Assessment of Normative Range and Deriving Cut-Off Values for Lupus Anticoagulant Testing: An Experience from a Tertiary Care Center in Southern India.

Lupus anticoagulant (LA) testing is a demanding laboratory test. The most common among recommended tests for LA are dilute Russell viper venom time (dRVVT) and LA sensitive activated partial thromboplastin time (LA-aPTT). Although integrated test systems come with manufacturer assigned cut-off, but according to guidelines cut-off should be locally-derived. This study was carried out to find normative range of LA using LA-aPTT and dRV VT tests in south Indian population (n = 100) and using locally derived cut-off values to interpret patient samples (n = 152) in a one-year study period. Cut-off (mean+2.3 SD) for aPTT, LA-aPTT, dRVVT screen and dRVVT confirm assays were 37.5, 45.7, 45.5 and 41.3 s and for normalized ratio and percentage correction of ratio were 1.16 and 15.8% respectively. Among patients, 25 (16.4%) were positive of whom 14 were positive by normalized ratio, seven by LA-aPTT and four were positive by both. Four additional patients could be identified using local cut-off. While the specificity of all screening tests was high, sensitivity of only dRVVT screen was high. All testing laboratories should attempt to derive normative range and locally-derived cut-off. While dRVVT is a good screening test with high sensitivity, however combining LA-aPTT offers advantage of picking up cases negative by dRVVT.