Treatments and diagnostic tests

What are immunological tests?

There are immunological tests for many different medical conditions and purposes – for instance, to test for an allergy, to screen for bowel cancer or to find out if a woman is pregnant. They can be used to carry out routine tests in hospitals and laboratories, to do quick tests yourself at home, as well as in family doctors’ and specialists’ practices.

How do they work?

Certain substances or pathogens (germs) in your body can be detected with the help of immunological techniques. The things that can be detected include viruses, hormones and the blood pigment hemoglobin. The tests take advantage of the body’s immune system: In order to fight germs or foreign substances, the immune system produces antibodies. Antibodies are proteins that can bind to a specific germ or substance, just like a key fits into a specific keyhole. They “catch” the germs or substances, neutralize them and attract other immune cells.

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, urine or stool, they bind to the matching substance or germ if found in the sample. This reaction shows that the germ or substance is present.

What happens during the test?

As mentioned above, immunological tests contain specific antibodies that bind to the substance or germ that is being looked for. In some tests this reaction is visible to the naked eye. For example, in tests to determine your blood group, the blood coagulates (clumps together) on the test card. In other tests, the reaction has to be made visible using a fluorescent dye or an enzyme.

Immunological tests can generally be divided into rapid tests and laboratory tests.

Laboratory tests

In laboratory tests, sensitive devices measure the amount of bound antibodies based on the extent of a light or color reaction. The greater the reaction, the more of the substance or germ is present. Laboratory tests take longer than rapid tests but they are also more accurate.

Rapid test

In 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 are not as sensitive as laboratory tests and can't determine exactly how much of the substance or germ is present.

Illustration: Paper strip for rapid testPaper strip for rapid test

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. The antibodies continue to bind to the substance you are looking for, and this reaction causes a change in color.  If enough of the liquid sample is used, it flows all the way along the paper strip until it reaches a control line at the other end. If the control line changes color too, the test was carried out properly.

Illustration: Paper strip tests: positive and negative reactionsPaper strip tests: positive and negative reactions

What are immunological tests used for?

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

  • Bowel cancer screening: This test looks for the blood pigment hemoglobin, a sign of blood in stool. Blood in stool can be caused by various things, such as hemorrhoids, polyps or even bowel cancer.
  • Allergy tests: to detect antibodies against allergy-triggering substances like grass pollen or certain foods.
  • Detecting germs 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 hepatitis C, HIV or HPV viruses. Pregnant women can have a blood test to find out whether they are protected from (immune to) toxoplasmosis.
  • 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.
  • 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.
  • Pregnancy test: Women can use this rapid test to find out whether their urine contains the "pregnancy hormone" beta-hCG.
  • 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 also be detected in this way. These include sleeping pills (benzodiazepines), amphetamines and morphine.
  • 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.

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

The antibody principle is also applied in doping tests, food hygiene tests, and tests for toxic substances.