A patient admitted to the hospital with thrombosis is given the standard heparin dosage. Twenty-four hours after the heparin is started, the PTT is 38 seconds (reference range, 25 to 37 seconds). Both normal and abnormal controls are within their limits. Which of the following is most likely?

A) laboratory error
B) patient has low antithrombin
C) heparin-induced thrombocytopenia
D) patient should be switched to low–molecular-weight heparin (LMWH)

Ans: B) patient has low antithrombin