Does hemolysis affect activated Partial Thromboplastin Time (aPTT) results?

Impact of Hemolysis on Activated Partial Thromboplastin Time (aPTT)

Hemolysis significantly affects aPTT results, with free hemoglobin causing shortened aPTT values that could lead to inappropriate clinical decisions, particularly in anticoagulation management. 1

Effects of Hemolysis on aPTT

Mechanism of Interference

• Hemolysis releases intracellular components including free hemoglobin that directly interferes with coagulation testing
• According to the International Society on Thrombosis and Haemostasis, elevated free hemoglobin shortens aPTT results 1
• This shortening effect can mask true coagulation abnormalities or suggest inadequate anticoagulation when it may be adequate

Magnitude of Effect

• The degree of interference correlates with the severity of hemolysis:
  • Mild-to-moderate hemolysis can produce statistically significant shortening of aPTT 2
  • More severe hemolysis (≥0.8 g/dL hemoglobin) causes even greater shortening of aPTT values 2
  • A cut-off value of 0.95 g/dL hemoglobin has been identified as clinically significant for aPTT results (75% sensitivity, 62.5% specificity) 2

Clinical Implications

Anticoagulation Monitoring

• For patients on heparin therapy, falsely shortened aPTT due to hemolysis may lead to:
  • Overestimation of anticoagulation effect
  • Inappropriate dose reduction or discontinuation
  • Increased risk of thrombotic events 1

ECMO and Critical Care Settings

• In patients on extracorporeal membrane oxygenation (ECMO), hemolysis is a common complication
• Falsely shortened aPTT can lead to inappropriate anticoagulation management in these high-risk patients 1
• The 2023 International Society on Thrombosis and Haemostasis guidelines specifically warn about this interference 1

Liver Disease Patients

• In patients with liver disease, hemolysis may further complicate interpretation of already complex coagulation profiles 1
• Traditional tests like aPTT already have limitations in assessing true hemostatic balance in these patients 1

Detection Methods and Analyzer Considerations

• The detection method influences the degree of interference:

  • Photo-optical methods are more susceptible to hemolysis interference 2, 3
  • Mechanical/electromechanical detection systems may be less affected 4

• Different reagents show variable sensitivity to hemolysis:

  • Some APTT reagents (like STA®-Cephascreen®) show minimal interference 3
  • Others (like STA®-PTT Automate) demonstrate clinically significant interference 3

Recommendations for Laboratory Practice

• For routine screening in non-anticoagulated patients:

  • Mildly hemolyzed samples may still provide clinically useful results 5, 6
  • The observed differences, while statistically significant, may not always be clinically meaningful 6

• For patients on anticoagulation therapy:

  • Hemolyzed samples for aPTT should generally be rejected due to potential for clinical mismanagement 3
  • Consider alternative monitoring methods when hemolysis is present:
    • Anti-Xa assay may be preferable for heparin monitoring in hemolyzed samples 1
    • However, be aware that severe hemolysis can also affect anti-Xa activity 1

• When hemolysis cannot be avoided:

  • Document the presence and degree of hemolysis
  • Consider using mechanical detection methods rather than photo-optical methods 4
  • Interpret results with caution, particularly in anticoagulated patients

Conclusion

Hemolysis has a significant impact on aPTT results, typically causing falsely shortened values that could lead to inappropriate clinical decisions. This effect is particularly concerning in anticoagulation management where accurate aPTT results are critical for dosing decisions. While some studies suggest mild hemolysis may not always necessitate specimen rejection, the safest approach for patients on anticoagulation therapy is to obtain a new, non-hemolyzed sample whenever possible.