An uncommon cause of metabolic alkalosis that has been seen as a presenting feature of CF as well as a complication in those with known disease. It is accompanied by chronic salt depletion and sometimes failure to thrive without severe dehydration. Principal findings are hypokalaemic hypochloraemic metabolic alkalosis, sometimes with hyponatraemia. This may be preceded by anorexia, nausea, vomiting, respiratory exacerbations, fever and weight loss.
Check venous sample in blood gas machine for bicarbonate. However after salt replacement, the metabolic abnormality resolves and weight gain follows rapidly. Treatment is with sodium and/or potassium chloride supplements, which may be required for many months. Unexplained failure to thrive should always have urinary electrolytes checked, a spot urine Na+
<20 mmol/l indicates low total body sodium that needs correcting. A serum potassium at the lower end of the normal range may still be associated with body depletion.
It is quite usual for a newborn screened infant under 3 months to have low urine Na levels. The normal range is less well defined so if they are thriving, we do not treat this with sodium supplements.