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?

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When a patient has a deficiency of antithrombin,heparin may be administered in therapeutic or even higher dosages,but it will not exert an anticoagulant effect in vivo or be detected by the PTT.This is called heparin resistance.

A Patient Admitted to the Hospital with Thrombosis Is Given