What is the maximum allowable platelet (platelet count) count when centrifuging blood for routine coagulation tests?

Maximum Allowable Platelet Count for Routine Coagulation Tests

For routine coagulation tests, the maximum allowable platelet count in centrifuged blood is 10 x 10³/μL (option C).

Platelet Count Requirements for Coagulation Testing

Evidence-Based Standards

• The International Society on Thrombosis and Haemostasis (ISTH) recommends that platelet-poor plasma (PPP) should have a platelet count below 10 x 10³/μL for accurate coagulation testing 1.
• This low platelet count is essential because residual platelets in plasma can significantly affect coagulation test results by releasing phospholipids and other factors that interfere with accurate measurements 1.

Preparation Methods

To achieve the required platelet-poor plasma with counts below 10 x 10³/μL, specific centrifugation protocols are recommended:

• Double centrifugation method: The gold standard approach involves:

  • First centrifugation: 2500 x g for 15 minutes at room temperature
  • Collection of supernatant (avoiding the buffy coat)
  • Second centrifugation: 2500 x g for 15 minutes at room temperature 1

• Single high-speed centrifugation: In some laboratory settings, a single centrifugation at higher speeds may be sufficient for certain tests:

  • For PT and APTT: A single centrifugation may be adequate if platelet counts are below 200 x 10³/μL 2
  • For specialized coagulation tests: Platelet counts must be below 10 x 10³/μL 2

Clinical Implications

Impact on Test Accuracy

• Platelet counts above 10 x 10³/μL can lead to:
  • Shortened clotting times
  • Falsely elevated coagulation factor levels
  • Inaccurate assessment of hemostatic status 1
  • Incorrect estimation of thrombin generation 1

Test-Specific Requirements

Different coagulation tests have varying sensitivity to residual platelets:

• Basic tests (PT, APTT): May tolerate slightly higher platelet counts but still require PPP
• Specialized tests (protein S activity, thrombin generation assays): Absolutely require platelet counts <10 x 10³/μL 2

Laboratory Workflow Considerations

Rapid Centrifugation Options

Recent studies have explored more efficient centrifugation protocols:

• High-speed centrifugation (3000-4440 g for 5-7 minutes) may achieve adequate platelet depletion for routine tests 3, 4
• However, specialized coagulation assays still require the more stringent platelet count threshold of <10 x 10³/μL 2

Quality Control

• Laboratories should verify platelet counts in their PPP preparations periodically to ensure they meet the <10 x 10³/μL requirement
• This is particularly important when modifying centrifugation protocols or implementing automated systems 4

Common Pitfalls to Avoid

• Using platelet-depleted plasma (PDP with counts <40 x 10³/μL) instead of true PPP (<10 x 10³/μL) for specialized coagulation tests
• Inadequate centrifugation time or speed
• Disturbing the buffy coat during plasma collection
• Delays in processing samples (should be processed within 1 hour of collection) 5

The 10 x 10³/μL threshold represents the established standard for maximum allowable platelet count in properly prepared samples for routine coagulation testing, with particular importance for specialized coagulation assays.