Phlebotomist Forgetful Frank collected a tube of blood for an aPTT in a sodium citrate tube. The tube was delivered to the laboratory at 4:30 p.m. Which statement best describes the expected results?

• A. Sodium citrate is a preservative as well as an anticoagulant, so the aPTT result should be accurate
• B. An aPTT collected in sodium citrate will give falsely long results because some factors are unstable in this anticoagulant
• C. A falsely prolonged aPTT may occur because some factors deteriorate rapidly at room temperature
• D. Exposure of the plasma to erythrocytes for several hours has probably activated the factors, so the aPTT will be falsely short

Answer: (C)

The main idea here is that coagulation test results are highly sensitive to how a blood sample is handled before testing. Some clotting factors, especially those in the intrinsic pathway like factor VIII, are labile at room temperature and lose activity if the plasma sits unrefrigerated for several hours. In this scenario, the citrate tube was collected and then not tested until late in the day, so the plasma likely remained at room temperature for an extended period. As these factors deteriorate, the clotting process takes longer to complete when the aPTT is measured, giving a falsely prolonged result. Citrate’s role is to prevent clotting by chelating calcium, not to preserve factor activity over time, so the delay—not the citrate itself—is what leads to the longer apparent aPTT. The other statements don’t reflect the real preanalytical issue: citrate isn’t a preservative for factor stability, and prolonged exposure of plasma to red cells isn’t the mechanism that would shorten the aPTT.