Evaluation of a new venom-based clotting assay of protein C

Congenital protein C deficiency significantly increases the risk of venous thromboembolism, a serious and potentially lethal condition. Protein C levels can be determined by chromogenic, clotting and antigenic assays, each type of assay has differences in specificity and sensitivity to protein C deficiency. In principle, clotting-based assays of protein C are preferred over chromogenic assays, as they can detect some rare mutations that are missed by the chromogenic assay, however, clotting-based assays may be prone to inaccuracy because of poor specificity. We have evaluated a new venom-based clotting assay of protein C, and optimized it for use on Sysmex CA-1500 analyser. The assay was linear from 0 to 130 U/dl, a normal plasma demonstrated good inter-assay precision, with a coefficient of variation of 4.8%. The assay compared well with antigenic- and venom-based chromogenic protein C assay in normal individuals, subjects with lupus anticoagulant, and subjects with FV Leiden. Median protein C levels by clotting, chromogenic and antigen for the three subject groups were 108 U/dl, 108 IU/dl and 109 IU/dl for normal subjects, 94 U/dl, 106 IU/dl and 103 IU/dl for subjects with lupus anticoagulant, and 102 U/dl, 104 IU/dl and 100 IU/dl for subjects heterozygous for FV Leiden. Comparing levels of clotting protein C with protein C antigen by ratio (clotting/antigen), the three groups showed small differences that did not quite reach statistical significance, (mean ratios ranged from 0.95 to 1.01, anovaP = 0.0561), the lowest ratio was with the lupus anticoagulant group. Comparing clotting assay with chromogenic assay by ratio (clotting/chromogenic), the three groups did show a statistically significant difference (P = 0.0033) which was due to a difference in mean ratios between normal and lupus anticoagulant groups (ratios 1.00 and 0.91, respectively, P < 0.01). There was no statistical difference in any of the groups when comparing chromogenic protein C with protein C antigen (mean ratios ranged from 1.02 to 1.05, P = 0.3925). In a normal sample, the clotting-based protein C level was unaffected by increasing FVIII level by up to 1000 IU/dl, using intermediate purity FVIII concentrate. The new assay is considered to be a suitable assay for the routine diagnosis of protein C deficiency.