A 37-year-old female patient has a PTT of 76 seconds (reference range, 25 to 37 seconds). Her plasma is mixed 1:1 with normal plasma. An immediate PTT performed on the mix is 74 seconds. Which of the following is the most likely diagnosis?

a. Hemophilia A
b. Factor VIII inhibitor
c. Intrinsic factor deficiency
d. Lupus anticoagulant

ANS: D
A prolonged PTT that remains prolonged after an immediate repeat of the PTT on the 1:1 mix is seen in patients (either male or female subject) with the lupus anticoagulant. A patient with hemophilia A would most likely be a male subject (female hemophilia A is rare). In addition, the immediate 1:1 mix should correct, as it also would with an intrinsic factor deficiency. The PTT on an immediate 1:1 mix in the presence of a factor VIII inhibitor would also essentially correct.