Polymerase chain reaction (PCR) is a modern high-informative method of molecular-genetic diagnostics, which is actively used for diagnostics of various contagious and genetic disorders. American biochemist Kary Mullis won the Nobel prize for inventing PCR.
How It Works
PCR diagnostics have existed for over 30 years. Today, doctors worldwide consider it one of the most accurate methods for identifying infections. The method is performed by making multiple copies of microscopic concentrations of pathogen DNA fragments in a biological sample of the patient. PCR is used not only for diagnostics of contagious and genetic disorders but also for establishing paternity.
PCR is Used Most Often In?
Urogynecology
Contagious genitourinary diseases are associated with mild symptoms and chronic nature, which often lead to infertility and miscarriage in women. PCR can help to identify almost any type of sexually-transmitted infection, such as chlamydia, cycoplasma, ureaplasma, trichomoniasis, Gardnerella, candidiasis of any type, and gonococcus. The PCR test also works for viruses that affect both male and female urogenital tracts, especially herpes and papillomaviruses. The smear is taken from the vagina with no pain to the patient. The text is nearly 98 percent accurate with results available in 1-2 days.
Neonatology
PCR diagnostics is an effective tool for identifying neonatal infections during the intrauterine stage and with newborn babies. The list of dangerous infections includes cytomegalovirus, toxoplasmosis, rubella and herpes viruses, chlamydia, and mycoplasma. Using the blood test to identify these infections with newborn babies is not effective since the immune system of a child develops during several months while the specific antibodies can develop too slowly and in insufficient quantities for identification.
Additionally, the blood of newborn babies can include maternal antibodies of the IgG class, which can permeate the placenta barrier. The PCR test in neonatology is made in amniotic fluid or chorionic villi, which requires a biopsy. The test is effective but is associated with a low risk of miscarriage. It is used only if the medical professional considers the potential risks as low compared to potential benefits.
Blood Transfusions
PCR diagnostics is also used to avoid the risk of transfusion of infected blood or plasma to patients. It makes it possible to identify infections—even those that have a long incubation period and can’t be identified with standard tests. In July 1999 the PCR test became a standard requirement for all donor blood and plasma in the countries of the European Union. It assures the donors that they are not carriers of the diseases and confirms to the recipient that the blood they receive is safe.
Diagnostics of Contagious Diseases
This is the category where COVID-19 resides. The PCR test can help diagnose COVID-19 only within a limited time frame, between the 3rd and the 9th day after the onset of the disease. This is the period associated with first symptoms such as cough, fever, difficulty breathing, and tightness in the chest. If the testing is done before symptoms start showing, the chance of a mistaken diagnosis is close to 50 percent.
In addition to identifying COVID-19, the PCR test can diagnose hepatitis viruses, AIDS, Salmonella infection, Helicobacter pylori, cholera, brucellosis, and many other infections. It can also help to identify the pathogen of atypical pneumonia and chronic bronchitis. PCR can not only make diagnostics but also identify a specific stain. The PCR method is also used for early diagnostics of tuberculosis.
What Is the Problem with False Positives
False positives are the primary issue with PCR diagnostics. There is a chance to amplify copies of the DNA of both living and dead microorganisms, which is why PCR should be done twice after a certain interval (depending on the diagnosed pathogen).
There is also a chance of cross-reaction. The DNA fragments are not always unique; similar fragments can be found in molecules of various microorganisms. Thus it is possible to assume one for another and make a mistake.
Mutations represent the final risk factor. Viruses are continually changing, resisting the countermeasures against them. During therapy, a virus adapts to it and makes the test non-informative if the mutation is not traced in time.
Are False Negatives Possible?
A false negative result of the PCR test is also possible in cases of wrong sampling and external influence on the sample during transportation and analysis. Special attention should be paid to medicines and methods of their intake from the moment symptoms first appear to the moment the patient seeks professional medical help.
For example, if a patient self-medicates with antibiotics, it can lead to a temporary reduction in the number of pathogen microorganisms down to an unidentified level. Even still, after a short time, the disease would return.
In the case of improper preparation in taking a smear from the mucous membrane, for example, the sample material will contain the residual surface film from the medicines instead of cells of the mucous membrane when affected by disinfectant sprays or liquids.
What is the Correct Approach to PCR Diagnostics
The first stage includes taking samples of biomaterial, which should be done only with sterile equipment and sterile tubes. Failure to observe temperature requirements during storage or transportation of the samples makes them unusable.
In order to avoid false positives or false negatives, it is required to avoid alcohol and antimicrobial agents 24 hours before the test as well as to avoid smoking, drinking, eating, brushing the teeth, using mouth fresheners, gargling, making inhalations, and taking any medicines at least one hour before the test.
