Detection of procoagulant phospholipid interfering in tests for lupus anticoagulant

Excess platelets shorten most clotting tests for lupus anticoagulant (LA). Often it is not clear if a shortened, normal or slightly prolonged result in a test masks a weak LA in combination with activated platelets, which express procoagulant phospholipid (PPL). Our aim was to investigate a new LA-insensitive factor Xa-activated clotting time (XACT) test for detecting PPL in plasma specimens submitted for LA testing. In most clotting tests for PPL, specimens are mixed with human platelet-free plasma (PFP) to correct for factor defects. Such tests are usually very sensitive to prolongation by LA, which act against PPL-human clotting factor complexes. We found that phospholipid-free plasma from pigs could be used instead of human platelet-free plasma as substrate plasma without reducing sensitivity of XACT to PPL. However, the pig plasma-based system was significantly less affected by most LA. Activated platelets were detectable despite the presence of most LA. Since some LA still had significant prolonging effect on the XACT despite the use of pig plasma we investigated this further. ELISAs for IgG and IgM anti-beta2GP1 and anti-prothrombin antibodies were carried out on 23 specimens. We did not find that LA in plasmas displaying anti-prothrombin antibodies had less prolonging effect on the test based on pig plasma than that using human platelet-free plasma. Similarly, there were no subtyping trends apparent among results from anti-beta2GPI-positive samples. Our results do not support the concept that anti-prothrombin-dependent LA might be more species specific than anti-beta2GPI-dependent LA.