Limitations

The mixing study is only a qualitative screening assay used to distinguish factor deficiency from the effects of an inhibitor and cannot be used to rule out either entity. Occassionally, both factor deficiency and inhibitor may coexist.
When the baseline Protime is only minimally prolonged, the results may be misleading.
Factor inhibitors that affect only PT and not PTT are rare.

Mild factor defects or low titered/time and temperature inhibitors may be difficult to discern and usually require further investigation.
Interference from heparin, heparin flushes and coumadin should be ruled out.

• Oral anticoagulants depress the production of factors II, VII, IX and X in the liver by inhibiting the action of Vitamin K. The PT is sensitive to the levels of factors II, VII, and X and is used to monitor patient therapy with oral anticoagulants.
• Many commonly administered drugs (Novoseven) may affect the PT results. These should be considered when unusual or unexpected abnormal results are obtained. Unexpected abnormal results should be followed by further coagulation studies to determine the source of the abnormality.
• Inhibitors, such as lupus anticoagulation may interfere with the test. INRs may not reflect the exact degree of anticoagulation. Hirudin or other direct thrombin inhibitors in therapeutic dose results in prolonged prothrombin times.