A Minor Variation of the APTT Mixing Test Can Be Used to Identify Defects in the Contact Mechanism

Abstract number: OR084

Exner T, Low J, Joseph JE

A significant proportion of unexpectedly-prolonged APTT screening results are due to defects in the contact mechanism, especially minor decreases in factor XII. Generally such defects are unimportant and except for factor XI deficiency, do not contribute to bleeding risk. We have developed a simple new variation of the APTT mixing test for identifying defects in the contact mechanism. This method can be applied to any plasma which gives a prolonged APTT screening test result to identify whether the abnormality acts ‘above’ or ‘below’ factor XI. In the modified APTT method the test plasma is mixed with APTT reagent and preincubated as usual for 3–5 minutes during which time contact activation should occur. However, instead of then simply recalcifying with M/40 calcium chloride, a half-volume of normal plasma is added together with the calcium chloride. Any defect in contact mechanism in the test plasma yields a prolonged result because contact activation has not been fully achieved. However any defect in factors other than those in the contact mechanism yields a normal result, the correction being induced by the added normal plasma. The converse test wherein normal plasma is preincubated with APTT reagent to generate normal contact product and then test plasma is added with the calcium chloride is useful to confirm the result. Inhibitors are defined in the usual way by failing to correct on mixing with normal plasma. The method gave good discrimination of deficiencies of factor XII and XI from those of factors VIII and IX. In practice, results were found to depend on the type of APTT reagent and contact activator used. Significant heterogeneity was found among new APTT reagents in sensitivity to Fletcher factor deficiency. Addition of Polybrene to the calcium chloride was useful for shutting down contact activation after recalcification and gave more clear-cut results with all reagents.