RELIABILITY OF A MODIFIED TISSUE FACTOR-DEPENDENT FACTOR-V ASSAY FOR ACTIVATED PROTEIN-C RESISTANT FACTOR VA USING A CALCIUM-CONTAINING THROMBOPLASTIN

The original tissue factor-dependent factor V assay for activated prot ein C resistant factor Va (Blood 1995; 85: 1704-1711) has been modifie d to use a calcium containing thromboplastin and to express results as an observed to expected ratio (Obs./Exp.). The latter permits establi shing a normal range independent of variations due to differences in r eagents. Comparing Obs/Exp ratios with DNA analysis in 72 persons reve aled that an Obs/Exp ratio of 0.6 distinguished without overlap normal s from heterozygotes for FV R506Q. Three homozygotes had a ratio of <0 .1. Application of this Obs/Exp cut-off ratio of 0.6 to a total of 226 plasma samples tested to date discriminated without overlap between n ormals and heterozygotes. We conclude that this assay - readily adapta ble to any dedicated coagulation laboratory and capable of yielding re liable results in all clinical circumstances in which testing is indic ated - can distinguish between normals and heterozygotes for the FV R5 06Q mutation without the need for confirmatory DNA analysis.