Gram stain–guided diagnosis of pertussis in an elderly patient with suspected macrolide treatment failure

Macrolide-resistant Bordetella pertussis has emerged as an important clinical concern, particularly in East Asia, where it may contribute to treatment failure. A 73-year-old woman with asthma and eosinophilic chronic rhinosinusitis presented with a persistent cough that was unresponsive to azithromycin. Gram staining of sputum revealed abundant gram-negative coccobacilli, prompting suspicion of Bordetella pertussis . Routine cultures were negative, and the diagnosis was confirmed by an immunochromatographic antigen assay and loop-mediated isothermal amplification (LAMP). Given the lack of apparent clinical response to azithromycin, possible macrolide resistance was considered, and the patient was successfully treated with trimethoprim–sulfamethoxazole. This case highlights the potential role of Gram staining as an early diagnostic clue and underscores the importance of non-culture diagnostic methods and awareness of antimicrobial resistance in the management of pertussis.