Comparison of Ion Selective Electrodes for the Measurement of Sodium and Potassium in Human Milk

Accessible and cost-effective measures of human milk biomarkers are needed for lactation screening and management. Sodium (Na), potassium (K), and the sodium-potassium (Na/K) ratio in human milk show promise as indicators of secretory activation and mammary gland health.

This validation study compares the accuracy and precision of handheld and benchtop ion selective electrodes (ISE) as compared to inductively coupled plasma mass spectrometry (ICP-MS) for the measurement of Na and K in defatted and whole milk.

Milk samples (N = 27) were assessed for Na and K in defatted and whole milk using ICP-MS, handheld, and benchtop ISE instruments. Bland-Altman analysis was performed to compare Na, K and Na/K measurements in whole versus defatted milk and between instruments.

All three assay methods provided high precision for Na and K concentrations in both whole and defatted milk. Na/K ratios were similar between whole and defatted milk for both ICP-MS and the handheld ISE. Measurements of Na and Na/K were systematically higher with both ISE instruments as compared to ICP-MS. However, the systematic difference was approximately three times greater in the benchtop ISE as compared to the handheld ISE. This resulted in identification of a larger number of cases of elevated mammary epithelium permeability (MEP) with the benchtop ISE as compared to handheld ISE or ICP-MS.

Meaningful assessment of Na and Na/K in whole milk is possible using the handheld ISE. Its portability and ease of use suggest potential for assessing Na and K in human milk at the point-of-care for use in both research and clinical applications.