Evaluation of i-STAT creatinine assay

We evaluated a new, 2-min blood creatinine method using the hand-held i-STAT analyzer. Good results have already been reported using this analyzer for 10 methods including electrolytes, TCO2, pH, PCO2, bicarbonate, glucose, hemoglobin and urea for uremic blood, hemodialysate and peritoneal effluent.

Evaluation included study of imprecision and accuracy.

Imprecision studies gave excellent results, including those for reproducibility of 6 solutions with a mean of 10 repeats and coefficients of variation (CVs) of 0.4-3.4%, and also the mean of the differences between 33 duplicate blood specimens which was 2.2% of the specimen mean. To assess accuracy, we compared results of 149 tests by i-STAT and Beckman Synchron CX7 methods. The difference between the two means was 2.6% and the mean of all differences was 10.9% with i-STAT results higher, especially when blood creatinine values were < 100 micromol/l (1.1 mg/dl) indicating the need for a slightly higher upper limit of the normal range. The correlation coefficient between the two methods was 0.99, the slope 1.0 and the intercept -5.0 micromol/l (-0.06 mg/dl). We assessed the recommended creatinine correction for variation in PCO2 above and below 40 mm Hg, but our results did not suggest the need for such a correction in our range of 27-64 mm Hg; omission would remove a major method disadvantage. Assays of hemodialysate and peritoneal effluent were also satisfactory.

The i-STAT creatinine method is simple and rapid and our evaluation showed satisfactory accuracy and precision. However, results were on average slightly higher than for the Beckman Synchron CX7 method.