how is tuberculosis diagnosed?

People are referred to a clinic when they are suspected of having tuberculosis. In the clinic the patients are questioned about their history of TB exposure and infection. The clinicians also take in consideration other risk factors like the demographic factors such as the country of origin, age, whether they have other diseases or medical conditions and their ethnic group. These factors may increase the patient’s risk for exposure to TB or to drug-resistant TB. It is very important to know if a person has HIV because this increases the risk of a latent TB infection progressing to the TB disease.

There are 2 types of TB that can be diagnosed: latent tuberculosis and active tuberculosis. For latent tuberculosis (meaning you have been infected with the bacteria but don’t show any symptoms) mainly the methods mantoux tuberculin skin test or interferon gamma release assay (IGRA) are used. You are screened for latent TB if you have been in close contact with someone that is known to have an active TB infection, or if you have spent time in a country where TB is common. People with latent tuberculosis cannot spread the virus to others and are given medication to prevent the infection from developing into the disease.

In the Mantoux tuberculin skin test (TST) a small amount of tuberculosis antigens (also known as tuberculin) is injected in the skin. An antigen is a substance that causes your immune system to produce antibodies against a disease. If the patient has had TB before, this causes an allergic reaction visible as a red patch where the antigens have been injected. Because of HIV or immune-reducing medicines the reaction is not completely reliable, and the result doesn’t clarify if a person still has TB.

Therefore, if the result of the mantoux test is positive, it is recommended to also get an interferon gamma release assay. This test measures the reaction of the patient’s immune system to M. tuberculosis as well, but this test is said to be more specific than the mantoux test, due to fact that the mantoux test has a high risk of getting false results. The IGRA test is becoming more widely available and takes 24 hours. To determine whether someone has active TB other tests are required.

There are different ways of finding out whether a person has active TBC.

The first one is by making an X-ray of the chest to try detecting deformities in the chest. When someone has pulmonary tuberculosis, irregularities are usually visible. However, because abnormalities can also suggest other diseases a chest radiograph is not a good method to definitively diagnose TB.

This picture shows an X-ray of someone that has advanced tuberculosis; the infections in both lungs are marked by white arrows. The black arrow-heads show the formation of a cavity (a gas filled area).

Instead of an X-ray it is also possible to opt for a CT scan, which is more detailed. But if you have extrapulmonary TB (which means outside of the lungs), doctors can also decide to make an MRI scan or an ultrasound scan of the affected part in your body.
To properly examine the inside of your body doctors might choose to go into your body with a long tube that has a camera attached to it, to look for irregularities.

If there are signs in the scans that show you might have TB the clinic will take a sample of your sputum (the thick fluid produced in your lungs that comes up when you cough). And, if possible, an attempt is made to cultivate the tuberculosis bacterium. Sputum cultures are tests that help identify and detect certain bacteria. The samples can also be used to test all kinds of resistance against drugs. This helps the doctor choose the appropriate type of medication.  A positive culture for tuberculosis confirms that you have the TB disease. The downside of this test is that culturing requires bio safety facilities that are expensive to build and require specially trained people to perform the procedure. It can also be expensive in terms of transportation costs and time taken off work. In addition, the diagnosis can take weeks because the bacteria of TB grow at a very slow rate.

In low and middle income countries the cultures are too expensive so in those countries they prefer smear microscopy instead. However, this is only accurate in about 50% of the cases and in only 20% of the children and HIV-infected people. This is why this method is not really relevant anymore in the more developed countries.
You may also have a lumbar puncture. In this test they take a small sample of cerebrospinal fluid (CSF), a fluid that is situated in the base of your spine. The CSF also surrounds the brain, so if the sample is checked and is positive for TB, this means the TB bacteria has infected your nervous system, meaning the brain and spinal cord.

Young children are more likely than older (15+) children and adults to develop a deadly form of TB. However, it is more difficult to diagnose the TB in children with a laboratory test. This is because the children find it difficult to cough up the sputum. And, if they do manage to collect the sputum specimens, the results are less likely to be positive due to the fact that the TB disease in children is caused by a smaller number of bacteria.

Therefore, the diagnosis in children is often done without consulting the laboratory. The diagnosis is based on the visible signs and symptoms associated with the TB disease and the history of contact with someone with the disease. If this is positive they have undergo a medical evaluation including a positive TST or IGRA, and an X-ray.

However, only approximately half of the children with TB show symptoms. When children don’t show any symptoms the only sign of TB infection is a positive reaction to the TB skin test, TB blood test or scan. The TB skin testing is considered to be reliable in children, and is preferred over TB blood tests for children that are less than 5 years old.

They are still searching for new ways to diagnose tuberculosis because the tests are either too insensitive or expensive, and there is always room for improvement.

New approaches to diagnose TB are the bio sensing technologies. A variety of portable, quick, and sensitive biosensors with immediate “on-the-spot” interpretation have been developed for m. tuberculosis. The detection is based on different recognition systems; it is basically a device that is able to convert a biological response into an electrical signal. There are different types of biosensors.

Piezoelectric Quartz Crystal Biosensors are used for the direct detection of m. tuberculosis. This has been rising in popularity because it is very sensitive, while having low cost and size, and being easy to operate.

Electrochemical Biosensors can be divided in immunosensors and DNA biosensors.
Electrochemical immunosensors detect the m. tuberculosis antigen and are very sensitive and accurate. These sensors can also be used in detecting varieties of cancer.
Electrochemical DNA biosensor can be used for early, sensitive and quick diagnosis and detection of m. tuberculosis. The sensor identifies a specific DNA sequence.

Magnetoelastic Biosensors are reasonably cheap and are very effective in monitoring and detecting the growth in a culture and samples of the m. tuberculosis.

The last method that has been growing in popularity is the nucleic acid amplification test, or NAAT for short. This diagnostic method can detect the known mutations for drug-resistant TB. It does this by, as the name suggests, amplifying the nucleic acid (including DNA and RNA). The amplified mycobacterium tuberculosis direct test or MTB for short is said to be very accurate and quick in use.

 source of the picture: http://phil.cdc.gov/phil/quicksearch.asp

 written by: Marleen de Jonge