Laboratory diagnosis of lupus anticoagulants effect of residual platelets in plasma, assessed by Staclot® LA and silica clotting time

Residual platelets in plasma are considered detrimental after freezing-thawing, as phospholipids released from ruptured platelets may quench lupus anticoagulants (LA). We aimed at assessing the effect of residual platelets after freezing-thawing plasmas tested with two procedures for LA. Blood from 52 patients suspected of having LA were centrifuged at 2.500 g. Plasmas were subdivided into 2 aliquots. One, was filtered to remove residual platelets and both were frozen and stored at -70degrees C. Silica clotting time (SCT) at low and high phospholipid concentrations and Staclot(R) LA with and without Hexagonal phospholipids were performed on thawed plasmas. Plasmas were considered LA-positive when both SCT and Saclot(R) LA performed on filtered plasmas were diagnostic for LA. Forty-two of 52 plasmas fulfilled the diagnostic criteria and were retained for subsequent analysis. SCT on non-filtered plasmas was diagnostic for LA in 42 of 42 plasmas. Though the median (range) percentage correction recorded after phospholipids addition for filtered plasmas, i.e. 67% (36%-83%) was reduced to 54% (25%-81%) for non-filtered plasmas (p<0.001), it was still above the cut-off (i.e., 20.9%). Staclot(R) LA on non-filtered plasmas was diagnostic for LA in 42 of 42 plasmas. Though the median (range) clotting time difference recorded after phospholipid addition for filtered plasmas. i.e., 40.8 (10-103.5) s was reduced to 31.7 (2.8-88.8) s for non-filtered plasmas (p<0.001), it was still above the cut-off (i.e., 1.7 s). In conclusion, residual platelets do not affect the diagnostic efficacy of SCT and Staclot(R) LA. However, the fact that the percentage correction for SCT and the clotting time difference for Staclot(R) LA are reduced by residual platelets. suggests that weak LA may be lost upon freezing-thawing non-filtered plasmas.