Dr Astrid wrote the following article which was published in Medical Forum Magazine in 2015. This article was written primarily for an audience of doctors and health care workers, and is quite technical.
Tuberculosis prevalence in Australia is one of the lowest in the developed world, approximately 5.5 cases per 100,000. Most TB cases occur in overseas born individuals reactivating latent TB within two to five years of arrival in Australia. Pulmonary TB is the commonest form. The diagnosis of active TB is microbiological and requires acid-fast bacilli (AFB) to be identified from sputum or lymph node biopsies. TB exposure or latent TB can be detected by either a Mantoux test or an interferon-gamma release essay (IGRA), such as the QuantiFERON Gold in-Tube assay (Cellestis Pty Ltd, Victoria, Australia).
QuantiFERON is an in vitro blood test of cell-mediated immune response that measures T cell release of interferon-gamma in response to stimulation by antigens unique to Mycobacterium tuberculosis (ESAT-6, CFP-10 and TB7.7). Unlike the Mantoux, it is highly specific (98%) and does not cross react with BCG vaccination or non-tuberculous mycobacteria.
An individual is considered positive for Mycobacterium tuberculosis infection if the interferon-gamma response to TB antigens is above the cut off (> 0.35 IU/mL). The test is highly sensitive and specific but as rates of latent TB in Australia are low, the risk of false positive results is approximately one in six.
The assay test cannot distinguish between latent tuberculosis infection and active TB disease (Table 1). A positive result may not necessarily indicate active TB, and a negative result may not rule out active TB.
Asymptomatic individuals
A common scenario is a student who requires TB clearance for their practicum and who has a QuantiFERON test. The most important aspect of testing is to establish the pre-test probability of the test being positive. An Australian born individual without travel to TB endemic countries and who does not have known TB exposure is unlikely to have latent tuberculosis infection. Therefore, a positive result needs to be interpreted within this context and the possibility of a false positive test considered. Conversely, a positive test in an individual who comes from a TB endemic country is likely to be truly positive.
Symptomatic individuals
Active tuberculosis most commonly presents with a persistent cough (> 3 weeks), fevers and night sweats. Anyone with a persistent cough who has lived in a TB endemic country warrants consideration for active TB. In this situation a sputum AFB examination and chest x-ray are the preferred first line investigations.
The WA TB Control Program (former Perth Chest Clinic) offers a free service for anyone with latent or suspected active TB. Review by a consultant specialising in tuberculosis management can be arranged by contacting the service on 08 9222 8500. For further information go to http://www.health.wa.gov.au/acc/tb/
References available on request