Validity of an ion selective electrode for measuring human milk sodium and potassium ion levels at point-of-care in lactating mothers with inflammatory breast conditions - International Breastfeeding Journal
Background Elevated human milk sodium (Na+) levels and Na+ to potassium (K+) ratios are commonly used in research to define breast inflammation in lactating mothers. Portable Na+ and K+ ion selective electrode probes (ISEPs) allow for immediate point-of-care testing by clinicians, potentially accelerating diagnosis, and treatment of inflammatory conditions of the lactating breast (ICLB). We determined validity of ISEPs compared to the reference method inductively coupled plasma – optical emission spectrometry (ICP-OES) in mothers with ICLB, and acceptability of ISEP point-of-care testing of mothers tested with clinical ICLB symptoms. Methods Human milk samples were collected from 43 mothers with ICLB, between December 2021 and September 2022. Mothers were recruited from private physiotherapy practices, a public women’s hospital, and the local community, in Melbourne, Australia. Human milk Na+, K+ and Na+:K+ ratio levels were tested at point-of-care (ISEPs), and later, in the laboratory (ICP-OES). Adjusted limits of agreement with Bland-Altman plots compared the two measurement methods, with rank linear mixed effects models establishing their relationship. Mother’s ISEP acceptability was assessed via two survey questions (0–10 numerical rating scale (NRS) and open text response) and analysed via descriptive statistics and thematic analysis. Results Adjusted limits of agreement (lower limit mean (95% CI) to upper limit mean (95% CI)) between the two measurement methods were: -6.12 (-7.75, -4.49) to 6.12 (4.49, 7.75) mM for Na+; 7.37 (5.82, 9.47) to 25.6 (23.5, 27.7) mM for K+; and -0.82 (-0.85, -0.79) to 0.80 (0.77, 0.82) mM for Na+:K+ ratio. For Na+:K+ ratio, 100% of values fell within the limits of agreement. For Na+ and Na+:K+ ratio, ISEPs and ICP-OES shared a substantial amount of variability (Na+: conditional R2 = 0.87, p < 0.05; Na+:K+ ratio: conditional R2 = 0.94, p < 0.05). Mother’s acceptance of ISEP testing was high with a median (Q1, Q3) NRS score of 10 (10, 10). The most frequent theme was that the testing was ‘quick and easy, and unproblematic’ (n = 30). Conclusions Point-of-care human milk ISEP Na+:K+ ratio measurement in mothers with ICLB is valid and is rated as a highly acceptable clinical assessment tool by mothers with ICLB.
