A Lower Calcium Concentration Prolongs More Frequently Dilute Prothrombin Time (dPT) of Patients with Lupus Anticoagulant (LA) and Anti-B2GPI Antibodies (aB2GPI)

Abstract number: P0614

Martinuzzo¹ M, Cerrato¹ G, Adamczuk¹ Y, Iglesias Varela¹ ML, Forastiero^1,2 R

^{1,2 11}Institute of Cardiology and Cardiovascular Surgery, Favaloro Foundation, Buenos Aires, Argentina ¹¹Institute of Cardiology and Cardiovascular Surgery, Favaloro Foundation, Buenos Aires, Argentina

It has been demonstrated that lowering calcium concentration, the coagulation time of dRVVT and dPT in plasmas from patients with LA and aB2GPI is prolonged. However, this procedure needs to be validated in a large number of patients. The aim of this study was to evaluate the response of plasmas from 284 consecutive patients referred for LA detection since July to December 2004 in the dPT performed with recombinant thromboplastin (Innovin) diluted 1 : 50 at two calcium concentrations (15 and 5 mM). A standardized home-made ELISA was used to detect IgG and IgM aB2GPI. Fifty-one of 284 patients had LA by using APTT, dRVVT, conventional dPT (Innovin at 1 : 500 dilution) and KCT. The patient/normal (P/N) ratios of dPT were calculated and the prolongation of the ratio at 5 mM calcium was evaluated. Considering a prolongation of the P/N ratio higher than 20% (5 mM compared to 15 mM), 23% (54/233) of plasmas from LA-patients, 19% (4/21) of LA+/aB2GPI-, and 50% (15/30) of LA+/aB2GPI+ (P= 0.003 vs aB2GPI-), presented such an increase. The prevalence was even greater (58.5%) when only medium and high titres aB2GPI were considered. When analysing the median of P/N ratios, LA-patients varied from 0.94 to 1.09 (15 and 5 mM), LA+/aB2GPI- from 1.41 to 1.20, LA+/aB2GPI+ from 1.46 to 2.05 and LA+/aB2GPI+ at medium or high titres from 1.40 to 2.55 (P= 0.03). In conclusion, a lower calcium concentration in dPT test induced a further prolongation of coagulation times in plasmas LA+/aB2GPI+ (particularly at medium/high titres). However, the sensitivity of the modified dPT to distinguish aB2GPI-dependent LA is not very high, the simultaneous presence of antibodies to prothrombin must be considered, and the expression of the results concerning the degree of prolongation and the cut-off values must be carefully evaluated. Multicentre studies should be done to validate this information.