A common cause for a prolonged aPTT is due to sample contamination with heparin. It is important prior to performing LA testing to demonstrate the sample has no heparin. A thrombin time (TT) is the most sensitive test for residual heparin and will be prolonged at 0.1 U/mL of heparin. A TT can also be prolonged if a patient has a fibrinogen deficiency.
In order to discriminate between heparin and fibrinogen, a reptilase time can be performed. A reptilase time will be abnormal with a fibrinogen deficiency and normal if the sample is contaminated with heparin.
