What is a PCR Test? A Comprehensive Guide

The PCR test, or polymerase chain reaction test, has been an integral part of medical diagnostics, especially in recent times during the COVID-19 pandemic. Indeed, to a considerable extent, the PCR tests have saved many lives while also managing most public health emergencies. In fact, the PCR test is not some modern technological marvel; it has transformed molecular biology for decades.

Apart from COVID-19, PCR also applies to a wide range of other areas—the definition, diagnosis, and management of infectious diseases and genetic disorders, the advancement of forensic science, and environmental research. These applications rely on its ability to detect even the smallest traces of genetic material with unparalleled accuracy. In this article, we focused on the definition of a PCR test, the different types, and their use both in healthcare and beyond.

What is a PCR Test?

A PCR test is a very specific form of diagnosis used to test for the presence of genetic material from a specific organism, such as a virus, bacteria, or other pathogens. PCR tests allow the identification of diseases at a very high level of precision because they amplify small amounts of DNA or RNA from a sample even when the sample contains very small quantities of the pathogens.

PCR tests are three simple steps that cycle infinitely to generate millions of copies of the target DNA:

• Denaturation: The DNA strands are separated using heat in the sample.
• Annealing: Short synthetic DNA sequences are primer molecules that bind to a particular segment of DNA for amplification.
• Extension: The enzyme Taq polymerase synthesizes new DNA strands from the template strands by adding complementary nucleotides to them.

These cycles of denaturation, annealing, and extension exponentially increase the amount of DNA, making it possible to detect it by analysis. For organisms such as the SARS-CoV-2 virus, which is RNA-based, reverse transcription starts with converting RNA into DNA before the amplification step. 

6 Types of PCR Tests

PCR testing comes in different forms, each serving a specific purpose. Let’s examine the most common types of PCR tests and their use in diagnoses like COVID-19.

1. Conventional PCR

This is the classical mode of PCR testing, wherein the resulting amplified DNA is analyzed through means like gel electrophoresis following the completion of the reaction. Compared to some currently available methods of PCR, detection is less effective, time-consuming, and does not allow for real-time monitoring.

2. Real-Time PCR (qPCR)

More commonly referred to as quantitative PCR, this method is applied in almost all clinical diagnostics, such as tests for COVID-19. It employs DNA amplification rendered to real-time detection by fluorescent dye or probe incorporation to measure emitted fluorescence after each cycle. This method detects target DNA or RNA fast and sensitively and is, therefore, widely used for the diagnosis of respiratory viruses.

3. Reverse Transcriptase PCR (RT-PCR)

It is a method of detecting pathogens of RNA origin and, hence, has great utility in viral diagnostics, such as for SARS-CoV-2, the causative agent of COVID-19. The first step in this test is converting RNA to complementary DNA (cDNA) before normal PCR amplification. For this reason, the abbreviation RT-PCR has been almost synonymous with the COVID test, especially during the pandemic.

4. Digital PCR (dPCR)

A highly sophisticated PCR test, dPCR dissects the sample into thousands of tiny droplets, wherein amplification takes place within each one. It provides far greater precision and sensitivity, thereby allowing its use in low-abundance targets such as early cancer detection or monitoring of minimal residual disease.

5. Multiplex PCR

Multiplex PCR can amplify many target sequences simultaneously. This makes a lot of sense regarding efficiency when diagnosing co-infection or used to test for different pathogens. With a single swab sample, the assay can detect influenza, RSV, and SARS-CoV-2, hence simplifying respiratory diagnostics.

6. Nested PCR

In this case, two sets of primers are used to achieve high specificity in two subsequent reactions. The first reaction amplifies a DNA region-wide, whereas the second reaction narrows the focus to a more specific target in the amplified DNA. Thus, nested PCR is highly accurate and powerful for detecting low concentrations of DNA in complex samples.

Read also: Multivitamins for Travelers: Supporting Health on the Go

Applications of PCR Tests

No match with any multinucleated polymorphic test in versatility makes PCR testing a necessary tool in multiple fields:

• Infectious Disease Diagnostics: Testing for disease-causing pathogens such as tuberculosis, hepatitis, and HIV is simply done by PCR tests. Accuracy helps in early diagnosis and treatment.
• COVID-19 Testing: Most of all, the PCR test became commonly known during the pandemic because it was efficient in detecting SARS-CoV-2, even in those who had no symptoms. 
• Genetic Testing: PCR is an important tool in the detection of mutations associated with genetic diseases, such as BRCA mutations in breast cancer or cystic fibrosis.
• Forensic Science: In cases being pursued for criminal offenses, evidence such as hair, blood, and saliva can be amplified by PCR for DNA to establish the identity of a suspect or victim.
• Oncology: PCR detects genetic markers seen in certain cancers and provides a guide for developing personalized treatment options.
• Environmental Studies: PCR testing in drinking water, soil, and food samples also monitors contamination due to the presence of microorganisms.

Advantages of PCR Testing

High Sensitivity and Specificity: It can detect the tiniest amounts of genetic material through PCR tests for accurate diagnosis, even in low pathogen loads.

• Rapid Results: Thus, modern PCR methods, such as real-time PCR, multiply results by the hour and are lifesavers in emergency cases.
• Wide Applicability: PCR tests can now meet a wide range of diagnostic needs, from infectious diseases to genetic testing.
• Scalability: Automated systems are currently used in PCR testing, allowing the processing of thousands of samples daily, as in the case of COVID-19 spreads.

The Final Words

The tool that has revolutionized medical diagnostics and made molecular biology the major arena of modern medicine is the PCR test. It can detect and amplify almost any bit of nucleic acid with such precision that it has opened doors for some of the most revolutionary applications in health, sciences, and so on. In fact, this PCR test has truly emerged as one of the key tools allowing mass testing for COVID-19 detection globally, as did most things enabled by this technology paradigm, to help countries manage the situation better.