Tuberculosis

Tuberculosis disease is caused by a bacterium that can affect any part of the body, but most commonly the lungs. Only TB of the lung or throat can be infectious and is generally transmitted by prolonged and/or frequent contact with an infected individual who is coughing bacteria droplets into the surrounding air. The bacteria are breathed in through the lungs, but can travel in the blood to other organs which become infected. Diagnosis of these non-pulmonary forms can be more difficult, as the person will not have the cough so commonly associated with TB.

A few years ago TB was thought to be a disease of the past, particularly in the developed world. However,the disease has now made a dramatic comeback for a number of reasons. It is now often described as a global epidemic.

Tuberculosis infection (latent TB infection) occurs when people carry Mycobacterium tuberculosis bacilli in their body but the bacteria are being controlled by the infected person's immune system and so are still in small numbers. These bacteria do not cause disease or any TB symptom. Individuals with latent TB infection are not infectious. They would be negative to most TB tests including culture, Nucleic Acid Amplification Tests (NAAT), and smear microscopy. Small nodules are occasionally seen on chest x-ray. Until recently the only test capable of identifying LTBI has been the tuberculin skin test.

Tuberculosis disease (active disease) occurs when the bacterial load is increased and overcomes the body's immune defence. Tuberculosis disease may be infectious and can be identified by one or more of the following symptoms:

• severe cough for 2 or more weeks (often involving blood loss)
• weight loss
• night sweats 
• poor appetite
• weakness or fatigue (tiredness)
• chills
• fever
• pain in the area infected

Active disease can often (but not always) be identified by culture, smear microscopy, NAAT or chest x-ray. The number of people with active TB at a given time is just the tip of the iceberg, as many more are infected with TB and are therefore at a risk of developing the disease.

To view the relationship between active disease and latent infection click on the thumbnail.

TB control methods are based on three strategies:

1. Vaccination using BCG is used in the developing to world and has limited use in the developed world.
2. The diagnosis and treatment of active disease is important for individuals who have TB and it also reduces the onward transmission of the disease.
3. The diagnosis and treatment of Latent TB Infection (LTBI).

Most of the cases of active disease occur through the conversion of LTBI to active disease. Therefore the European framework for tuberculosis control and elimination in countries with a low incidence recently recommended that "Tuberculosis control and elimination strategies must aim at diminishing the incidence and prevalence of latent infection to reduce the pool of those with tuberculosis infection from which future cases of tuberculosis will emanate¹."

Latent infection can only be reduced by firstly identifying those who are infected. This is achieved by testing specific groups with a:

• Greater risk of transmitting the disease (healthcare workers, military personnel, immigrants, contacts of index cases)
• Higher risk of progression to active disease (immunocompromised subjects, including HIV infected, renal patients, transplant patients, haematological disorders and those taking immunosuppressive drugs).

Once people with latent infection have been identified they can be given preventative therapy which will eliminate the Mycobacteria tuberculosis and so prevent conversion to active disease and eliminate the onward progression of the disease.

¹Broekmans J, et al Eur Respir J 2002; 19: 765-775

Diagnostics for Tuberculosis:  Global Demand and Market Potential/TDR, FIND SA.  WHO 2006: p. 21.

Previously, the only test available to diagnose latent TB infection has been the tuberculin skin test (TST). The test has remained more or less unmodified for the last 60 years. Despite its inaccuracies, the TST is still being used to identify patients latently infected with tuberculosis.

The T-SPOT.TB blood test offers a 21st century alternative the TST providing greater accuracty, convenience and logistical advantages.

People who are suspected of having active TB may be given a number of different tests to confirm the damage caused by the infection or to identify the organisms responsible for the infection.

Skin test
The tuberculin skin test described previously is sometimes used for active tuberculosis, with the same limitations. The skin test can only identify if a person has or does not have TB infection, it doesn't differentiate between latent and active TB. If the test is negative it is used as a rule out test indicating that the symptoms are not caused by TB.

The skin test often remains positive for life after initial infection with TB, and therefore the test can be false positive in patients successfully treated for TB in the past.

Radiological examinations - Chest x-ray and CT scan
Chest x-ray is used to check for lung abnormalities in people who have symptoms of TB disease. The results of a chest x-ray may be suggestive of TB; however the technique is not specific as many other diseases can produce similar features. Even experienced radiologists often have difficulty in determining the cause of some abnormalities. Therefore the results from a chest x-ray cannot confirm that a person has TB disease (i.e. the test is imperfect as a ‘rule in' test). Like the skin test it is difficult to distinguish past, cured TB from current active disease since scarring in the lungs remains after a previous TB infection (even if the patient is completely cured).

TB chest x-ray (left) and CT scan (right)

The chest x-ray also has poor sensitivity; in the early stages of disease, the damage to the lungs may not yet have become sufficiently marked to be detectable so people who have active TB are missed. It is therefore an imperfect ‘rule out' test. Clearly the chest x-ray is completely useless as either a rule in or a rule out test if the TB is not in the lungs. So in the 40% of all cases of active TB where the disease is not found in the lungs (extra or non-pulmonary TB) the chest x-ray is of no use.

In some hospitals Computerised Tomography (CT Scan) and Magnetic Resonance Imaging (MRI) have proved useful for imaging tuberculosis lesions, particularly those in the brain and spine. CT scans are therefore often used to identify non-pulmonary TB.

Smear Microscopy
The simplest laboratory test is the examination of sputum for the detection of acid-fast bacilli (AFB). This diagnostic test is cheap (reagents cost less than £3 per test) and is performed in minutes. However, the WHO estimates that it only identifies 35% of patients with active TB. As the test identifies cells suspended in liquid samples it has particular difficulty in detecting many forms of non-pulmonary TB which occur in a variety of organs in the body. This test will also identify acid fast bacilli other than M. tuberculosis so its specificity is not 100%.

Despite these shortcomings it is still the front line tool for active TB diagnosis, partly because the more definitive culture techniques take longer and because it can also determine if a person is infectious. It is argued that while AFB bacilli are found in the sputum the patient can continue to pass on the disease to other people.

In some countries (Germany and Japan for instance) patients are kept in hospital until there are no AFB visible in the sputum as they are then considered not to be infectious. Sputum smear is thus one method that can be used to monitor an active TB patient's response to therapy.

Ziehl-Neelsen acid fast stain

The MTB are the small red rod shaped organisms. Auramine fluorescence stains are a newer adaptation of this test. It requires fluorescence microscopy but is a little more sensitive as the fluorescence is easier to see than the coloured stain.

Culture
Culture techniques are still seen as the gold standard for active TB as they are extremely sensitive, so long as live mycobacteria can be obtained in the sample. It is therefore the "gold standard" test which is used for comparison with other methods when calculating sensitivity in active disease. M. tuberculosis can be cultured from a variety of specimens including sputum, Central Spinal Fluid (CSF), pleural effusion,  bronchoalveolar lavage (BAL), gastric aspirate etc and can thus be used to detect pulmonary as well as
non-pulmonary disease. By assessing the effect of antibiotics on the cultured bacilli, this technique can also identify the antibiotic susceptibility of the particular strain of TB infecting the patient. It is therefore the main method for identifying if a person has multi-drug resistant (MDR) TB. However, it is not always possible to obtain mycobacteria in the sample, especially in non-pulmonary TB so culture is not a sensitive test. If performed correctly it should have very high specificity and can distinguish M. tuberculosis from other mycobacteria. A drawback of this test is the time to result which can be anything from 2 to 6 weeks.

NAAT
Nucleic acid amplification tests (NAATs), such as polymerase chain reaction (PCR), are a relatively new development in active TB testing. Even though NAAT techniques can magnify even the smallest amounts of genetic material, the sample used still has to contain a certain number of TB bacilli and this is not always possible, particularly with non-pulmonary TB where sensitivity can be as low as 60%. To increase the number of bacilli in the sample, and hence improve the sensitivity of the test, the laboratory will often culture the sample, to allow the bacilli to multiply, before carrying out the PCR test. This can take several
days or weeks. The main use of NAAT is not to diagnose TB per se, but to rule out infections caused by atypical mycobacteria in a sputum smear positive patient, before culture results are known. This helps treatment to be initiated quickly, with the therapy then being tailored to the patient based on the culture results obtained 6weeks later.

More recently NAATs have been used to identify MDR TB as mutations in the DNA of MTB which confer drug resistance have been discovered. These methods are quicker than culture but generally only identify resistance to rifampicin and isoniazid.