Abstract
During the early COVID‑19 pandemic, diagnostic pathways were disrupted by limited access to SARS‑CoV‑2 testing, evolving clinical knowledge, and substantial diagnostic uncertainty. We report a case in which Mycoplasma pneumoniae (M. pneumoniae) and SARS‑CoV‑2 were both detected early in the clinical course of a previously healthy 35‑year‑old man in April 2020. The patient met all six Japanese clinical criteria for atypical pneumonia, and chest computed tomography demonstrated multifocal ground‑glass opacities compatible with atypical pneumonia. Clarithromycin was initiated on Day 4. An M. pneumoniae particle agglutination (PA) titer of 1:640 was recognized on Day 6, supporting possible M. pneumoniae infection, although serology alone cannot confirm acute disease. Later that afternoon, a positive SARS‑CoV‑2 PCR result was reported, prompting reframing of the working diagnosis toward COVID‑19. Antimicrobial therapy was temporarily discontinued and subsequently adjusted. This case illustrates how early‑pandemic diagnostic constraints, delayed availability of microbiological results, and overlapping radiological findings created diagnostic complexity. Although cognitive biases such as anchoring may have contributed to the diagnostic shift, available information is insufficient to establish them as definitive causes. Instead, this case highlights the challenges of interpreting limited diagnostic data in a resource‑restricted setting. As contemporary COVID-19 disease severity has attenuated and atypical bacterial infections have resurged, maintaining awareness of possible co‑detection remains important for accurate clinical assessment.