What are the recommended time limits for specimen acceptability in coagulation testing for Prothrombin Time (PT), activated Partial Thromboplastin Time (aPTT) on intravenous (IV) heparin, and aPTT without coagulation medication?

Time Limits for Specimen Acceptability in Coagulation Testing

For coagulation testing, PT specimens can be stored at room temperature for up to 24 hours, aPTT specimens from patients on IV heparin should ideally be processed within 1 hour, and aPTT specimens from patients not on anticoagulation can be stored at room temperature for up to 4 hours.

Matching the Time Limits

PT (Prothrombin Time)

• A. 24 hours when capped at room temperature
  • Multiple studies confirm PT specimens remain stable for 24 hours at room temperature when properly collected and stored 1, 2, 3
  • PT results show minimal clinically significant changes during this timeframe, with percentage changes typically less than 10% 4

aPTT (Activated Partial Thromboplastin Time) on IV Heparin

• B. Remove from cells within 1 hour (ideally)
  • Patients on heparin therapy require more careful handling of specimens due to the continued activity of heparin in the collection tube 5
  • Heparin can continue to affect coagulation parameters in vitro, potentially leading to falsely prolonged aPTT results if not processed promptly
  • The differences in aPTT values between initial testing and delayed testing are larger in heparinized samples compared to normal samples 5

aPTT (No Coagulation Medication)

• C. 4 hours when centrifuged, capped, at room temperature
  • For patients not on anticoagulation therapy, aPTT specimens show acceptable stability for up to 4-8 hours 4
  • After 8 hours, studies show that mean percentage changes in aPTT results remain under 10%, but by 24 hours, changes exceed 10% and may become clinically significant 4

Important Considerations for Coagulation Testing

Specimen Collection and Processing

• Use appropriate anticoagulant: tripotassium ethylenediamine tetraacetate (K3EDTA) or heparin for specific tests 6
• Mix blood well with anticoagulant to prevent clotting
• Properly label specimens with date, time of collection, and unique patient identifier

Storage Conditions

• Maintain specimens at room temperature (64°-72°F [18°-22°C]) 6
• Avoid extreme temperatures that could allow freezing or overheating (>99°F/37°C) as this may cause cellular destruction and affect results 6
• For aPTT specifically, refrigeration at 4°C may cause more pronounced changes compared to room temperature storage 1

Common Pitfalls to Avoid

1. Delayed processing of specimens from patients on heparin therapy
2. Improper mixing of specimen with anticoagulant
3. Exposure to extreme temperatures
4. Failure to check for clots or aggregates in the specimen tube
5. Using inappropriate collection tubes (polypropylene tubes may provide better stability than glass tubes) 2

Special Circumstances

• For patients with suspected heparin-induced thrombocytopenia (HIT), additional testing for anti-PF4 antibodies may be required 6
• In patients requiring monitoring of anticoagulation therapy, point-of-care tests to assess INR should not be used for patients on NOACs (Non-vitamin K antagonist oral anticoagulants) 6

By adhering to these time limits for specimen acceptability, laboratories can ensure accurate coagulation test results that properly guide clinical decision-making for patients requiring anticoagulation monitoring or assessment of coagulation status.