Reducing variability in tissue sodium and potassium measurement: validation of microwave digestion for cardiovascular research

Tissue sodium accumulation affects cardiovascular pathophysiology; however, current tissue element trace determination methods show high variability, limiting translational studies. We evaluated within-sample variability and tested whether microwave digestion reduced variability compared with dry ashing and whether flame atomic emission spectrometry (FAES) reduces variability compared with inductively coupled plasma-optical emission spectrometry (ICP-OES). Skin and muscle samples from rats, mice, and humans were divided into three parts and digested separately. Within-subject variability was tested by repeating the microwave digestion workflow on the second and third tissue pieces. Certified reference material (CRM) was used to assess recovery. Agreement between digestion and detection techniques was assessed using Bland-Altman analysis and intraclass correlation coefficients (ICCs). Interassay CVs for the microwave digestion/FAES workflow were 3.3 ± 2.8% for water, 9.1 ± 8.5% for sodium, and 14.9 ± 12.1% for potassium. CRM recovery was 98.9% for sodium and 91.5% for potassium. Dry ashing and microwave digestion agreement was good for sodium (ICC = 0.78) and excellent for potassium (ICC = 0.91). The dry ashing quantified less sodium than microwave digestion (-2.3 ± 23.7%), correlating to tissue weight (<i>r</i> = 0.54, <i>P</i> < 0.001) and negatively to sodium concentration (<i>r</i> = -0.63, <i>P</i> < 0.001). ICP-OES and FAES displayed excellent agreement for both elements (ICC > 0.90). The use of dry ashing and intrasample composition emerged as the primary driver of variability. Microwave digestion reduces variability and bias relative to dry ashing, whereas FAES maintained analytical concordance with ICP-OES, enabling more reproducible, faster, and easier quantification of tissue sodium and potassium in human and rodent studies relevant to cardiovascular physiology.<b>NEW & NOTEWORTHY</b> Tissue sodium is increasingly linked to cardiovascular pathophysiology, yet current measurement methods are slow, costly, and inconsistent. In this study, we quantified the variability of sodium and potassium assessment across human and rodent tissues. We show that ashing contributes to measurement bias, whereas microwave digestion offers a faster, more reproducible, and accessible workflow compared with dry ashing. This approach enables standardized electrolyte analysis to advance cardiovascular research.