Select the next best test to run given the following PTT results on a patient's plasma:
Initial PTT: 75 seconds
Immediate 1:1 mix with normal plasma: 38 seconds
Incubated 1:1 mix with normal plasma: 36 seconds
a. Dilute Russell's viper venom time
b. Factor assays
c. Factor V Leiden
d. Bethesda titer
ANS: B
Factor assays should be performed on this plasma to determine the factor deficiency (or deficiencies) present. The PTT corrects on a 1:1 mix. If this were the lupus anticoagulant, then the prolonged PTT should remain prolonged on the 1:1 mix; thus no reason exists to perform the dilute Russell's viper venom time to confirm the presence of the lupus anticoagulant. If it were a factor VIII inhibitor, which can fully correct on the immediate 1:1 mix, then it would prolong after incubation of the 1:1 mix. Then it would be appropriate to perform a Bethesda titer. Factor V Leiden does not result in a prolonged PTT.
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