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To evaluate urinary solutes in terms of their effect on body fluid tonicity, it is necessary to consider: whether that solute serves as an effective osmole in terms of ECF-ICF fluid shifts, its ability to accumulate in the body even if it is an osmotically effective particle, and the original number of body particles from which it derived. Thus, urea can be excluded since it is an ineffective osmole and only urine cations and anions need be considered. With respect to the former, one must separate the proportion of dietary versus endogenous potassium in this analysis as their effects differ. With respect to urine anions, urine chloride need not contribute to the loss of 'particles that count' when its excretion is accompanied by ammonium (i.e. equivalent to a bicarbonate gain). Thus, in the example cited at the beginning of this article, the excretion of hyperosmolar urine may not change body fluid tonicity if all the urinary potassium was of dietary origin and all the bicarbonate generated was retained as such. Finally, it is necessary to integrate defence of ECF volume (sodium balance), potassium balance, acid-base balance and intercompartmental fluid shifts to understand the overall renal response to defend tonicity. While there is utility in measuring urinary osmolality (assessment of medullary physiology, ADH action, water abstraction, concentrating power), in situations where body fluid tonicity is deranged, it is necessary to evaluate the urine sodium concentration along with the concentration of other solutes which may serve as effective osmoles in the urine.

Interpretation of the Urine Electrolytes and Osmolality in the Regulation of Body Fluid Tonicity