A simple procedure that increases the specificity of the activated protein C resistance test in samples containing antiphospholipid antibodies

It is well known that plasmas with lupus anticoagulants (LA) may give false low activated protein C (APC) ratios, and these false positive tests are not necessarily corrected by mixing with factor V deficient plasma (FVdef). In the present study, we show that repeating the test after mixing 1+1 with pooled normal plasma (NP) instead of mixing with FVdef confirm the presence of the Leiden mutation (FV-Leiden) in patients with antiphospholipid antibodies (APA). Samples from sixteen patients with a low APC-ratio were examined. Eight samples contained APAs, including five samples with LA. Results: The APC-ratios of eleven samples, including four with APAs, became normal when retested after mixing the plasma 1+1 with NP. All of them were heterozygous for FV-Leiden. One additional patient, who had partial correction of the APC-ratio, proved to be homozygous for the Leiden mutation. The remaining four samples, ail of which had high positive tests for APAs, remained unchanged. One of these four was heterozygous for FV-Leiden, while the other three had normal FV. Conclusion: Normalisation of a low APC-ratio after dilution 1+1 with NP confirms the diagnosis of FV-Leiden. PCR analysis could be reserved for cases not affected by this procedure. (C) 1997 Elsevier Science Ltd.