How much blood must be discarded from a heparinized arterial line to obtain accurate coagulation studies, specifically activated partial thromboplastin time? The published literature provides insight into the question and guidelines for practice in adult critical care. This article reviews and integrates findings from 14 research studies published from 1971 to 1993 on discarding blood from arterial lines for coagulation studies. Investigators compared activated partial thromboplastin time values from arterial and venous blood samples using various discard volumes, sites and sizes of catheters, and heparin flush concentrations. Similarities and differences in arterial and venous activated partial thromboplastin time were reported. Studies have demonstrated that adequate discard volume for activated partial thromboplastin time is 6 times the catheter dead space. These results should not be generalized to systemically heparinized patients, pediatric patients, or other types of heparinized lines such as pulmonary artery, central venous, or Hickman catheters.

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