum clot firmness) | Greatest width; ultimate clot strength, roughly 80% platelet-dependent | 50-70 mm | Platelet number or function | Platelet concentrate (and DDAVP where appropriate) |
| LY30 | ML (maximum lysis) | Percent amplitude lost 30 minutes after MA | 0-8% | Hyperfibrinolysis | Antifibrinolytic (tranexamic acid) |
Where Viscoelastic Testing Is Used
Cardiac surgery and cardiopulmonary bypass, liver transplantation, trauma resuscitation, and post-partum hemorrhage. Its value is speed and specificity: within 15 to 20 minutes it tells the team which product to give rather than triggering an empiric shotgun transfusion.
Limitations the Examination Tests
- Viscoelastic assays are performed on whole blood at 37 degrees Celsius and do not reproduce in-vivo hypothermia or acidosis unless deliberately programmed.
- They are insensitive to von Willebrand disease and to aspirin and P2Y12 inhibitors on the standard channel; a dedicated platelet-mapping channel is required for antiplatelet drugs.
- They do not replace the PT/INR for warfarin monitoring and are not standardized against an International Sensitivity Index.
- A heparinase-coated cup run in parallel with a standard cup isolates residual heparin: if R normalizes in the heparinase cup, the prolongation was heparin, not factor deficiency.
2. Heparin Neutralization and Heparin Interference
Heparin is the most common pre-analytical contaminant in coagulation testing, usually from a line draw or an inadequately cleared catheter dead space. It prolongs the APTT and the thrombin time and can spuriously prolong factor assays and lupus anticoagulant testing.
Recognizing Heparin: Thrombin Time versus Reptilase Time
| Result Pattern | Interpretation |
|---|---|
| Thrombin time prolonged, reptilase time NORMAL | Heparin (or another direct thrombin inhibitor). Reptilase, a snake venom enzyme from Bothrops atrox, cleaves fibrinopeptide A directly and is not inhibited by the antithrombin-heparin complex. |
| Thrombin time prolonged, reptilase time ALSO prolonged | Hypofibrinogenemia, dysfibrinogenemia, or high fibrin degradation products |
| Both normal | Fibrinogen and thrombin-fibrinogen interaction are intact |
Neutralization Methods
| Method | Chemistry | Practical Use |
|---|---|---|
| Heparinase (bacterial enzyme, e.g., Hepzyme) | Enzymatically cleaves the heparin polysaccharide chain | Added to plasma before repeat testing; if the APTT corrects, heparin was the cause. Also used as the heparinase TEG cup. |
| Polybrene (hexadimethrine bromide) | Cationic polymer that electrostatically binds and inactivates heparin | Built into "heparin-insensitive" thrombin time and dilute Russell viper venom time reagents so that heparinized samples can still be evaluated for a lupus anticoagulant |
| Protamine sulfate | Strongly basic protein forming a stable inactive salt with heparin | In-vivo reversal after cardiopulmonary bypass; also the basis of the protamine titration assay used to quantify heparin |
Important limits. Neutralization methods are designed for unfractionated heparin. They only partially reverse low-molecular-weight heparin and do not neutralize fondaparinux or the direct oral anticoagulants. Absorbent products such as activated charcoal are used instead to remove direct oral anticoagulants before lupus anticoagulant testing. Neutralizing reagents also have a saturation ceiling; grossly heparinized specimens should be recollected from a peripheral venipuncture rather than rescued chemically.
3. Chromogenic Anti-Xa Assays
The anti-Xa assay is the reference method for measuring heparins and direct factor Xa inhibitors, and it is unaffected by lupus anticoagulants, factor deficiencies, or an elevated baseline APTT.
- Principle. Patient plasma is mixed with a known excess of factor Xa. Heparin bound to the patient's antithrombin inhibits some of that Xa. Residual Xa cleaves a chromogenic substrate, releasing para-nitroaniline, which is measured at 405 nm. Colour is therefore inversely proportional to the anticoagulant concentration.
- Calibration is drug specific. Unfractionated heparin, low-molecular-weight heparin, fondaparinux, and each direct Xa inhibitor require their own calibrator; results are not interchangeable.
- Typical therapeutic targets. Unfractionated heparin 0.3 to 0.7 IU/mL; therapeutic low-molecular-weight heparin peak 0.5 to 1.0 IU/mL drawn about 4 hours after a subcutaneous dose; prophylactic low-molecular-weight heparin 0.2 to 0.5 IU/mL.
- When it is preferred over the APTT. Baseline APTT already prolonged (lupus anticoagulant, factor XII deficiency), heparin resistance from high factor VIII or fibrinogen, antithrombin deficiency, pregnancy, obesity, and paediatric dosing.
- Pre-analytical trap. Anti-Xa specimens must be centrifuged and separated promptly, because platelet factor 4 released from residual platelets neutralizes heparin in vitro and falsely lowers the result. A specimen left on the cells for hours reports a spuriously subtherapeutic level.
A trauma patient's thromboelastography tracing shows R time 6 minutes, K value 2 minutes, alpha angle 61 degrees, maximum amplitude 62 mm, and LY30 of 22%. Which product is indicated?
A specimen drawn from a central venous catheter has an APTT of 92 seconds and a thrombin time of 68 seconds, while the prothrombin time is 12.8 seconds (normal). Which follow-up result would confirm heparin contamination?
Which statement about heparin neutralization reagents is correct?
An anti-Xa result on a patient receiving therapeutic unfractionated heparin is unexpectedly reported as 0.12 IU/mL despite an appropriate infusion rate. The specimen sat uncentrifuged on the bench for 5 hours. What is the most likely explanation?