Can a Mantoux test detect HIV

Laboratory and diagnostics - detecting tuberculosis

The key test in tuberculosis diagnosis is still the direct detection of Mycobacterium tuberculosis pathogens from a patient's sputum (sputum when coughing).

The microscopic examination of the sputum is an old, relatively simple method with which approx. 50% of all tuberculosis sufferers can be identified. However, patients with a low number of germs in their sputum are more often overlooked, which makes diagnosis particularly difficult in the context of tuberculosis-HIV co-infection. Apart from the one-off investment costs for light microscopes, the examination is extremely inexpensive.

To increase the sensitivity of the method, the bacteria can be grown on special culture media. Culture is still the most sensitive and therefore the most reliable method, but this method is expensive. Specially equipped laboratories with high security for infection control and well-trained staff are required. Unfortunately, this diagnostic method takes time, because after the culture has been set up, it takes about 4 weeks before the result can be evaluated. After the cultures have grown, the bacteria can be tested for effectiveness against individual drugs. In countries with high rates of resistance, the establishment of tuberculosis cultures is therefore imperative.

A groundbreaking new development is the further development of molecular diagnostic methods (PCR). For several years there has been a laboratory device the size of a coffee machine, GeneXpert, which can automatically detect the DNA of the tuberculosis bacteria in the sputum in special analysis cartridges. The sensitivity of the detection is far superior to sputum microscopy and comes close to that of culture. The whole process only takes about 2 hours. This method can also be used to investigate whether the tuberculosis bacteria detected are resistant to one of the most important drugs, rifampicin. This gives valuable clues for further diagnosis and therapy. Unfortunately, the devices are quite expensive and the price and the regular supply of new diagnostic cartridges are still a problem in many places.

X-ray diagnostics

An X-ray can go a long way in making a good diagnosis, but it is not the sole diagnostic tool. Pulmonary tuberculosis (pulmonary TB) usually leads to changes in the chest overview, but the changes are not very specific. In extensive examinations, the findings differed to a considerable extent, even among experienced radiologists. Even if the x-ray findings are completely normal, tuberculosis cannot be ruled out. In addition, in countries with low incomes, such an X-ray can easily result in costs equivalent to a weekly salary.

Diagnosing a tuberculosis infection (latent tuberculosis)

The following tests can only diagnose Mycobacterium tuberculosis infection, not tuberculosis.

For a long time, the Mantoux test (Mendel-Mantoux tuberculin skin test) was the standard method for detecting tuberculosis infection. A standardized amount of proteins from the cell wall of mycobacteria was injected under the skin. In the event of an infection, the immune system had already come into contact with the tuberculosis pathogen and formed antibodies. These antibodies can react with the injected antigens. However, this test is positive for BCG-vaccinated people (BCG stands for Bacillus Calmette-Guerin, a weakened strain of the human pathogen tuberculosis pathogen Mycobacterium bovis), sick people, formerly healed tuberculosis patients and latently infected people and is also positive in HIV-positive patients and in highly acute infections often negative.

For a few years There are more suitable immunological methods for the detection of a tuberculosis infection, all that is needed is a blood sample. These IGRA tests (interferon gamma release assay; e.g. the Quantiferon test, TB spot test), are based on the fact that, after initial contact with tuberculosis bacteria, certain defense cells, the T lymphocytes, respond to M. tuberculosis-Antigens release increased interferon gamma. Cross-reactions with the BCG vaccination therefore do not occur.

It must be emphasized again that both a positive IGRA test and a positive tuberculin skin test are only one latent tubercular infection can determine. Both tests cannot detect tuberculosis as an active disease.

Histological examinations are rarely performed, especially since a number of countries and regions do not have trained pathologists available.

The immediate CSF test (the examination of the cerebrospinal fluid) is the decisive diagnostic measure when meningitis is suspected.