Diagnostic discordance and differential diagnoses of community-acquired pneumonia in the emergency department

BACKGROUND: Community-acquired pneumonia (CAP) is a common reason for presentation to the emergency department (ED), but accurate diagnosis is challenging due to overlapping and often nonspecific clinical features. Data on the frequency and spectrum of alternative diagnoses in patients initially diagnosed with CAP in the ED are limited. The aim of this study was to evaluate the diagnostic concordance of an initial ED diagnosis of CAP, characterize common alternative discharge diagnoses, and analyze clinical, laboratory, imaging utilization, and vital sign parameters associated with confirmed CAP. METHODS: We conducted a retrospective analysis of diagnostic concordance and differential diagnoses in 1,385 adult patients who presented to the ED of Hannover Medical School and were initially diagnosed with CAP. Initial ED diagnoses were compared with discharge diagnoses. RESULTS: The cohort comprised 37% female and 63% male patients with a median age of 71 years (IQR 58-79). Comparison between initial ED and discharge diagnoses demonstrated that suspected CAP was confirmed in 52% (n = 663) of cases. The most common differential diagnoses were other pulmonary conditions (n = 297, 41%), cardiac conditions (n = 150, 21%), and renal conditions (n = 95, 13%). Patients with confirmed CAP showed significantly higher C-reactive protein (CRP) levels (82 mg/L [IQR 37-159] vs. 57 mg/L [IQR 22-127]; p < 0.001) and slightly higher body temperature (37.1 °C [IQR 36.4-38.0] vs. 36.8 °C [IQR 36.2-37.7]; p < 0.001) than those with alternative diagnoses. CONCLUSION: Only about half of patients initially diagnosed with CAP in the emergency setting had a confirmed diagnosis at discharge, indicating that CAP in this context should be regarded as a working diagnosis rather than a definitive entity. Isolated laboratory, imaging, and vital sign parameters showed limited ability to distinguish CAP from alternative diagnoses, highlighting the importance of structured reassessment after admission to confirm or revise the initial diagnosis and to reduce misclassification and unnecessary antibiotic use.