Heparin contamination of specimens is a common cause of an unexpected activated plasma thromboplastin time (aPTT) prolongation. In one study, heparin contamination accounted for 39% of unexpected aPTT prolongations in patients who were not receiving heparin or Coumadin® treatment. Heparin contamination may account for an unexplained PTT prolongation even when a heparinized line is carefully flushed to remove heparin prior to specimen collection.
Heparinase (Hepzyme®) can be used to determine if the PTT prolongation is due to the presence of unfractionated or low molecular weight heparin. In addition, while patients are receiving heparin, it is sometimes necessary to perform coagulation tests that are affected by heparin. In such cases, heparinase can be used to remove heparin from the specimen so that coagulation tests can be performed without heparin interference.
To determine if a prolonged PTT is due to heparin, the aPTT is measured before and after heparinase treatment. One mL of platelet poor plasma is added to one vial of heparinase and incubated at room temperature for 15 minutes. Heparinase is a bacterial enzyme that degrades unfractionated and low-molecular weight heparin by cleaving them at multiple sites including within the pentasaccharide sequence. The pentasaccharide sequence is the antithrombin binding site that is required for heparin anticoagulation. Heparinase degradation yields small fragments of about 1000 daltons that lack anticoagulant activity. Heparinase can degrade up to 2 units/mL heparin. If the heparin level in plasma is greater than 2 USP units, but less than 4 USP units, it can be completely neutralized by two sequential treatments.
If heparin is the explanation for the prolonged aPTT, the aPTT will become normal after treatment of the specimen with heparinase. If the PTT shortens significantly but remains prolonged after heparinase, a coagulation abnormality may be present in addition to heparin contamination. If a markedly prolonged aPTT (eg, >150 seconds) shortens significantly but remains slightly prolonged after heparinase, a small amount of residual heparin is a possible explanation, because the initial amount of heparin contamination was very high. If a second heparinase treatment of the specimen produces a normal PTT, heparin is the confirmed explanation.
For patients receiving oral anticoagulants, thrombolytic therapy, or patients with circulating inhibitors or anti-phospholipid antibodies, the medication and clinical histories should be considered when interpreting results. These patients may have underlying conditions that result in prolongation of the APTT and/or PT in the absence of heparin. Thrombin Times should not be performed on samples that have been sequentially neutralized.
Specimen requirement is a blue top (sodium citrate) tube of blood.
Newman RS and Fagin AR, "Heparin Contamination in Coagulation Testing and a Protocol to Avoid It and the Risk of Inappropriate FFP Transfusion,"Am J Clin Pathol, 1995, 104(4):447-9.