ELISA and PCR: just about complex laboratory methods for diagnosing infections

In laboratory diagnostics there are many methods for detecting the required element in the biomaterial. Two fundamentally different methods are most often used to diagnose infectious diseases: ELISA and PCR. We suggest that you familiarise yourself with these methods to understand the pros and cons of each of them.

ELISA (enzyme-linked immunosorbent assay) is the detection of antibodies to infectious agents in the patient's blood. Antibodies (immunoglobulins, Ig) are proteins that our immune system produces in response to infection. Antibodies are produced for each pathogen. Antibodies "tag" microbes, and by the presence of such "tags" on the surface of microbial cells, the body recognises the target for destruction. Antibodies to the same pathogen may differ in the timing of their appearance: 
IgM – during the acute period
IgG – if the process is chronic, or as a permanent immunity (depending on the type of pathogen)
IgA – in case of chronic exacerbation
The ELISA method looks for "traces" of a reaction to an invading infection, not the infection itself! Because of the low cost and the use of blood as biomaterial, the method is used as a screening method. For example, if parasitosis is suspected, patients give blood to determine antibodies to ascarids, giardia, etc. If the result came back positive, it is not necessary to immediately proceed to the treatment of parasites - it is necessary to compare the clinical picture, the results of ELISA and, possibly, the results of other methods of examination. It is worth remembering that antibodies can remain in our body for a very long time after treatment, even after the infection has left our body, i.e. so-called "serological scar" remains. 
Let's take an example: there are thousands of stars in the night sky. Many of these stars have long since perished, but their light still reaches us, centuries later. In essence, we see only the trace of a star. The light is there, but the star is long gone. But if there is no light, it doesn't mean that the star wasn't there.
It's the same with antibodies. The presence of antibodies does not guarantee the presence of the infectious agent you are looking for. The absence of antibodies does not mean that there is no infection. If the result of the ELISA test is positive, it means that it is necessary to look for the infection with more specific and expensive methods - PCR, bacteriological cultures. A false positive result is possible if several microbes have a similar antigenic structure, in which case "similar" antibodies will be produced against different microbes.

PCR is one of the most scientific methods. It detects the DNA of a microorganism. The genetic material is individual for each microbe, which means that the method has the highest specificity and cannot be "labelled". The principle of PCR is that the desired genetic code is repeatedly doubled in a test tube. If even an insignificant amount of this code is present, an increase in its amount will occur and the analyser will be able to "catch" its presence. Thus, PCR is the "gold standard" for the detection of infectious agents. However, this method has its own disadvantages. Firstly, unlike ELISA, PCR can tell about the presence of the pathogen only in the place of biomaterial collection, which is limited to the swab (for example, from the urethra or nose). Secondly, due to its high sensitivity, PCR can detect even dead microbial particles, which is observed immediately after treatment. In this case, the result will be a false positive.

Summarising the above, here is a table comparing these two methods with each other:

Attention! These "classical" diagnostic methods have been modified in OLYMP CDL: PCR is performed in Real-time mode, ELISA is replaced by ECHL, which improves the quality of tests and minimises the probability of false results. 

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