Comparison of capillary and venous blood sampling for routine coagulation assays

Capillary blood samples are generally assumed as unsuitable for coagulation testing since it is recognized that contamination with tissue factor and dilution with tissue fluid affects the coagulation assay. However, limited data is available about coagulations assays in which capillary blood sampling is compared to the standard venous blood withdrawal method. The aim of this study was to perform a method comparison between capillary and venous blood sampling for routine coagulation assays.

Both venous and capillary (finger stick) blood samples were collected from 188 healthy volunteers and patients. In citrate plasma, International Normalized Ratio (INR), prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), fibrinogen, and D-dimer were measured according to routine protocols using the ACL-TOP 750 LAS (Werfen) coagulation analyzer. Regression analysis was performed and the mean relative difference between capillary and venous sampling was reflected to the total allowable error (TEa).

Strong correlations and acceptable variations, using the TEa as decision limit, were found for INR, PT, TT, fibrinogen, and D-dimer between capillary and venous sampling. However, capillary sampling resulted in significant shorter APTT values when using the standard APTT-SP Liquid reagent with a mean bias of -10.4% [95% CI -12.4 to -8.4].

Based on these results, capillary blood sampling proved to be an alternative blood withdrawal method for routine coagulation assays, with the exception of APTT, if a venipuncture is unavailable or undesired.