Problems when drawing blood from an intravenous line:
(1) contamination by heparin used to keep the line from clotting
(2) contamination by glucose or other substances being infused through the line (see 41.30.10)
(3) dilution of the blood sample
In general, the more blood that is wasted the less the degree of contamination. However, the more blood that is wasted the greater the blood loss by the patient.
Protocol to minimize contamination of the blood specimen:
(1) Flush the line with 5 mL of sterile normal saline.
(2) Draw and discard a volume of blood - one of the following:
(2a) 6 times the dead space for the catheter
(2b) 5-10 mL of blood (5 mL is the minimum recommended by NCCLS; 10 mL reduces the chances of contamination and matches a common syringe.
6 time the dead space of the catheter in mL =
= 6 * (((internal diameter in mm) / 20)^2) * PI() * (length in cm)
where:
• 20 in the denominator includes the conversion to centimeters (10) and the radius (2).
