A 67-year-old woman arrives at the emergency department with acute bleeding into the back of the throat. A coagulation screen is ordered with the following results:
Test Result Reference Range 1:1 Mix with Normal Plasma
Prothrombin time (PT 12 seconds 11-15 seconds
Partial thromboplastin time (PTT) 57 seconds 25-37 seconds 36 seconds (immediate)
Fibrinogen 450 mg/dL 150-400 mg/dL
What would be a reasonable next step to perform?
a. Perform factor IX assay.
b. Perform factor VII assay.
c. Incubate the 1:1 mix at 37° C for 1 hour and then repeat the PTT.
d. Perform TT.
ANS: C
Factor VIII autoantibodies are usually time and temperature dependent. Thus the 1:1 mix must be incubated at 37° C for 1 to 2 hours to give the antibody time to react with the factor VIII added by the normal plasma. Prolongation of the PTT on this incubated 1:1 mix indicates that an antibody to VIII is present. A factor VIII assay, when performed, will show a low factor VIII (less than 30%).
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