A specimen for a prothrombin time and activated partial thromboplastin time is collected in a 5 mL tube but only 2.5 mL of blood is present. What is the expected result?

• A. PT and aPTT both falsely short
• B. PT and aPTT both falsely long
• C. PT and aPTT both unaffected
• D. PT unaffected, aPTT falsely short

Answer: (B)

When a citrate-anticoagulated blood sample is underfilled, the citrate-to-blood ratio is higher than intended. Citrate binds calcium to prevent clotting, so excess citrate left in the tube continues to chelate calcium during the assay. Since both the prothrombin time (PT) and the activated partial thromboplastin time (aPTT) depend on available calcium to initiate clot formation, the tests run more slowly, yielding longer clotting times. This makes both PT and aPTT falsely prolonged, rather than reflecting the patient’s true coagulation status. In practice, underfilling a citrate tube will produce artificially long results for both tests.