By Dr Ananya Mandal, MD
Screening for tuberculosis mainly involves screening for the presence of tuberculosis and the presence of latent tuberculosis.
Screening for latent tuberculosis
Latent tuberculosis is infection with tuberculosis without manifestations of the infection. This is a difficult condition to diagnose mainly because there are no symptoms. Screening for the condition is the best way to pinpoint the diagnosis and treat the infection to prevent spread and flare ups.
Persons who have been in contact with infected tuberculosis patients may have latent tuberculosis and may need to be examined for latent tuberculosis. Persons who have moved from a country where there is a high prevalence rate of tuberculosis also need to be screened for latent tuberculosis. Staying in an endemic zone for tuberculosis for over three months raises the risk of latent and active tuberculosis significantly.
Screening for latent tuberculosis in United Kingdom is performed at a local TB clinic and the patient may be referred to the Health Protection Agency (HPA). The HPA protects the general population from spread of such diseases and infections. They may work in collaboration with the NHS and the TB clinic to screen and contain the spread of tuberculosis.
The Mantoux test
The Mantoux test is the commonly employed test to check for latent tuberculosis. It involves injecting a substance called PPD tuberculin into the skin over the inner side of the forearm. Those with a latent infection with tuberculosis have a skin that is sensitive to the PPD tuberculin and develop a hard red bump usually within 48 to 72 hours of having the test. If the test is strongly positive a Chest X ray is performed to confirm if the infection is active or frank tuberculosis.
Those without latent tuberculosis will have no reaction to the Mantoux test. Screening may be repeated in a year as tuberculosis may take a long time to develop. Those who have had a BCG vaccine may have a mild skin reaction to the Mantoux test. This does not mean they have latent tuberculosis and signifies that the body’s immune system is capable of recognizing tuberculosis.
Interferon gamma release assay (IGRA)
This is a relatively new test for screening. The IGRA may be used to help diagnose latent tuberculosis in individuals who have a positive Mantoux test, who have just moved from a country where tuberculosis is common, who are on medications that suppress immunity and who are working closely or living closely with infected tuberculosis patients.
Screening for tuberculosis before and after travel to a country where tuberculosis rates are high
This includes a complete medical examination for tuberculosis with medical history of suppressed immunity and exposure to persons with tuberculosis. This also includes a complete physical examination for symptoms of tuberculosis.
A chest X ray may be suggested in susceptible individuals where tuberculosis is suspected. Common tests for screening are with Mantoux test and use of IGRA when required. In suspected cases sputum or phlegm may be tested for tuberculosis as well.
Applicants over 15 years require a medical history, physical examination, and a chest X ray. Those with a chest X ray with suggestive findings or have signs and symptoms of tuberculosis or have human immunodeficiency virus (HIV) infection should provide three sputum specimens for examination under the microscope and culture test.
Applicants 2-14 years of age who live in countries with a high rate of tuberculosis (defined by World Health Organization WHO as rates over 20 per 100,000 population) need a Mantoux test and a IGRA test.
Tuberculosis screening medical examination for applicants ≥2 years of age in countries with a WHO-estimated tuberculosis incidence rate ≥20 cases per 100,000 population.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)