Causes of Abnormal Screening Results
Causes of Abnormal Screening Results | Specimen Collection and Transport Issues | Medication | Pathologic Conditions | Factor Deficiencies | Factor Inhibitors
Medication
Warfarin. Oral anticoagulants decrease the activity of factors II, VII, IX, and X. The PT is generally more sensitive to the effects of warfarin therapy than the aPTT, but both can become extended.5
Heparin. The PT is usually less affected by heparin than the aPTT and can sometimes be normal.5
Thrombin Inhibitors. New anticoagulant medications, such as hirudin analogues and argatroban, can inhibit thrombin and extend both the PT and aPTT.5
Other Drugs. Drugs that can cause a prolonged screening test include hydroxy-ethyl starch, hematin, and suramin.6 Taularidine, an additive in some intravenous medications, can lengthen the aPTT.6
Pathologic Conditions
Vitamin K Deficiency. Extrinsic pathway (VII), intrinsic pathway (IX and X), and common pathway (II) factors can be affected. Both aPTT and PT results can be extended; however, factor VII has a relatively short half-life and can become depleted more quickly than other factors in vitamin K deficiency (or during oral anticoagulant therapy). For this reason, the PT is more sensitive to vitamin K deficiency than the aPTT.5
Consumptive Coagulopathy. Nearly all factors can become depleted in conditions associated with extensive thrombosis or in disseminated intravascular coagulation (DIC). The PT may become extended before the aPTT.5
Diminished Factor Production. Nearly all factors can become deficient in severe liver disease or in malnutrition. Due to the short half-life of factor VII, the PT will often be extended first while the aPTT will not become extended until the disease becomes more advanced.5
Von Willebrand Disease. While von Willebrand factor (vWF) is not part of either the intrinsic or extrinsic pathways, vWF is the obligate carrier for factor VIII. Diminished factor VIII levels can produce extended aPTT results with normal PT.
Amyloidosis. This condition can produce an extended PT and/or aPTT due to diminished levels of factor X.6
Nephrotic Syndrome. Diminished levels of factors XI and XII associated with proteinuria can sometimes result in an extended aPTT with normal PT.5
References
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