Differential Diagnosis of Prolonged PTT with a Normal PT
Heparin in the specimen, which may be a result of heparin treatment of the patient
Treatment with low molecular weight heparin, argatroban, or lepirudin
A lupus anticoagulant (i.e., lupus inhibitor)
Deficiency of factor VIII (hemophilia A) or IX (hemophilia B)
Deficiency of factor VIII (mild or severe) associated with von Willebrand’s disease
Deficiency of factor XI, XII, or prekallikrein (Fletcher factor)
An inhibitor to factor VIII or IX
Suggested Additional Lab Testing
If heparin contamination in the sample is a possibility, ask for a repeat PTT or, if necessary, get a heparin measurement.
To help differentiate between coagulation inhibitors and factor deficiencies for prolonged PTT, perform PTT mixing studies (patient plasma plus normal plasma in equal volumes).
A patient positive for the lupus anticoagulant should have at least one positive lupus anticoagulant screening test and at least one positive confirmatory test that involves detection of phospholipid neutralization by the lupus anticoagulant.
Factor assays for factors VIII, IX, XI XII, and, in rare circumstances, an assay for prekallikrein (Fletcher factor) can determine if a deficiency in one or more of these factors explains the prolonged PTT.
Bethesda Unit assay is used to quantitate the amount of antibody to factor VIII in a patient with a PTT mixing study result suggestive of a factor VIII inhibitor. An evaluation for von Willebrand includes factor VIII, von Willebrand factor, and ristocetin cofactor. The factor VIII in von Willebrand patients may not be low enough to prolong the PTT.
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