Hospitalised adults with pneumonia are frequently misclassified as another diagnosis.

Using data from a large randomised controlled trial of adults hospitalised with acute respiratory illness, we examined the reliability of pneumonia diagnosis on discharge documentation. 50 (28.2%) of 177 patients with a pneumonia diagnosis had no radiological evidence of pneumonia. 67 (34.9%) of 192 patients with clinico-radiological evidence of pneumonia did not have a diagnosis of pneumonia listed; 'COPD exacerbation' or 'lower respiratory tract infection' was often listed instead. These patients more frequently had a respiratory comorbidity and lower oxygen saturations, CRP and temperature at presentation. Pneumonia diagnoses misclassification on discharge documentation may have clinical, financial, and research data implications.