Blood sampling for biochemical analysis from central venous catheters: minimizing the volume of discarded blood.

The purpose of the work was to establish a reliable procedure for blood sampling from central venous catheters with a minimal loss of blood. Based on previous studies of potassium measurements, the routine procedure had been to discard 12 ml of blood. In order to clear the catheter from substances from the infusion including potassium, the catheters were first flushed with 5 ml of saline. Repeated samples in fractions of 3 ml for hematological analyses (ethylenediaminetetraacetic acid plasma; seven patients), coagulation tests (citrate plasma; seven patients) and for serum measurements (eight patients) showed that a discarded volume of 3 ml corresponding to at least six times the dead-space of the catheters was sufficient after initial flushing with saline.