[Urine electrolytes: use and pitfalls to avoid]

In clinical situations of electrolytic or acid-base disorders, the measure of electrolytes concentration and osmolality in a urine sample, which is called the urinary spot, is a simple way to guide the diagnosis and therapy. The interpretation of results must take into account the patient's history and clinical examination. The urine sodium concentration and urine osmolality are key elements in the diagnosis of dysnatremias and in renal failure. The urine chloride concentration is useful in the diagnosis of acid-base disorders. The potassium excretion in the urine, which is regulated mainly by aldosterone, is particularly helpful in the case of hypokaliemia. Finally, the determination of the transtubular potassium gradient is a valuable tool for the diagnosis of hypoaldosteronism.