A 4-year-old boy has a prolonged aPTT of 53 seconds. Mixing with normal plasma at a 1:1 ratio yields an aPTT of 50 seconds. Which of the following is most likely?

A 4-year-old boy has a prolonged aPTT of 53 seconds. Mixing with normal plasma at a 1:1 ratio yields an aPTT of 50 seconds. Which of the following is most likely?

• A. Factor deficiency
• B. Inhibitor of other anticoagulant
• C. Vitamin K deficiency
• D. Liver disease

Answer: (B)

The key idea is how mixing studies distinguish a true factor deficiency from the presence of an inhibitor. In the intrinsic pathway, if the prolonged aPTT is due to a deficiency of a coagulation factor, adding normal plasma (which supplies the missing factor) should correct the time back toward normal. If an inhibitor is present in the patient's plasma, it can neutralize the added factors from the normal plasma, so the aPTT will not fully correct or will only partially improve. Here, the aPTT stays prolonged after a 1:1 mix (from 53 to 50 seconds, still not normal), indicating that an inhibitor in the patient’s plasma is interfering with coagulation rather than a simple factor deficiency. Vitamin K deficiency or liver disease would typically correct with mixing because the normal plasma provides functional factors, and a straightforward deficiency doesn’t persist after mixing. Thus, the finding most likely reflects an inhibitor of coagulation rather than a factor deficiency or impaired production.

The key idea is how mixing studies distinguish a true factor deficiency from the presence of an inhibitor. In the intrinsic pathway, if the prolonged aPTT is due to a deficiency of a coagulation factor, adding normal plasma (which supplies the missing factor) should correct the time back toward normal. If an inhibitor is present in the patient's plasma, it can neutralize the added factors from the normal plasma, so the aPTT will not fully correct or will only partially improve. Here, the aPTT stays prolonged after a 1:1 mix (from 53 to 50 seconds, still not normal), indicating that an inhibitor in the patient’s plasma is interfering with coagulation rather than a simple factor deficiency. Vitamin K deficiency or liver disease would typically correct with mixing because the normal plasma provides functional factors, and a straightforward deficiency doesn’t persist after mixing. Thus, the finding most likely reflects an inhibitor of coagulation rather than a factor deficiency or impaired production.