The urine osmolal gap: A clue to estimate urine ammonium in 'hybrid' types of metabolic acidosis

The urine osmolal gap is defined as the difference between measured urine osmolality and the sum of the concentrations of sodium, potassium, chloride, bicarbonate, urea and glucose. Normally, this gap is 80-100 mosmol/kg H₂O. A determination of the urine osmolal gap may be useful to ascertain the etiology of metabolic acidosis which is of the mixed wide and normal plasma anion gap type ('hybrid' metabolic acidosis). For example, with 'hybrid' metabolic acidosis, a low urine osmolal gap will suggest the absence of excessive organic aciduria (ketoacidosis) and the basis of the normal plasma anion gap type of acidosis will be determined by the urine anion gap or 'net charge'. Where 'hybrid' metabolic acidosis has occurred due to wide anion gap metabolic acidosis with loss of organic acid anion in the urine, the urine osmolal gap will be high and can be used in a semi-quantitative fashion to estimate the sum of urinary ammonium plus ketone body anion concentrations.