Background: Retrospective case series suggest that abnormalities in fluid, electrolyte, and acid-base homeostasis may occur among infants with a febrile urinary tract infection. Potentially inaccurate laboratory methods of sodium testing have often been used. Methods: Between January 2009 and June 2016, we managed 80 previously healthy infants (52 males and 28 females) ≥4 weeks to ≤24 months of age with their first episode of acute pyelonephritis. Ionized sodium, ionized potassium and ionized chloride were determined by direct potentiometry, as recommended by the International Federation of Clinical Chemistry. Bicarbonate was calculated from pH and carbon dioxide pressure. Results: Electrolyte or acid-base abnormalities were disclosed in 59 (74%) of the 80 infants: hyponatremia (n = 54), hypobicarbonatemia (n = 18), hyperkalemia (n = 14), hyperbicarbonatemia (n = 6), hypochloremia (n = 3), hypokalemia (n = 3), and hyperchloremia (n = 1). None of the patients was found to be hypernatremic. Patients with and without electrolyte or acid-base abnormalities did not differ with respect to age, sex distribution, and whole blood glucose. Blood tonicity was lower and poor fluid intake, frequent regurgitations or loose stools more common among infants with electrolyte or acid-base abnormalities. Conclusions: This prospective cross-sectional study shows that electrolyte or acid-base abnormalities, most frequently hyponatremia, occur in approximately 3 quarters of infants with acute pyelonephritis.

The first study on sodium level measured by the direct potentiometry in infants with bronchiolitis points out that the prevalence of hyponatremia is two-fold higher than so far reported.