Tuberculosis test

Tuberculin skin test (THT)

In the Mendel-Mantoux tuberculin test, a small amount of protein from the pathogen (tuberculin) is injected into the skin on the inside of the forearm. After three days at the latest, a result can be seen: A hardening (with reddening of the skin) has developed at the injection site. In the evaluation of this tuberculosis test, however, only the transverse diameter of the hardened skin site is considered, not the redness that occurs. Whether the tuberculosis test is considered positive depends on the risk factors of the tested patient:

≥ 5 mm

If the hardening is at least five millimeters long, the test is considered positive in those with abnormal radiographs (shadows, fluid retention), immunodeficiencies, and HIV positives. Even with people who have close contact with tuberculosis patients, or have had, this length is considered a positive finding.

≥ 10 mm

For the elderly, the homeless, drug addicts, diabetics, kidney disease, and the like, the test is positive if the hardening is at least ten millimeters long. The same applies to those who come from countries with a particularly high number of TB-infected people.

≥ 15 mm

A response of at least 15 millimeters in length suggests a fresh tuberculosis infection that needs to be treated (in individuals without a specific risk). On average, however, it takes eight weeks after infection with pathogens until the tuberculosis test strikes.

A positive test result is also seen in vaccinated people within five to ten years after the vaccination against TBC. But even with them, a hardening of more than 15 millimeters in diameter signals a tuberculosis infection.

If there are no skin lesions, the test for tuberculosis is negative. But even then, the doctor can not safely rule out tuberculosis. For in the first four to six weeks after a TB infection, the test may still be negative, if the infected has not formed antibodies until then. In addition, even in severe cases (as in miliary tuberculosis), the test result is false negative in about half of the cases. Such false negative tuberculosis tests also occur despite an infection:

• a congenital or acquired immunodeficiency such as AIDS
• a treatment with drugs that suppress the immune system (immunosuppression)
• malignant tumors of the lymphatic system (Hodgkin’s disease, non-Hodkin’s lymphoma)
• a sarcoidosis, a disease of the organic connective tissue
• virus infections such as rubella, measles, chickenpox, influenza
• Live vaccines (chicken pox, measles, mumps, rubella, yellow fever), about six weeks later
• very old age

If a doctor has the suspicion of a false negative result, ie a diameter of less than six millimeters, the German Central Committee for the Control of Tuberculosis recommends to perform an interferon gamma test.

Tuberculosis Rapid Test

In recent years, rapid tests for tuberculosis diagnostics have been developed. The aim of the research on faster test procedures was and still is to be able to initiate the right treatment as early as possible.

Xpert MTB / RIF

In this tuberculosis test, the sputum of patients is examined. More specifically, genes of the pathogen Mycobacterium tuberculosis can be detected therein. The so-called polymerase chain reaction (PCR) is used: The individual components of the genetic material, the genes, are duplicated and can thus be finally determined.

Tuberculosis pathogens are resistant to some drugs. This resistance (resistance) is also due to the genes of the respective Tbc bacterial strains. The California-developed Xpert MTB / RIF rapid test also detects such resistance to the tuberculosis drug rifampicin.

The special feature of this tuberculosis test is its simplicity and speed. After about 90 minutes, a special device provides information about infection and possible resistance to rifampicin. Scientists hope that this method replaces time-consuming bacteriological investigations. The pathogens must be bred on special nutrient media. This can take up to three months to reach a result.

In 2010, the rapid tuberculosis test achieved very good results in one study. The World Health Organization decided at the end of 2011 to invest nearly 26 million euros in this tuberculosis test by 2015. The aim was to improve tuberculosis diagnostics in African and Asian countries and reduce the incidence of disease. However, a study published in 2013 from Africa showed that the test has little effect on the incidence of TB. Although sufferers are treated faster, but based on experience, doctors started early without therapy even with this rapid test. Further studies are now looking at the actual benefits, especially with regard to possible rifampicin resistance, of this tuberculosis test.

Antibody Test

In addition, over the last few years, several generic tuberculosis tests have come on the market, which are supposed to detect a TB infection even faster. A small amount of blood is tested for tuberculosis antibodies. Antibodies are proteins of the immune system and are formed in an infection against the attacker.

In these rapid tests, a patient’s blood is dropped on a cassette with a thin paper (membrane). On this there are other proteins that bind to tuberculosis antibodies. If these are present in the blood of the person examined, after about 15 minutes usually 2 colored stripes appear on the paper. Thus, the test is generally (depending on the version) as positive.

Manufacturers such as experts, for example the German Central Committee for the Control of Tuberculosis, point out the limited benefit of these tuberculosis tests. Only with an active tuberculosis infection could one substantiate the suspicion. Particularly low-bacterial TB diseases are not recognized. In addition, a therapy with antibiotics affects the result negatively. Antibody rapid tests are usually fixed on one or a few pathogens. As a result, these tuberculosis tests give false negative results in infections with other mycobacteria. A negative test result does not rule out tuberculosis.

If suspected infection, experts recommend known methods of clinical tuberculosis diagnostics, such as the tuberculin skin test, the direct detection of bacteria or the interferon-gamma test. You still do not see any profit in it, a quick one Tuberculosis test to use routinely.