Which finding is most indicative of a pseudo-outbreak?

• A. Positive cultures from symptomatic patients with diverse susceptibility patterns
• B. Positive cultures from asymptomatic patients with identical susceptibility patterns
• C. A documented chain of hospital-acquired transmissions
• D. Widespread environmental contamination with multiple organisms

Answer: (B)

Pseudo-outbreaks occur when an apparent cluster of infections is produced by laboratory or reporting artifacts rather than real illness. The hallmark is finding positive cultures in patients who have no symptoms, and all those isolates share identical antimicrobial susceptibility patterns. That uniformity across asymptomatic cases suggests a common source of contamination or a lab processing error affecting multiple samples, not true patient-to-patient transmission. In contrast, a real outbreak would more likely involve symptomatic patients and diverse susceptibility patterns among isolates, and investigations would find transmission pathways or environmental sources. While environmental contamination or transmission chains are concerns in genuine outbreaks, they don’t match the scenario of asymptomatic patients with the same susceptibility profile, which is the signature of a pseudo-outbreak.