[A frequent problem in the laboratory control of heparinization: contamination of blood specimens with exogenous heparin].

If blood for monitoring of heparin therapy is collected through indwelling catheters it may be contaminated by exogenous heparin. A prospective study comparing thrombin times in plasma obtained by venipuncture and by collection through heparin perfused catheters showed that 26 out of 77 catheter samples were contaminated. In 16 of these 26 cases, overestimation of the heparin effect would have led to incorrect dosage recommendations. It is concluded that blood for laboratory monitoring of heparin treatment should be collected by venipuncture.