Comparison of analytical performance of i-Smart 300 and pHOx ultra for the accurate determination of pleural fluid pH

BACKGROUND: Pleural fluid pH is an essential test for diagnosing complicated parapneumonic effusion. We evaluated the performance of two blood gas analyzers in measuring pleural fluid pH. METHODS: The i-STAT G3+ (Abbott) was used as a reference analyzer to evaluate the pH values obtained from other methods: the i-Smart 300 (i-SENS), the pHOx Ultra (Nova Biomedical), using a clot catcher to filter off microclot, and pH indicator paper. Within-device precision was performed using quality control materials. We compared pleural fluid pH (n = 86) by the above methods and analyzed the concordance rate at the level of the medical decision point, pH 7.2. RESULTS: The within-device coefficient of variations of pH were below 0.1% for all blood gas analyzers tested. The slopes of the regression equations for the i-Smart 300, pHOx Ultra, and pH indicator paper against the reference analyzer were 0.850 (95% confidence interval, CI, 0.800–0.896), 0.714 (95% CI, 0.671–0.766), and 1.105 (95% CI, 0.781–1.581), respectively. The kappa values for the i-Smart 300, pHOx Ultra, and pH indicator paper against the reference analyzer were 0.883 (95% CI, 0.656–1.110), 0.739 (95% CI, 0.393–1.084), and 0.464 (95% CI, 0.102–0.826), respectively. CONCLUSIONS: The i-Smart 300 and pHOx Ultra demonstrated good analytical performance and diagnostic accuracy when determining pleural fluid pH compared with that by the i-STAT G3+, whereas the pH indicator paper showed unsatisfactory results.