Moore et al. BMC Public Health 2010, 10:612 http://www.biomedcentral.com/1471-2458/10/612 However, the introduction of highly active anti-retroviral therapy (HAART) in the mid-1990s has since led to greater immune-competence in these patients. The increases observed here were in older age groups and mainly from pulmonary sites, inconsistent with dissemi- nated infection in HIV/AIDS patients; the percentage of reports from blood specimens also fell considerably. The changes seen may therefore be associated with increases in immune-suppressive therapies for other conditions in older age groups. Ninety-two per cent of M. kansasii reports were from pulmonary specimens, which is consistent with the pre- sentation of such infections resembling pulmonary tuberculosis [7]. The increases seen in the number of pulmonary M. kansasii reports may have important implications for both patients and public health as treat- ment and contact tracing for tuberculosis may be inap- propriately/unnecessarily initiated. This highlights the need for prompt submission of specimens for identifica- tion, and awaiting speciation prior to large-scale contact investigation [15]. Interpreting the trends The HPA’s reporting guidelines state that only clinically significant isolates should be reported and these criteria did not change over the study period, so a change in guidelines cannot explain the observed trend. It is possi- ble, however, that the guidelines have not been ade- quately followed and so the increases observed may not reflect clinical significance. The correct reporting of only clinically significant isolates appeared to vary by region and species - just 1% of all M. gordonae reports, rarely considered clinically significant, came from the North East, yet the region accounted for the highest proportion of reports of all NTM, suggesting greater adherence to guidelines in this region. The relatively small increase in M. malmoense reports, which guide- lines state may always be considered clinically signifi- cant, appears to indicate that some of the larger increases in other species, where clinical significance is less defined, may be accounted for by increased ten- dency of laboratories to report non-clinically significant isolates. In a region of the Netherlands, just 25% (53/212) of all pulmonary NTM isolated met the American Thoracic Society diagnostic criteria for clinical significance based on a review of medical records [12,13], highlighting the scope for over-reporting. However, this did vary by spe- cies: just 1/48 (2%) of M. gordonae isolates were consid- ered clinically relevant compared to 24/59 (41%) isolates of M. avium, for example. Changes in laboratory techniques may have contribu- ted to increased detection of NTM as the use of auto- mated liquid culture gradually became more widespread Page 5 of 6 over the study period. In the early 1990s, such methods were considered expensive and impractical for many laboratories [16] but by 2000, increasing numbers of laboratories were purchasing such systems [17], and the latest HPA standard operating procedures from 2006 state the use of automated liquid culture as the standard recommendation [18]. Experience of the authors sug- gests that M. gordonae and M. chelonae were rarely iso- lated prior to the use of liquid culture media. Since this change, moderate numbers have been isolated but very few are clinically relevant and are more likely to be con- taminants of samples, particularly where the patient has no chest x-ray changes. The large increase in M. gordo- nae reports was not specific to any particular age group, sex or site and is likely to be associated with increased detection and reporting of isolates that are not clinically significant. The introduction of DNA probe identification meth- ods [17] may have contributed to increasingly specific and accurate speciation. The apparent emergence of some species, for example M. abscessus, are due to changes in taxonomy. Other increases seen may be linked to a rise in the number of samples submitted for identification of M. tuberculosis as rates and awareness of tuberculosis have increased (rates of tuberculosis in England increased from 11.2 per 100,000 in 1999 to 15.4 per 100,000 in 2006) [19]. Limitations Some duplicate reporting by local hospital laboratories and specialist reference laboratories may be occurring, leading to an artefactual rise, especially in the North East. It is also possible that multiple isolations from the same patient were reported and so the numbers of iso- lates may not reflect numbers of patients. Having said this, multiple isolates should impact only on the num- bers and not on the trends observed, assuming the numbers of samples submitted per patient has remained similar. The lack of clinical data accompanying most reports did not allow us to assess their actual/true clinical sig- nificance, and it was therefore not possible to interpret whether trends were related to contamination or coloni- sation rather than clinical disease. Conclusions Reports of NTM in England, Wales and Northern Ireland continue to increase but the clinical significance of this increase remains unknown. A number of infec- tions inevitably require clinical care and a small propor- tion may lead to inappropriate treatment/contact investigation if tuberculosis is assumed. However, due to the non-transmissible nature of these infections, they have failed to attract the level of attention or policy