Mathematical expressions of measuring curves for concentration of clotting factors.

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time (t) to relative arbitrary substance concentration of coagulation factors (II + VII + X) abbreviated c(ra)) was originally made from a measuring curve corresponding to the equation log t = a(o) +a1 log c(ra).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