A PCR test (polymerase chain reaction) is the most accurate type of COVID-19 test available. It detects the genetic material of the SARS-CoV-2 virus from a sample taken from your nose or mouth, and it can identify an infection even when the amount of virus in your body is very small. That sensitivity is why PCR remains the preferred test when a reliable diagnosis matters most.
How a PCR Test Detects the Virus
The SARS-CoV-2 virus stores its genetic blueprint in RNA, a single strand of genetic code. A PCR test works by converting that viral RNA into DNA, then copying that DNA over and over until there’s enough to measure. The process runs through 20 to 30 heating and cooling cycles, and each cycle doubles the amount of viral DNA in the sample. After all those rounds, even a tiny trace of virus produces hundreds of detectable copies.
What makes the test specific to COVID-19 is the use of primers: short pieces of synthetic DNA designed to latch onto genetic sequences unique to SARS-CoV-2. If the virus isn’t present in your sample, the primers have nothing to bind to and no copying occurs. When copying does happen, fluorescent dyes attach to the newly made DNA and glow brighter with each cycle. The machine measures that glow in real time, which is why the test is sometimes called RT-PCR (reverse transcription PCR) or qPCR (quantitative PCR).
What the Results Tell You
A PCR result is reported as positive or negative, but behind that binary answer is a number called the cycle threshold, or Ct value. This is the number of copying cycles it took before the fluorescent signal crossed the detection threshold. A low Ct (fewer cycles needed) means there was a lot of virus in your sample. A high Ct (more cycles needed) means there was very little.
Lower Ct values are generally associated with higher infectiousness, while higher values suggest a lower risk of spreading the virus to others. As a rough rule, every 3-point increase in Ct represents about a 10-fold decrease in the amount of viral material. Most labs don’t include the Ct number on your results report, though. A single Ct value without clinical context isn’t reliable enough on its own to determine whether you’re contagious.
One important quirk of PCR’s extreme sensitivity: you can continue testing positive for weeks or even months after your initial infection. Some people show positive results for up to 90 days. That doesn’t necessarily mean you’re still contagious. It means the test is picking up leftover fragments of viral genetic material that your body hasn’t fully cleared.
PCR vs. Rapid Antigen Tests
Rapid antigen tests (the at-home tests most people are familiar with) look for proteins on the surface of the virus rather than its genetic material. They’re fast and convenient, but considerably less sensitive. In a CDC study comparing the two, antigen tests caught only about 47% of infections that PCR detected. On days when people had no symptoms, antigen sensitivity dropped to just 18% compared to PCR.
Antigen tests performed better when symptoms were present, rising to about 56% sensitivity, and peaked at 77% on days when people reported a fever. That’s still a significant gap. This is why the CDC recommends using a PCR test when possible for patients at higher risk of severe COVID-19, particularly when timely antiviral treatment depends on getting a reliable diagnosis quickly.
The tradeoff is speed and access. Antigen tests give you results in 15 minutes at home. PCR samples typically need to be processed at a laboratory, with results taking anywhere from several hours to a couple of days depending on demand and location.
How Samples Are Collected
The original gold standard for PCR testing was the nasopharyngeal swab, a long, flexible swab inserted deep into the back of the nasal cavity. It’s uncomfortable but effective. Over time, less invasive options became widely available: anterior nasal swabs (the shallow nose swabs similar to what you’d use with an at-home test) and saliva samples.
A large meta-analysis published in JAMA Internal Medicine found that saliva-based PCR testing had similar sensitivity and specificity to nasopharyngeal swabs, particularly in outpatient settings. Nasopharyngeal swabs showed a sensitivity of about 85% and specificity of nearly 99%. Saliva performed comparably. This means you can generally trust a PCR result regardless of which collection method is used, though your testing site may have a preferred approach based on their equipment and protocols.
Accuracy Across New Variants
As SARS-CoV-2 has mutated through variants like Delta and Omicron, a reasonable concern is whether PCR tests can still detect the virus. The short answer: yes, with rare exceptions. Most PCR tests are designed to target multiple sections of the virus’s genome simultaneously. If a mutation disrupts one target, the other targets still catch the virus.
There have been specific cases where a single gene target failed. Certain Omicron subvariants carry a deletion in the spike gene that causes what’s called S-gene target failure, where one of the test’s genetic targets can’t detect the virus. A similar issue occurred with a deletion in another gene region. In both cases, tests designed with multiple targets continued to work correctly overall. The FDA identified a small number of single-target tests that were expected to miss Omicron entirely, but these were specific products, not a system-wide problem.
The FDA continues to monitor authorized tests for potential impacts from new mutations. For the average person getting tested at a clinic or lab, the risk of a false negative due to a variant is very low.
Cost and Coverage
If you have Medicare, diagnostic PCR tests for COVID-19 are fully covered with no out-of-pocket cost when ordered by a healthcare provider and performed at a participating lab, pharmacy, clinic, or hospital. Medicare Advantage plan members should check with their specific plan for any cost-sharing details. For people with private insurance, coverage varies by plan, and the broad federal mandates requiring free COVID testing that were in place during the public health emergency have largely expired. If you’re uninsured or unsure of your coverage, calling the testing facility ahead of time can save you from an unexpected bill.