The effect of sodium citrate in arterial catheters on acid-base and electrolyte measurements

Objective: To compare the effects of heparin or sodium citrate used to anticoagulate indwelling arterial catheters on acid-base and electrolyte measurements. Design: Randomized controlled trial. Setting: Medical-surgical university-affiliate intensive care unit. Subjects: Twenty patients with indwelling arterial catheters. Interventions: Patients were randomly allocated to have ten 1-mL aliquots of blood sampled serially from an arterial catheter maintained with either heparin or sodium citrate. A sample then obtained by arterial puncture provided true measurement values. Acid-base and electrolyte measurements of whole blood were obtained from each sample by means of a Corning 860 analyzer. Measurements and Main Results: Contamination with sodium citrate lowered ionized calcium and pH but increased glucose and PCO2. Heparin produced negligible effects on those measurements. When sodium citrate was used, reliable measurements were not obtained for ionized calcium, pH, and glucose, even after 9 mL of blood had been discarded. However, reliable PCO2 measurements were obtained after 2 mL of blood was discarded. Conclusions: Sodium citrate used to maintain arterial catheters can contaminate blood samples. The result of that contamination can mimic severe hypocalcemia, metabolic acidosis, and mild hyperglycemia. Failure to recognize the effects of sodium citrate on acid-base and electrolyte measurements may lead to changes in treatment that could affect patient outcome adversely.