What are immunological tests?

Immunological tests take advantage of the body’s immune system: In order to fight germs or foreign substances, the immune system produces antibodies. These are proteins that can catch and stop the "attackers" from harming you. Like a key that fits a lock, different special antibodies are made for each germ or foreign substance.

The immunological tests used in laboratories are made by producing artificial antibodies that exactly match the substance or germ in question. When these antibodies come into contact with a sample of blood, mucous membrane, urine or stool that has the matching substance or germ in it, they bind to it. So the reaction shows that the specific germ or substance is present.

In some tests, the reaction can be seen just by looking at it – like the blood sticking together on a test card for determining your blood group. In other tests, the reaction is made visible using a fluorescent dye or an enzyme.

That depends on whether it's a laboratory test or a rapid test:

In laboratory tests, sensitive devices measure how many antibodies become attached. The intensity of the light or color reaction shows how much of the particular substance was found in the test. Laboratory tests take longer than rapid tests, but they are also more accurate.

In immunological rapid tests, the antibodies are usually found on paper strips (test strips), but sometimes glass is used too. Rapid tests are easy to use and provide instant results. But they're not as sensitive as laboratory tests and can't determine exactly how much of the substance or germ is present.

Rapid tests work based on the principle of "lateral flow" (flowing sideways):

When a liquid sample (such as urine) is placed on one end of the test strip, the antibodies on the test strip bind to the substance you are looking for if it is present. Then the liquid slowly moves along the absorbent paper towards the other end of the strip. If the sample has enough of the particular substance you're looking for and it reacts with the antibodies, the test line changes color.

There is another line at the end of the paper strip called the "control line." It changes color if enough liquid has been applied to the test strip. So this color change means that the test was carried out properly.

Immunological tests are widely used. Their areas of application include:

• Allergy tests: These tests are used to detect antibodies against allergy-triggering substances like grass pollen or certain foods.
• Determining your blood group: When blood transfusions are done, the person donating the blood and the person receiving the blood have to have the same blood group. Immunological tests can be used to determine the blood groups before a blood transfusion.
• Bowel cancer screening: This test looks for the blood pigment hemoglobin, which is a sign of blood in stool. Blood in stool can be caused by various things, such as an inflammation of the lining of the stomach (gastritis), hemorrhoids, polyps or bowel cancer.
• Diagnosing heart attacks and thrombosis: Shortly after a heart attack or if someone has thrombosis, higher levels of a certain protein are found in the blood. These can be detected using an immunological test.
• Rapid tests for drugs and medication: Immunological tests can also be used to look for recreational drugs such as cannabis, ecstasy and cocaine. Medical drugs that affect the central nervous system can be detected in this way too. These include sleeping pills ( benzodiazepines), amphetamines and morphine.
• Detecting germs that are causing an infection: If it is thought someone has bacterial tonsillitis or scarlet fever, the test looks for Streptococcus bacteria. In the case of Lyme disease following a tick bite, there are tests that can detect the Borrelia bacteria that cause it, and there are tests that can detect the antibodies to Borrelia bacteria. Immunological tests can also be used to detect viruses. Examples include SARS-CoV-2 (COVID), hepatitis C, HIV and HPV viruses. Pregnant women can have a blood test to find out whether they are protected from (immune to) toxoplasmosis.
• Pregnancy test: Women can use this rapid test to find out whether their urine contains the "pregnancy hormone" beta-hCG.
• Urine test: If sugar, blood, proteins or inflammatory cells are found in urine using this rapid test, it could be a sign of diabetes, a urinary tract infection or kidney damage.

Immunological tests can also be used to diagnose congenital or acquired diseases of the immune system, differentiate between different forms of rheumatoid arthritis, or monitor the progression of an existing medical condition, such as certain types of cancer (in prostate cancer the PSA levels in blood are monitored).