Comparison of PT and aPTT values drawn by venipuncture and arterial line using three discard volumes

Blood samples obtained through heparinized arterial catheters are used routinely for a variety of laboratory tests. Accuracy of coagulation studies performed from samples obtained in this fashion continues to be questioned, particularly in regard to the minimum discard volume necessary to clear the catheter of heparinized solution.

To examine differences between prothrombin time and activated partial thromboplastin time values obtained from blood drawn by venipuncture and from an indwelling intra-arterial line using three discard volumes.

Prothrombin time and activated partial thromboplastin time samples were drawn by venipuncture from 41 critically ill adult patients. Simultaneously, three consecutive blood samples of 2.3 mL were drawn from the arterial line after an initial discard volume of 3 mL (discard volumes of 3.0, 5.3 and 7.6 mL).

Significant differences were found between arterial and venous prothrombin time values for the 3-mL discard volume group, as well as between arterial and venous activated partial thromboplastin time values for all three discard volume groups (paired t-test, Bonferroni correction).

We recommend that when drawing prothrombin time and activated partial thromboplastin time samples from an arterial line, a 5.3-mL discard volume be used.