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Reliability of an Ion-Selective Electrode as a Simple Diagnostic Tool for Mastitis

Background: Developing a simple quantitative tool for mastitis diagnosis is essential. The Ion-Selective Electrode for sodium has been reported to reliably measure sodium concentrations in human milk. Research Aim: To determine whether an Ion-Selective Electrode measurement of sodium:potassium ratios could serve as a diagnostic tool for mastitis and, if so, to determine the diagnostic cut-off value. Methods: A total of 107 milk samples, including 55 from milk bank donors and 52 from participants with mastitis, were studied. The sodium:potassium ratios were determined in 33 samples (without mastitis n = 15; with mastitis n = 18) by the Ion-Selective Electrode and ion chromatography. The remaining 74 samples (donor milk n = 40; participants with mastitis n = 34) were analyzed by Ion-Selective Electrode only. Values were averaged over three measurements for each method. Results: The median postpartum months of donors and participants with mastitis were 2 and 3 months, respectively. The mean (SD) sodium:potassium ratios without and with mastitis were 0.5 (0.1) and 1.7 (1.2), respectively. A positive correlation existed between sodium:potassium ratios obtained from the two methods (r = 0.98). Area under the curve values were 0.951 (95% CI [0.904, 0.986]) for the Ion-Selective Electrode (N = 107) and 0.978 (95% CI [0.926, 1.000]) for the ion chromatography (n = 33) methods. The optimal cut-off value for the Ion-Selective Electrode method was 0.60, with 86.5% sensitivity and 92.7% specificity. Conclusions: The Ion-Selective Electrode was sufficiently accurate for the diagnosis of mastitis. Cohort studies are needed to explore the relationship between sodium:potassium ratios and clinical outcomes.