Mycobacterium

Over on his site ForrestGump is tackling the A-Z of CST; by any measure a daunting task. His 2nd instalment concentrates on the mechanics of the bug;

TB is a very nasty bacterium found in every county in the world that results in around 2 million deaths per year.

It's probably worth taking a backward step by mentioning that TB is a bacteria and that bacteria are absolutely everywhere. As an example, there are 40 million bacterial cells in a gram of soil and a million bacterial cells in a milli litre of fresh water. Finding bacteria is no problem, establishing their identity can be one.

TB, or more properly Mycobacterium tuberculosis, belongs to the Mycobacterium family and are distinguished by their waxy protective covering. There are many members of the Mycobacterium family and for the purposes of TB they can be segregated into 2 groups, tuberculose and non tuberculose mycobacterium, or TB and NTM.

Unlike TB NTM are not transmissible (they are "environmental") however like TB can affect the lungs and soft tissues. Whilst TB control has reduced TB in developing countries cases of NTM continue to increase globally

The "isolation prevalence" of pulmonary NTM has significantly and rapidly increased in Ontario

and

The data show that the annual number of isolates of EM from both HIV positive and negative patients is increasing both absolutely and relatively to isolates of Mycobacterium tuberculosis.

and here

contacted 50 laboratories in different countries for the necessary information..There was an increase in the number of NTM isolated from clinical samples of patients.

Once undiagnosed, the incidence of NTM is increasing and is rivalling that of TB

In 2003, there were 657 cases of TB reported in the province of Ontario, corresponding to an incidence rate of 5.4 per 100,000. In contrast, we estimate that there were likely 420 new cases of pulmonary NTM disease in Ontario in the same year, corresponding to an incidence rate of 3.5 per 100,000.

The point of all this is that the Mantoux, or TB skin test, is unable to diffentiate between the various members of the mycobacterium family and in particular between TB and NTM.

The use of TB specific antigens in the QuantiFERON test means that bulk of NTM are excluded

These findings support the hypothesis that the QFT is the test of choice in populations with a high BCG vaccination rate or with an increased chance of exposure to non-tuberculous mycobacteria (NTM), or in people older than 40 years.

This is important as a false diagnosis is a waste of resource and presents the patient to unnecessary risk

In summary, our results show enough evidence to state that previous NTM sensitisation in children induces false-positive results in the TST for diagnosing LTBI and that the IFN-y tests could avoid both unnecessary chemoprophylaxis treatment among child populations and consuming resources searching the index case.