Blood gas analyzers are commonly used in critical care units to determine iCa levels in heparinized blood sample. Heparin has a chelating effect on all cations, which is more pronounced for iCa. Hence, we hypothesized that iCa levels will be artifactually low when measured from a heparinized blood sample vs. a non-heparinized blood sample.

Methods: 59 paired arterial blood samples, 0.5 ml each, were obtained anaerobically from indwelling arterial catheters in patients admitted to the pediatric intensive care unit using 1ml nonheparinized plastic syringes and 1 ml plastic syringes containing 50 U heparin (Mertell Medical Products inc., Temecula, CA, U.S.A.). Patients represented a mix of post-operative surgical and medical patients. Samples were assayed within 4 minutes of collection using a blood gas analyzer (GEM premier, Mallinckrodt Sensor System, Inc., Ann Arbor, Michigan, U.S.A) for pH, PaCO2, PaO2, iCa, sodium, potassium and hematocrit. Statistical analysis was performed with a paired Student's t-test for samples with and without heparin. Significance was defined as p <0.05. Values as mean± SD.

Results: iCa levels were lower in heparinized samples(P<.0001). Hematocrit levels were also lower in heparinized samples(p<.0003), but not clinically significant. No statistical difference between groups in pH, PaCO2, PaO2 was noted. 12 of 59 (20%) patients had heparinized blood iCa values <1.0 mmol/L (0.90±.07) with concomitantly normal nonheparinized blood iCa values(1.09±0.06).

Conclusions: The use of heparinized syringes lowers the measured iCa calcium level. This discrepancy has clinical importance since critically ill patients may be over treated with calcium supplementation for artifactual hypocalcemia. Since calcium infusion may be associated with complications, interpretation of ionized calcium results should be approached with caution. Table

Table 1 No caption available.
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