Quantifying Lupus Anticoagulant Interference in Intrinsic Coagulation Factor Activity Assays and Evaluating the Corrective Performance of the SCT-Based Method.

BackgroundLupus anticoagulant (LA) prolongs phospholipid-dependent coagulation tests and can falsely reduce intrinsic coagulation factor activities in one-stage assays. The quantitative relationship between LA intensity and the degree of assay interference remains insufficiently defined.MethodsOne hundred patients with LA-positive antiphospholipid syndrome or systemic lupus erythematosus were studied. FVIII:C, FIX:C, and FXI:C were measured using one-stage assays, and a modified assay incorporating SCT-confirm reagent was applied to reduce LA interference. The pseudo-lowering degree (PLD) was calculated to quantify correction efficacy. Associations between LA characteristics and reduced factor activities were analyzed, and activity levels or PLD values across LA-intensity subgroups were compared.ResultsUsing one-stage assays, reduced FVIII:C, FIX:C, and FXI:C were noted in 27%, 23%, and 26% of patients, respectively. LA intensity was unrelated to reduced FVIII:C but significantly associated with decreased FIX:C (P = .006) and FXI:C (P < .001). Moderate LA positivity showed higher FIX:C levels than weak positivity, whereas markedly lower FXI:C was seen only in strongly LA-positive patients. The SCT correction assay restored FVIII:C, FIX:C, and FXI:C to normal levels in approximately 66.7%, 82.6%, and 92.3% of affected cases, respectively. In strongly LA-positive patients, PLD values of FVIII:C, FIX:C, and FXI:C were significantly higher than in weak or moderate subgroups.ConclusionsFVIII:C, FIX:C, and FXI:C exhibit differential susceptibility to LA interference. The SCT-based correction approach effectively mitigates this effect, especially for FXI and offers a practical improvement over conventional one-stage assays to avoid misclassification of intrinsic factor deficiencies in LA-positive patients.