What is tuberculosis?
Tuberculosis (TB) is a serious bacterial infection with Mycobacterium tuberculosis (M tuberculosis). Active infection causes serious illness and death, and can be difficult to treat. It can also spread to other people. Symptoms of active infection can include:
- Fever, night sweats
- Chest pain
- Cough up sputum and/or blood
- Weakness, fatigue
- Weight loss, loss of appetite
Active TB often involves the lungs, but can involve any part of the body including the skin (cutaneous TB).
What is tuberculosis screening?
TB screening refers to the process of examining and testing a patient that does not have any symptoms of active TB. Screening is undertaken to identify and treat latent TB, so that it can be effectively treated before it becomes active infection.
What is latent tuberculosis?
Latent TB occurs when patients carry live M tuberculosis bacteria without knowing they are infected, because their immune system has walled off the bacteria. In healthy people, TB bacteria can remain alive in the body without causing disease for many years. Latent TB does not infect other people.
If the immune system is broken down, for example if the normal cytokine TNFα is removed by certain treatments, latent TB may become active TB.
Who is at risk from tuberculosis?
Active or latent TB infection is not common in New Zealand and other developed countries, but continues to be reported in certain population groups. Risk factors for TB infection include:
- Close contact with a patient with TB
- Living or visiting a country or community where TB is common
- Living in a crowded community, including an institution such as a rest home, long-stay hospital, prison
- Working in hospitals and healthcare environments
When should tuberculosis screening be undertaken?
TB screening should be undertaken in people at risk of tuberculosis and in people at risk for converting undetected latent TB to active TB. For example:
- Treatment with corticosteroids, biologics especially Tumour Necrosis Factor-alpha (TNFα) inhibitors, and immune suppressing agents such as azathioprine, ciclosporin, mycophenolate or chemotherapy
- Illness, particularly human immunodeficiency virus (HIV) infection, leukaemia, lymphoma, diabetes, alcoholism and renal failure
- Debility (eg old age, young age, and longterm or heavy smokers)
What TB screeening tests are performed?
After taking a history and general examination, the main TB screening tests are:
- Chest X-ray
- Tuberculin skin test
- TB blood test (in adults)
Skin testing and TB blood tests is usually positive in patients with latent TB; they demonstrate prior infection and a good level of immunity to it. The tests are also positive in patients with active TB.
The test results may require expert interpretation. Other tests may also be required, depending on symptoms.
Latent TB is suspected on chest X-ray if any of the following features is/are present:
- Fibrotic area
- Apical scarring
- Blunted costophrenic angle
- Pleural thickening
Patients with active TB may have cavitations or other signs on X-ray. A patient with latent TB may have a normal chest X-ray.
Tuberculin skin testing
The tuberculin skin test or Mantoux test contains a purified protein derivative (PPD) of TB bacteria, so it is also called a PPD test. The test material does not contain live bacteria and cannot cause TB. The test is undertaken by specially trained laboratory technicians and other healthcare workers.
- The test is usually undertaken on the inner aspect of a forearm
- The selected area is first cleaned using an alchol swab
- 0.1 ml tuberculin (F TU) is injected intradermally, raising a bleb 6–10 mm in diameter
The test site is inspected between 2 and 3 days later. The diameter of hard skin thickening is recorded in millimetres. The test is classified as negative (no induration or less than 5 mm induration ) or abnormal (more than 5 mm induration). Abnormal tests may take weeks to subside and can leave a mark or scar.
TB blood test
The usual TB blood test is interferon gamma release assay (IGRA). There are several options, including QuantiFERON Gold™. One advantage of IGRA over skin testing is that it is not positive after BCG vaccination. A disadvantage is the difficulty and expense of conducting the test accurately.
Test results may be positive (infection is likely), netative (it is unlikely), indeterminate or borderline (uncertain).
My test is negative. Can I be sure I don't have TB?
Even if initially negative, TB tests should be repeated from time to time in patients who are at risk of being infected with TB and in those who are at risk of latent TB converting to active TB.
What happens if the test is positive?
If the test is positive, the result should be evaluated by a physician. Further tests and follow-up may be appropriate.
Treatment of TB is recommended in patients with active TB and in those at risk for conversion of latent TB to active TB, for example if they have a chronic immunodeficiency illness or treatment with immune suppressive drug.
Usual treatment is either isoniazid for 6 months, rifampicin for 3 months, or a combination. Guidelines vary for different populations, and if the patient is pregnant or has liver disease (see references).
Should the TB tests be repeated?
TB testing should be repeated every year or so in patients on immune suppressive medications or in other patients at significant risk of active TB.