PB1127 - Performance of different pairs of APTTs with high and low sensitivity to phospholipids to diagnose lupus anticoagulant.

Background: Some pairing of sensitive and insensitive to phospholipids APTT have been shown to be effective in lupus anticoagulant (LA) detection .

Aims: To verify if the ratio obtained with sensitive APTT / insensitive APTT to phospholipids is a useful tool for the diagnosis of LA.

Methods: Population: 120 normals to establish the cut-off point for each of the APTT ratio studied. 73 consecutive patients who attended the laboratory for the LA diagnosis. No patients were anticoagulated at moment of the study. Also, 4 patients with acquired haemophilia (AH) and 12 FVII,IX,XI /XII artificial single deficiency plasmas were used.

Methods: Diagnosis was made according to ISTH guidelines, using the Russell viper venom assay and the silica APTT assay (both screening and confirmatory) with locally established cut-off point. APTT was performed with PTT-LA (a reagent sensitive to the presence of LA) and with Pathromtin (P), Cephascreen (Ceph), CK prest (CK p) (reagents insensitive to the presence of LA). All assays were performed in a coagulometer with viscosity clot detection. The results were expressed as a sensitive APTT/insensitive APTT ratio. 97.5th and the 99th percentile cut-off points were calculated for ratios (n=120). The degree of concordance was evaluated using the kappa coefficient.

Results: From 73 patients investigated for LA, 37 were negative for both tests; 36 were positive: 25 by both tests; 11 only positive by APTT-silica assay. Fig 1 shows the results with sensitive/insensitive APTT ratio and table 1 the correlation measured by kappa coefficient for both percentiles. All samples with factors deficiency or AH were negative for LA with all APTT combination.

Conclusion(s): The sensitive/insensitive APTT ratio represents a simple, inexpensive and sensitive strategy for the diagnosis of AL through the APTT pathway, especially when the cut-off point is established as the 97.5th. It should be used in conjunction with the Russell Viper Venom Test.