Cuesta M, et al. - In this prospective, single-center, observational study, researchers described what causes hyponatremia in CAP. The study sample consisted of all patients with CAP and hyponatremia (≤ 130 mmol/L) during a 9-month period. In 143 admissions with CAP (Study 1), the prevalence of each subtype of hyponatremia and the associated mortality were determined. A sub-cohort of patients with SIAD (n=10) to record the natural history of SIAD related with CAP (Study 2) was prospectively followed. Most commonly, hyponatremia in CAP is secondary to SIAD or hypovolemia. Hypervolemic hyponatremia is less common, but the prognosis is worse. Prospective observation shows that antimicrobials normalize concentrations of plasma AVP and sodium in SIAD. Failure to reverse hyponatremia indicates underlying lung disease, such as bronchiectasis.