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A Polymerase Chain Reaction (PCR) is a molecular biology technique that seeks to amplify a fragment of genetic material millions of times. The PCR technique has countless applications and is becoming more and more widespread. Currently, due to its reliability and precision, it is considered the benchmark for diagnosing infection by the SARS-CoV-2 virus, which is responsible for causing COVID-19. 

What is it?

It consists of replicating a highly specific fragment of genetic material millions of times. To diagnose a SARS-CoV-2 infection, very specific fragments of the genetic material from the virus have been selected to prevent the test from confusing this virus with others. PCR diagnosis is complex. Trained personnel are needed to perform and interpret the test correctly. 

What is it used for?

This technique is used to detect any microorganism present in samples from patients who have been infected. For example, it is used to detect SARS-CoV-2 in saliva samples. As it detects the genetic material of the SARS-CoV-2 virus, the test is highly sensitive (very few false negatives) and highly specific (very few false positives).  

How is it done?

The basic steps for making a diagnosis using a PCR test are the following:  

  • Sample collection. To detect SARS-CoV-2, a nasopharyngeal swab sample is recommended. For this, a swab is used to collect a sample from both the nasal and pharyngeal areas. This swab collects cells from the upper airway (nose and nasal cavity, as well as the mouth and the beginning of the throat) to test for the presence of viral RNA and to determine whether a person has a SARS-CoV-2 infection or not. Other samples (e.g., saliva, sputum, etc.) can be used, although this depends on the context in which the test is to be performed (epidemiological surveillance, hospitalised patients, rapid testing of positive cases, etc.). 
  • When the sample is received in the laboratory, it is inactivated. This allows samples to be handled and avoids the biological risk of transmission to staff. Automated platforms are then used to analyse the sample. A series of steps are followed on these platforms, starting with the extraction of the viral RNA (genetic material). Next, a PCR technique is used to amplify many copies of the RNA fragment to be analysed so that it can be detected. 

How long does it take?

This depends on the workflow of each laboratory. The PCR can be processed using semi-automated equipment to extract the genetic material and then real time PCR (rt-PCR) equipment. In these cases the result may be available within 5 to 8 hours. In addition, there is a wide variety of automated platforms on the market that allow the PCR technique to be performed in less time, as they automatically extract the genetic material, and then amplify and detect it all in the same device. In this case the test results may be ready in 1 to 5 hours. These times vary depending on the urgency of the test, the number of samples processed in the laboratory, the quality of the samples received, and other factors. 

How do you prepare yourself?

You do not need to prepare before the SARS-CoV-2 PCR test. Nasopharyngeal swabs are easily accessible and easy to collect. 

Special situations

There are absolutely no contraindications for a nasopharyngeal swab. However, special care should be taken if the person has anatomical defects in the craniofacial region, recent trauma involving the bones of the face, or a coagulation disorder. In these cases, a different type of sample may be considered.

Who performs the test?

The nasopharyngeal swab is taken by healthcare personnel, and the PCR test is performed by professionals with expertise in clinical analysis and/or microbiology. 

What will I feel during the test?

The nasopharyngeal swab procedure is quick, and usually harmless and painless. However, in some people it may cause discomfort, coughing or sneezing, and occasionally nausea. 

Substantiated information by:

Juan Carlos Hurtado
Mariana Fernandez-Pittol

Published: 5 February 2021
Updated: 5 February 2021


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