A 60-year-old male has markedly prolonged PT and aPTT when tests are performed on plasma collected in citrate, while a CBC performed on EDTA shows normal platelet counts. The best explanation is which statement?

• A. Coagulation reactions require platelet factor 3; availability is reduced when platelets are low
• B. The ratio of anticoagulant to blood is critical; the volume of anticoagulant must be decreased when the hematocrit exceeds 55%
• C. PT and aPTT are unaffected by specimen collection
• D. EDTA contamination causes prolonged coagulation times

Answer: (B)

The important idea is that PT and aPTT in citrate plasma rely on a precise anticoagulant-to-blood ratio. Citrate chelates calcium to prevent clotting, and labs use a standard amount of citrate for a given blood volume to achieve proper dilution. When the hematocrit is high (over 55%), there’s less plasma in the same volume of blood. With a fixed amount of citrate, the citrate-to-plasma ratio becomes too high, leading to excessive calcium chelation and falsely prolonged coagulation times. Therefore, the volume of anticoagulant should be decreased to restore the correct ratio in samples with high hematocrit. The normal CBC on EDTA argues against a true platelet deficiency affecting factor III, and this pattern is about pre-analytical citrate handling rather than a true hematologic abnormality.