# Pulmonary Infection Caused by Mycobacterium malmoense and the Difficulties in Distinguishing It From the Pulmonary Mycobacterium tuberculosis Infection

## TL;DR

This paper reports a rare case of pulmonary infection caused by Mycobacterium malmoense, which was challenging to differentiate from tuberculosis.

## Contribution

The study highlights the diagnostic challenges and the importance of proper sample concentration in Mtb TRC tests.

## Key findings

- M. malmoense infection was confirmed through culture after initial tests were inconclusive.
- Proper sample dilution is crucial for accurate Mtb TRC test results.
- The case underscores the need for careful interpretation of diagnostic tests for mycobacterial infections.

## Abstract

Mycobacterium malmoense is a rare species of non-tuberculosis mycobacteria detected in Japan that causes pulmonary infection and cervical lymphadenitis. Here, we report a case of pulmonary infection caused by M. malmoense, which was difficult to distinguish from Mycobacterium tuberculosis (Mtb) infection. A 64-year-old Japanese woman with a history of pulmonary tuberculosis had bloody phlegm, a cough, and discomfort in her chest. Chest computed tomography revealed a cavity, infiltration, and a nodule. A smear test for acid-fast bacilli was positive, a Mycobacterium avium complex transcription reverse-transcription concerted (TRC) test was negative, and an Mtb TRC test was withheld because the internal control was negative. After diluting the specimens, the internal control tested positive, and the sample tested negative. We diagnosed pulmonary M. malmoense infection based on a culture test. In conclusion, attention should be paid to the concentration of bacteria in Mtb TRC test samples, ensuring that the internal control provides expected results.