Effect of heparin on arterial blood gases

To evaluate changes in arterial blood gas samples caused by the addition of liquid heparin, 50 patients had three simultaneous blood samples drawn, each with one of three amounts of heparin. The liquid heparin decreased statistically the PCO2, PO2, HCO3, and base excess, while the pH remained unchanged. By using a 2-cc blood sample with a 5-cc glass syringe and a 11/2-inch, 18-gauge needle to draw the heparin solution up to the 2-cc mark, and then completely evacuating it, we found that 0.025 cc of solution remained to coat the syringe. Although this remaining solution would cause a 1.25% error in the blood gas results, the error would be acceptable because it is generally less than the standard deviation of the laboratory results. Excess liquid heparin statistically exaggerated or produced false results consistent with a metabolic acidosis with respiratory compensation. We recommend that the complete evacuation of liquid heparin from the sampling syringe be included when performing an arterial blood gas analysis.