Calculation of Lupus Ratio. Lupus Ratio (LR) may be calculated from APTT, RVVT or dPT-based clotting times. A LR test is performed on 1:1 mixtures of patient plasma and normal plasma (NP). The clotting time is measured twice, once with a reagent with a low cephalin concentration (a), then with a reagent with a high cephalin concentration (b). The ratio between these clotting times is divided by the corresponding ratio of NP (c:d). This final ratio is the LR of that plasma (a/b : c/d).