The prevalence and clinical significance of pulmonary infection due to non-tuberculous mycobacteria in Hong Kong

Abstract

An unexpected increase in positive sputum cultures of non-tuberculous mycobacteria (NTM) was noted in Hong Kong in 1990 compared to previous years, in contrast to a steady decline in the number of positive cultures of Mycobacterium tuberculosis. A retrospective case note study was therefore undertaken to ascertain the clinical importance of the rise in NTM isolates. A representative sample of 183 of the 675 patients with NTM isolates from sputum during 1990 was identified. Cases were assigned to groups according to whether there was evidence of progressive pulmonary disease due to NTM (group 1), persisting colonization without evidence of progressive disease (group 2) or transient isolation of NTM without evidence of progressive disease (group 3). Of 168 cases with adequate clinical and radiological records, 28 (16·7%) represented progressive disease due to NTM and 6 (3·6%) represented persisting colonization. The remainder were both transient and clinically insignificant. Most patients (71%) with progressive pulmonary disease due to NTM had pre-existing lung damage, and 50% had received anti-tuberculous therapy for documented M. tuberculosis previously. The commonest organism involved was the M avium complex. Eighty-six percent of patients with progressive disease and 83% of those with persisting colonization had at least one smear positive sputum specimen, whereas only 2% of patients with sputum contamination had a smear positive sputum sample ( P<0·0001). M. malmoense, M. xenopi and M. fortuitum all appear to be rare causes of significant NTM disease in Hong Kong. However, M. chelonei caused four cases of progressive disease within the study population, two of which were fatal. Although positive sputum cultures of NTM are relatively common in Hong Kong, the majority do not appear to represent clinically significant infection. The spectrum of organisms identified differs from that reported in other regions of the world.