A SARS test detects the virus SARS-CoV-2, the pathogen responsible for COVID-19. There are three main types: molecular tests (like PCR) that identify the virus’s genetic material, antigen tests (rapid tests) that detect viral proteins, and antibody tests that check your blood for signs of a past infection. Each works differently, has different accuracy, and is useful at different points during or after an illness.
Molecular Tests: The Most Sensitive Option
Molecular tests, most commonly called PCR tests, work by detecting tiny amounts of the virus’s genetic material (RNA) in a sample from your nose or throat. The test copies that genetic material over and over in cycles until there’s enough to measure. If the virus is present, even in very small quantities, the test will eventually find it.
The number of copying cycles needed to detect the virus is called the cycle threshold, or Ct value. A low number means there was a lot of virus in the sample, which generally correlates with being more infectious and potentially more ill. A high number means the test had to work harder to find virus, suggesting a lower viral load. Clinicians don’t typically share this number with patients, though. Your result comes back as positive, negative, or indeterminate.
PCR tests are considered the gold standard for accuracy, but they have a practical drawback: they can remain positive for weeks after you’ve recovered and are no longer contagious. That’s because they’re sensitive enough to pick up leftover fragments of viral RNA that pose no risk to others. This makes them excellent for diagnosing an active infection but less useful for deciding when you’re safe to end isolation.
Rapid Antigen Tests: Fast but Less Sensitive
Rapid antigen tests, including the at-home kits widely available over the counter, detect specific proteins on the surface of the virus rather than its genetic material. They work like a pregnancy test: you swab your nose, mix the sample with a liquid solution, and apply it to a test strip. Results appear within 15 minutes as lines on the strip. One line means negative, two lines mean positive.
The trade-off for speed is sensitivity. Compared to PCR, antigen tests catch about 47% of infections overall. That number improves significantly when you compare antigen results to viral culture (which measures whether the virus is actually alive and capable of infecting someone) rather than PCR. Against culture, sensitivity rises to around 80%. In practical terms, antigen tests are better at telling you whether you’re contagious right now than whether you’ve been infected at all.
A faint line on a rapid test still counts as positive. It may mean you have a lower viral load, that you’re near the beginning or end of your infection, or that the swab didn’t collect a great sample. Regardless of line intensity, the result is the same: you’re positive.
When to Test for the Most Accurate Result
Timing matters more than most people realize. Viral loads tend to rise from the day symptoms start and peak around the fourth day. This means antigen tests are least reliable right when you first feel sick. On the first day of symptoms, rapid test sensitivity ranges from about 36% to 71%. By the fourth day, it climbs to 79% to 91%.
If you test negative on a rapid test during your first day or two of symptoms, COVID has not been ruled out. You should test again around day four if symptoms persist. This is especially relevant if you’re considering antiviral treatment, which works best when started early. A negative rapid test on day one can create a false sense of security during the very window when treatment would be most effective.
PCR tests are more forgiving with timing because of their higher sensitivity, but they still perform best within the first week of symptoms.
Nose Swabs: Shallow vs. Deep
Most at-home tests use a shallow anterior nasal swab, where you rotate the swab just inside your nostril. Clinical settings sometimes use a nasopharyngeal swab, which goes much deeper into the back of the nasal cavity and is noticeably less comfortable.
For antigen testing, the two collection methods perform about the same. Studies comparing them in community testing centers found sensitivity differences of only a few percentage points, with shallow swabs sometimes matching or even slightly outperforming deeper ones. For PCR testing, deeper swabs are 12% to 18% more sensitive, but this difference mainly matters when viral loads are very low. At the viral load levels where antigen tests can detect the virus at all, swab depth makes little practical difference.
Antibody Tests: Checking for Past Infection
Antibody tests (also called serology tests) don’t detect the virus itself. Instead, they look for immune proteins your body produces in response to infection or vaccination. There are two main types of antibodies these tests measure: IgM, which appears first and signals a recent infection, and IgG, which develops later and can persist for months or longer.
Neither antibody is reliably detectable in the first week of illness. During the first seven days after symptoms start, IgM antibodies are found in only about 37% of infected people, and IgG in about 31%. Between days 8 and 14, detection improves to roughly 73% for IgM and 65% for IgG. After two weeks, both become highly detectable: IgM at about 96% and IgG at 99%.
Because of this delay, antibody tests are not useful for diagnosing an active infection. They’re designed to tell you whether you were previously infected, which can be helpful for epidemiological tracking or understanding your immune history.
Combination Tests for Multiple Viruses
When respiratory viruses are circulating simultaneously, combination (multiplex) tests can check for several pathogens at once. The CDC developed a multiplex PCR assay that simultaneously detects and distinguishes between influenza A, influenza B, and SARS-CoV-2 from a single nasal or throat sample. These tests use different fluorescent markers for each virus, so the lab can identify exactly which infection you have. Multiplex panels are particularly useful during flu season, when symptoms of COVID, flu, and RSV overlap heavily and treatment decisions depend on knowing which virus is responsible.
SARS Tests Before COVID-19
The term “SARS test” predates COVID-19. During the original SARS outbreak in 2003, caused by a closely related virus called SARS-CoV, diagnosis relied on a combination of PCR testing, antibody detection, and actual virus isolation in lab-grown cells. The fundamental approach was similar to today’s methods, but the scale was entirely different. The 2003 outbreak infected roughly 8,000 people worldwide, so mass testing infrastructure never developed. COVID-19 drove the creation of rapid at-home tests, widespread PCR networks, and multiplex panels that simply didn’t exist for the original SARS.