Plasma tissue factor antigen levels in capillary whole blood and venous blood: effect of tissue factor on prothrombin time.

To measure the amount of tissue factor released during specimen collection and its potential effect of shortening the prothrombin time, we measured tissue factor and prothrombin time in twenty-three paired venous and capillary blood samples from anticoagulated patients and in ten paired samples from controls. We also compared venous prothrombin time determined by a plasma-based assay with venous and capillary prothrombin time determined with a whole blood assay. Venous specimens were obtained using a two-syringe technique; capillary specimens were obtained by fingerstick after wiping the first drop of blood. Plasma tissue factor was determined by an enzyme-linked immunoabsorbant assay. The patients' mean venous tissue factor (235 +/- 101 pg/ml) and capillary tissue factor (268 +/- 106 pg/ml) were higher than those of the controls (161 +/- 42 pg/ml and 187 +/- 63 pg/ml, respectively, P < 0.05). These differences disappeared after adjusting for age. Capillary tissue factor levels were higher than venous tissue factor (244 +/- 102 pg/ml vs. 213 +/- 93 pg/ml), with a mean difference of 31 pg/ml (P = 0.0001). In addition, whole blood prothrombin time was lower in the capillary than in the venous samples (17.7 +/- 5 sec vs. 18.3 +/- 5.4 sec, P = 0.004). However, there was no correlation between capillary-venous differences in tissue factor and capillary-venous differences in the whole blood prothrombin time. Whole blood capillary and venous prothrombin times highly correlated with the plasma-based venous prothrombin time (r = 0.98, P < 0.0001). These results demonstrate that obtaining blood by fingerstick does not result in a clinically significant release of tissue factor. In addition, we did not observe any interference of plasma tissue factor with the whole blood prothrombin time assay. A direct relationship between tissue factor and age was observed.