Reverse transcriptase PCR (RT-PCR) is a laboratory technique that detects and measures RNA by first converting it into DNA, then amplifying that DNA into millions of copies. It’s the standard method for diagnosing RNA viruses like SARS-CoV-2, HIV, hepatitis C, and influenza, and it’s also widely used in research to measure gene activity in cells.
The technique works in two stages. The first stage, reverse transcription, uses an enzyme to build a DNA copy from an RNA template. The second stage is standard PCR, which rapidly multiplies that DNA so it can be detected. Without the reverse transcription step, PCR alone cannot work with RNA, since it only amplifies DNA.
How the Two-Stage Process Works
In the first stage, an enzyme called reverse transcriptase reads a strand of RNA and builds a matching strand of DNA from it. This DNA copy is called complementary DNA, or cDNA. The enzyme gets its name because it reverses the normal flow of genetic information: cells typically use DNA to make RNA, but this enzyme does the opposite.
To start the process, a short piece of genetic material called a primer attaches to the RNA, giving the enzyme a starting point. The mixture is heated to around 70°C for five minutes to unravel any tangles in the RNA’s structure, then quickly cooled on ice to keep it straightened out. The reverse transcriptase enzyme then works at about 42°C for an hour, steadily building the cDNA strand. A final burst of heat at 94°C for five minutes destroys the enzyme, stopping the reaction.
In the second stage, the cDNA enters a standard PCR cycle. This involves repeated rounds of three temperature changes: heating to 95°C to separate the two DNA strands, cooling to 42–65°C so primers can attach, and warming to 72°C so a DNA-building enzyme extends the new strand. Each cycle doubles the amount of DNA. After 25 to 35 cycles, a single piece of cDNA has been copied into thousands to millions of identical fragments, enough to detect and measure.
One-Step vs. Two-Step RT-PCR
There are two ways to set up the process, and the choice between them depends on what you need from the results.
In one-step RT-PCR, both the reverse transcription and the PCR amplification happen in the same tube without opening it between stages. All the reagents go in at the start, and the machine runs through both processes automatically. This is faster, requires less hands-on work, and reduces the risk of contamination from pipetting between tubes. It’s the preferred format for high-throughput diagnostic testing, like processing hundreds of COVID-19 samples in a day. The downside is that the cDNA generated during the reaction can’t be saved for additional experiments.
In two-step RT-PCR, the reverse transcription happens first in one tube, producing cDNA that can be stored and used for multiple different PCR reactions afterward. This format also allows more flexibility in how the cDNA is generated, since you can use different types of primers (gene-specific primers, random primers, or primers that target the tail end of messenger RNA). Two-step protocols tend to offer higher sensitivity, making them the better choice for research applications where precision matters more than speed. Labs doing viral diagnostics research often prefer the two-step approach for this reason.
What Ct Values Tell You
When RT-PCR is paired with real-time fluorescent monitoring (a combination called RT-qPCR), it produces a number called the cycle threshold, or Ct value. This is the number of amplification cycles it takes for the target RNA to reach a detectable level. The relationship is inverse: a low Ct value means the sample contained a lot of RNA, while a high Ct value means very little was present.
In COVID-19 testing, for example, a Ct value below 20 generally indicated a high viral load, while values above 30 suggested much lower amounts of viral RNA. Research during the pandemic found that 66.4% of patients with Ct values of 31 or higher cleared the virus within 14 days of initial detection. Higher Ct values have also been linked to non-replicating viral RNA, meaning the detected genetic material may no longer represent an active infection. This distinction matters because a positive RT-PCR result doesn’t always mean someone is contagious.
Diagnostic Accuracy and Limitations
RT-PCR is often described as the “gold standard” for detecting RNA viruses, but its real-world performance depends heavily on sample quality and timing. In laboratory conditions, many diagnostic RT-PCR tests can detect as few as 500 to 5,000 copies of viral RNA per milliliter, pushing analytical sensitivity close to 100%. Clinical sensitivity, however, approaches only about 80%, with specificity in the range of 98–99%.
That gap exists because of factors outside the test itself. A swab might miss the virus if collected too early or too late in an infection, or if the sample isn’t taken from the right spot. RNA is also fragile and degrades quickly if samples aren’t handled properly. Contaminants like proteins or residual chemicals from the extraction process can interfere with the enzymes, either blocking the reverse transcriptase from building cDNA or preventing the DNA polymerase from amplifying it. Labs assess RNA purity by measuring how the sample absorbs ultraviolet light at different wavelengths, though these ratios don’t always catch every type of contamination that could inhibit the reaction.
Common Uses Beyond Viral Testing
While viral diagnostics brought RT-PCR into public awareness during the COVID-19 pandemic, the technique has been a core tool in biological research for decades. Its primary research application is measuring gene expression, meaning how actively a particular gene is being used by a cell at a given moment.
Every cell in your body contains the same DNA, but different cells use different genes. A liver cell activates liver-related genes, while a brain cell activates a different set. When a gene is active, the cell produces messenger RNA from it. By extracting RNA from a tissue sample and running RT-PCR, researchers can determine which genes are turned on and how strongly. This has been applied to studying how plants respond to drought stress, how cancer cells differ from normal tissue, how immune cells activate during infection, and how organisms respond to environmental toxins. In one set of studies, researchers profiled over 1,400 genes involved in regulating other genes in a model plant, identifying dozens that were active only in roots or only in shoots.
Commercial RT-PCR kits exist for detecting a wide range of pathogens beyond SARS-CoV-2, including HIV, hepatitis B and C, cytomegalovirus, human papillomavirus, dengue virus, influenza A and B, West Nile virus, and rotavirus.
Digital Droplet RT-PCR
A newer variant called digital droplet RT-PCR (RT-ddPCR) partitions a sample into thousands of tiny droplets, each containing either zero or one copy of the target RNA. After amplification, the machine counts how many droplets contain the target, giving an absolute number of RNA copies without needing a standard curve for comparison. Traditional RT-qPCR relies on comparing a sample’s signal to a set of known standards, which introduces variability.
RT-ddPCR is more precise at low concentrations, where traditional methods tend to be less reliable. In testing for waterborne viruses like rotavirus, RT-ddPCR showed a coefficient of variation below 15% across a wide concentration range, outperforming standard RT-qPCR in repeatability. It’s also more tolerant of substances in the sample that would normally interfere with the reaction. The practical benefit is that labs can get accurate counts of viral RNA even in samples where the target is scarce or the sample matrix is complex, like environmental water samples or bodily fluids with high protein content.