Table 1.
Variables That Influence Activated Partial Thromboplastin Time and the Antifactor Xa Assay and Interfere With the Accuracy of Unfractionated Heparin Dose Determination9
| Variables that prolong aPPT with no effect on the anti-Xa assay |
|---|
| Underfilled sodium citrate collection tube10,11 |
| Decreased clotting factor production from liver disease12,13 |
| Decreased clotting factor production from congenital deficiencies12,13 |
| Decreased production of factors not associated with increased bleeding risk (ie, factor XII, prekallikrein, HMW kininogen)12,13 |
| Increased clotting factor consumption (ie, DIC)12,13 |
| Presence of lupus anticoagulant or acquired factor inhibitors12,13 |
| Variables that shorten aPPT with no effect on the anti-Xa assay |
| Increased acute-phase reactants (factor VIII, fibrinogen)13,14 |
| Variable that overestimates UFH in the anti-Xa assay with no effect on aPPT |
| Hypertriglyceridemia (>690 mg/dL)15,a |
| Variables that underestimate UFH in the anti-Xa assay with no effect on aPPT |
| Hyperbilirubinemia (conjugated >29 mg/dL; unconjugated >14 mg/dL)15,a |
| Gross hemolysis (>1.5 g/dL)15,a |
Anti-Xa, antifactor Xa; aPPT, activated partial thromboplastin time; DIC, disseminated intravascular coagulation; HMW, high molecular weight; UFH, unfractionated heparin.
aValues for STA-Liquid Anti-Xa reagent (Diagnostica Stago) as cited. Other reagents may have different thresholds for interference.