Latent TB When infected with M Tuberculosis, but do not have active tuberculosis disease. Patients are not infectious. TB infections in Australia are predominantly due to reactivation of latent infection in people who were previously infected in their countries of birth or during their childhood when TB was more common in Australia. Simply put, the immune system ‘walls off’ the TB bacilli (in a granulomatous lesion), which can lie dormant for years. It is kept in this state by the cell-mediated immune system. Main Risk: around 10% of these people will develop active TB during some point in their lives – the greatest risk being within the first 2 years of being infected. Usually when their immune system is weakened.

Investigations – Mantoux Test Readily available test for identifying latent M. tuberculosis infection. Works via a hypersensitivity reaction by the cell-mediated immune system to purified proteins from M. Tuberculosis (called Tuberculin). Tuberculin is injected intradermally in the forearm and the resulting area of induration (not erythema) is measured hours later. Positive result is based on the size of the induration, considering the risk-status and prevalence of TB in certain patients. Previous vaccination with BCC affects the way results are interpreted – may give false positives. Mantoux test should be done to identify people with an increased risk of TB, who would benefit from treating the latent infection. – People with HIV, recent contacts of a person known to have clinically active TB, health care workers at increased risk, etc.

Investigations – QuantiFERON-TB Assay A recently produced blood test that is able to measure quantitatively the production of cytokine Interferon-γ by lymphocytes sensitised to mycobacterial proteins using an ELISA technique. Advantages: – Involves only 1 visit for a blood sample. – No injection technique/subjective interpretation problems – Does not boost responses measured by subsequent tests, which can happen with tuberculin skin tests – Is not affected by prior BCG vaccination.