Measurement of ionized calcium in canine blood samples collected in prefilled and self-filled heparinized syringes using the i-STAT point-of-care analyzer

BACKGROUND: Heparinized syringes are commonly used with point-of-care analyzers (eg, i-STAT) to measure ionized calcium (iCa(2+)); however there is little information about the validity of their use in canine patients. OBJECTIVE: To examine the suitability of prefilled (40 IU heparin/mL) and self-filled (150 IU heparin/mL) heparinized syringes for iCa(2+) measurements using the i-STAT analyzer. METHODS: Forty-seven blood samples were collected from 41 canine patients. Two milliliters of blood were collected into a 2-mL nonanticoagulated (NA) syringe, a commercially available preheparinized (PH) syringe (dry calcium-balanced lithium heparin, 40 IU/mL), and a 2-mL self-filled heparinized (SH) syringe (liquid sodium heparin, 150 IU/mL). iCa(2+) was measured in the sample using the i-STAT analyzer and a wet-reagent analyzer (KoneLab 30i) used as the reference instrument. Data were analyzed using paired t-tests, Pearson correlation coefficient, and Bland-Altman difference plots. RESULTS: There was no significant difference between the results obtained from NA and PH syringes using the i-STAT analyzer, and the correlation was excellent (r =.97). The i-STAT values from the SH syringes (mean, 1.07 mmol/L) were significantly lower (P<.001) than those from the NA syringes (mean, 1.38 mmol/L). iCa(2+) was significantly higher with the i-STAT analyzer than with the KoneLab analyzer for both the PH (mean i-STAT, 1.38 mmol/L vs mean KoneLab, 1.30 mmol/L) and SH (mean i-STAT, 1.07 mmol/L vs mean KoneLab, 1.03 mmol/L) samples. CONCLUSIONS: Blood samples collected in the PH syringes used in this study can be used interchangeably with those collected in NA syringes for measuring iCa(2+) using the i-STAT analyzer. SH syringes with high-concentration heparin products (5000 IU/mL) are unsuitable for measuring iCa(2+) because they cause clinically significant underestimations. Although there was good correlation between the i-STAT and KoneLab analyzers, the results should be interpreted using analyzer-specific reference intervals.