Pogorelić Z, Lukšić B, Ninčević S, et al. - Researchers conducted this prospective study to examine hyponatremia as a new biochemical marker associated with complicated appendicitis in the pediatric population. From January 2019 to May 2020, pediatric patients (n = 184) with acute appendicitis confirmed by histopathology were enlisted in a prospective cohort study. Medical history, demographic and clinical data have been recorded in the study protocol. Compared with patients with non-complicated appendicitis, the mean serum sodium level in patients with complicated appendicitis was significantly lower. In patients who were diagnosed with perforated acute appendicitis, the receiver operating characteristic curve of plasma sodium concentration exhibited an area under the curve of 0.983. Patients with complicated appendicitis were more likely to be younger than five years of age, have a duration of symptoms for > 24 h, sodium serum concentration ≤ 135 mmol/L, body temperature > 38.5 °C and CRP serum concentration > 62 mg/L. Hyponatremia is a novel and very discriminative marker in the pediatric population with complicated appendicitis and is thus recommended in the diagnosis and treatment planning of appendicitis.