Archer USMLE Step 3 Blog

Approaching Prolonged PTT and understanding Mixing Studies : 

ARCHER HEMATOLOGY REVIEW

Question :  On a mixing study, the PTT corrects initially but gets prolonged again after incubation for 2 hours. What is this condition?

This description of mixing studies is consistent with presence of a temperature- and time-dependent anti-VIII inhibitor. It just means that the inhibitor is a warm reacting IgG antibody that requires one or two hour’s incubation at 37°C to be detected. If the PTT corrects initially but prolongs again after incubation (meaning if thePTT is at least 15% longer than the normal reagent plasma’s incubated PTT ) –> anti-VIII should be suspected. This can be confirmed by obtaining a Factor VIII level which will be low in case of presence of inhibitor. Further coinfirmation of Factor VIII inhibitor can be obtained by Bethesda Titer. In such cases, you should also carefully review patient’s prior bleeding history –…