Abstract
Conversion of coagulation time (t) to relative arbitrary substance concentration of coagulation factors(II + VII + X) abbreviated cra) was originally made from a measuring curve corresponding to the equation log t=aa0 + a1 log cra. Three more suitable equations are presented and tested for fit: t=A0 + A1/cra; t=B0 + B1/cra + B2/c2ra; log t=b0 + b1 log cra + b2 (log cra)2. Their suitability for the Simplastin®-A and Thrombotest® methods is also investigated. The last equation yields the best fit and replaces the errorprone reading of log-log curves by a calculation suitable for routine use.