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.

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.