Frequency and Clinical Outcomes of Atypical Bacterial Pathogen Positivity in Adults Hospitalised with Community-acquired Pneumonia: A Prospective Study from a North Indian Emergency Department

Background: Atypical bacterial pathogens are recognised causes of community-acquired pneumonia (CAP), but contemporary data regarding their frequency and clinical significance among adults hospitalised with CAP in India are limited. Aim: This study examined atypical-pathogen positivity and compared clinical severity and early outcomes according to test results. Methods: A prospective observational study was conducted among adults hospitalised with CAP through the emergency department of a tertiary-care centre in North India from January to July 2024. Patients were tested for Mycoplasma pneumoniae and Chlamydia pneumoniae using serological assays and for Legionella pneumophila serogroup 1 using urinary antigen testing. Disease severity, organ-support requirements, and clinical outcomes through day 7 were compared according to atypical-pathogen positivity. Results: Among 50 patients, 5 (10.0%; 95% CI, 3.3–21.8) had laboratory evidence of an atypical bacterial pathogen: 4 (8.0%) had M. pneumoniae and 1 (2.0%) had C. pneumoniae; none tested positive for L. pneumophila. Severe CAP was present in 3/5 atypical-positive patients (60.0%) and 12/45 atypical-negative patients (26.7%) (risk ratio, 2.25; 95% CI, 0.95–5.34). Ventilatory support was required in 3/5 (60.0%) and 5/45 (11.1%), respectively (risk ratio, 5.40; 95% CI, 1.81–16.11), and invasive mechanical ventilation was required in 3/5 (60.0%) and 3/45 (6.7%), respectively (risk ratio, 9.00; 95% CI, 2.44–33.24). Vasopressor support was required in 40.0% and 11.1%, respectively. By day 7, clinical worsening occurred in 40.0% of atypical-positive and 24.4% of atypical-negative patients. No deaths occurred among atypical-positive patients; 3 of 45 atypical-negative patients (6.7%) died. Conclusions: In this prospective cohort of adults hospitalised with CAP, 10% had laboratory evidence of an atypical bacterial pathogen, predominantly M. pneumoniae. Atypical-pathogen positivity was observed in patients with severe illness and was associated with a greater observed requirement for ventilatory support. Given the small number of positive cases and wide confidence intervals, these findings are hypothesis-generating and require confirmation in larger studies using molecular diagnostics.