 How do COVID-19 tests work? RT-PCR explained. Viruses have been around for a long time. Adapting and taking over host cells to survive and thrive. They are tiny, efficient packages of protein-wrapped genetic material, RNA or DNA, that cause many animal and human diseases, some deadly. Quick detection is important to protect us all. But the size of viruses and the way they work makes them difficult to spot. In the 1980s, a method called Polymerase Chain Reaction, or PCR, was developed to identify a small fragment of a virus and create multiple copies, making detection easier. PCR originally used radioactive isotopes to mark these copies. The amount of radiation emitted by them showed how much viral material was present in a sample. Today, the glow of fluorescent dyes does this job. PCR is the gold standard in testing for COVID-19 and other viral diseases like Ebola. How does it work? A sample from the person's blood, saliva or mucus is taken and nucleic acids are extracted. This is a mix of the person's own genetic material and, if present, the virus's RNA or DNA. For RNA-based viruses, such as the one causing COVID-19, the genetic material must first be converted into double-stranded DNA for copying in a process called reverse transcription, or RT. Chemical reagents are added to build copies of the genetic material in the sample. These include probes with fluorescent dyes, which will mark any viral DNA found. In a PCR thermocycler machine, different temperature cycles trigger chemical reactions that replicate the original genetic material. In about an hour, billions of copies are made. If the virus is present, the copies will generate fluorescence. The more fluorescence in each cycle, the more viral material is present. The machine measures this increase and displays the information immediately. When done like this in real-time, PCR tests, including RT-PCR, are the fastest and most reliable methods for detecting viruses and other pathogens. Such tests are crucial in the early diagnosis of diseases to counter epidemics in humans and animals.