Tests for infectious diseases — explaining the complex in simple terms

Terms like "swab", "culture", and "DNA test" are familiar to many patients. But few people, even those interested in medicine, understand or can explain the essence of these studies. Therefore, it is useful — and not uninteresting — to clarify a little the purpose and methodology of these tests.

All infection tests taken from the mucous membranes of the human body are called a "swab" or, in professional terms, bacterioscopy. This test is simple, inexpensive, and does not require much time to obtain results. The essence of the method is diagnosing the infectious agent by examining it under a microscope. Of course, since a person performs this, much depends on the professionalism and experience of the laboratory worker. But in addition, the results can be affected by the fact that some bacteria cannot be detected even under a microscope. Therefore, a diagnosis cannot be made based solely on bacterioscopy.

Another diagnostic method, known as a culture (bacterial culture), has nothing to do with the everyday meaning of the word "culture." Its name comes from the word "to cultivate." This test is carried out as follows. Material is taken from the patient (blood, urine, saliva, etc.), which is placed in a special container with a nutrient medium. Favorable conditions promote the rapid growth of microbes. After some time (usually one to two days) bacterial colonies grow in such numbers that the doctor or lab technician can much more easily determine which infectious agent is involved, with a higher degree of accuracy. In addition, by exposing these colonies to different types of antibiotics, it is possible to determine which is most effective for that microorganism. However, this method also cannot provide 100% reliability. The number of bacteria in the biological material may be insufficient, and they may not have time to multiply and grow on the nutrient medium within the time frame of the analysis.

The next, more complex test is called the polymerase chain reaction (PCR) or, in simple terms, DNA testing. The essence of this test lies in acting on the microorganism at the genetic level. Here, thanks to a set of special reagents, the organism's DNA is amplified many times. The bacterial species can then be identified by medical instruments. Such diagnostics require only a tiny amount of biological material — a few microbes. According to statistical studies, medical tests using PCR provide accuracy in 97% of cases, which is a very high rate in modern diagnostic medicine.

Another method, called serological, is based on detecting antibodies in the patient's blood serum. When the pathogen enters the human body, the immune system activates defenses and produces antibodies that bind to the microorganisms and thus help remove them from the body. Using the serological method, it is possible to determine which antibodies have appeared in the patient and, based on that, identify the cause — the infectious agent. This test does not look for the microbe itself; the diagnosis is conducted indirectly.

It should be noted that a patient with an infectious disease often feels unwell, suffers from a high fever, and finds it difficult to go to the doctor or clinic and wait in queues. In such cases, it is much easier to request a doctor to come to the home. Many institutions and medical centers now provide this service. An experienced nurse can draw blood from the patient's vein at home (on a doctor's order) and deliver the sample to the laboratory.