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Life, as we know it, evolved in water that contained a steady concentration of salts. Consequently, the functioning of living cells is dependent on the provision of an environment with just this right concentration of electrolytes. The emergence of organisms from the sea and onto land was only possible because of the evolution of kidneys, which preserved the environment of the sea within the organisms. Without the kidneys providing volume, electrolyte and acid-base homoeostasis, no heart could beat, no muscle move, no thought be thought.1 Disturbances of this homoeostasis are thus serious conditions that pose grave dangers to the patient.2 3 Treatment is influenced by the correct identification of the underlying problem: hyponatraemia requires a different approach if it is due to an excess of water compared to a deficiency of sodium.4 We can get help in the assessment of these disorders by ‘asking the kidney’, that is, interrogation of kidney function. The kidney ‘speaks’ via the composition of urine. Thus, by determining this composition we can obtain important information about the nature of the underlying problem. However, the determination, as well as the interpretation of these values, has limitations and potential pitfalls.
In this article, we will concentrate on urine sodium and osmolality. We will briefly review the methodology used for measurement, but mostly reflect on the interpretation of results in the clinical setting and the underlying physiology.
How is urine sodium measured?
By far the most common method for urine sodium analysis is electrochemically using an ion-selective electrode (ISE). There are two types of ISE in general laboratory use: the direct and indirect ISE. The indirect ISE method was developed for plasma where the sample is diluted (in order to increase sample volume and reduce protein concentration) before it is presented to the measuring electrode. Direct ISEs were a later …
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