Interpretation of clot-based lupus anticoagulant assays-Normalizing clotting time against different denominators

The endpoint for all lupus anticoagulant (LA) assays is a clotting time in seconds. This study aimed to clarify the use of normalizing clotting time to ratio and how the use of different denominators is relevant.

Whether normalization could reduce reagent variability and possess better diagnostic performances was assessed; denominators included reference interval (RI) mean, local-obtained pooled plasma clotting time, standard plasma clotting time, and control plasma clotting time (CNPPct). Moreover, whether day-to-day variation in CNPPct would impact its application was studied.

If not normalized, significant difference existed among different reagent batches; if normalized (against any denominators), no statistically significant difference existed anymore. The validation of in-house RIs achieved a 100% success rate. Normalization against different denominators had different RIs, but the same diagnostic efficacies (when a prolonged LA1 is used to suggest further LA-related testings, normalized LA1 demonstrated a better sensitivity: 1.0 vs. 0.95). Normalization against a "daily" CNPPct (obtained alongside test plasmas day to day) demonstrated low inter-day variations (LA1: ~1%, LA2: ~1%), and it could employ the RI for normalization against a "fixed" CNPPct (obtained alongside normal plasmas when the RI was established).

Normalizing clotting time reduces reagent-batch variability and promotes the adoption of common RIs, and therefore reduces the necessity of establishing RI for new reagent batches. Normalized LA1 is more sensitive when used to suggest further LA-related testings, and therefore reduces the rate of missed LA diagnosis. All denominators are of the same application value. Day-to-day variation in CNPPct did not impact its application as a reliable denominator.