Wait two days after symptoms to take a COVID-19 test

Confusion often surrounds the timing of COVID-19 tests. Medicine cabinets across the U.S. are stocked with leftover test kits, but questions remain: When should you test if you have symptoms? How accurate are these tests? What steps should you take if you test positive?

COVID-19 testing

COVID-19, caused by the SARS-CoV-2 virus, emerged in late 2019 and rapidly spread worldwide. The virus primarily spreads through respiratory droplets. Symptoms typically appear 2-14 days after exposure and vary in severity.

Common symptoms include fever, cough, and fatigue. Some individuals may experience shortness of breath, muscle aches, sore throat, or loss of taste and smell. Severe cases can lead to pneumonia, acute respiratory distress syndrome, and death, particularly in older adults and those with underlying health conditions.

Preventive measures include wearing masks, practicing social distancing, and frequent hand washing. Vaccination significantly reduces the risk of severe illness. Testing is crucial for identifying and isolating cases to prevent further spread.

Rapid tests can detect active infections, though their accuracy depends on timing. Waiting two days after symptoms appear improves test reliability. Understanding and adhering to these guidelines helps control the spread and impact of COVID-19.

Waiting is key for COVID tests

A recent study from researchers at CU Boulder reveals that the timing of COVID-19 tests significantly impacts their accuracy.

The experts have developed a new mathematical model to guide the use of rapid tests not only for COVID but also for respiratory syncytial virus (RSV), the flu, and other infectious diseases.

“With COVID, our model shows that it’s best to wait two days after symptoms appear before using a rapid test. The virus is often undetectable before this point,” explained study first author Casey Middleton, a doctoral student in the Department of Computer Science and the IQ Bio program.

“For flu and RSV, however, testing should be done as soon as symptoms are felt.”

Evolution of COVID rapid testing

Rapid testing has dramatically evolved post-pandemic. Companies now offer all-in-one tests for COVID-19, influenza A and B, and RSV, available in some doctor’s offices and pharmacies.

At-home COVID testing has become routine, with many self-collecting nasal swabs to protect loved ones.

“If you’re deciding whether to attend a social event or visit elderly relatives, testing is a great idea,” said senior author Daniel Larremore, a professor of computer science at the BioFrontiers Institute. “However, each COVID variant behaves differently, affecting how they interact with tests.”

Study findings on test timing

The new computational model considers various factors, including Omicron variants and patient behavior. The results show that testing immediately when symptoms appear results in a false negative rate of up to 92 percent for COVID.

Waiting two days reduces this to 70 percent. Taking a second COVID-19 test on the third day significantly reduces the false negative rate. This approach catches about one-third of infections that the initial test might miss.

By waiting an additional day, the chances of accurately detecting the virus increase, helping to identify more true cases and prevent the spread of the disease.

“Our immune systems, previously exposed to COVID, react quickly, causing symptoms before the virus is detectable,” Middleton said. “New variants also grow more slowly in those with some immunity.”

For RSV and flu, the virus multiplies rapidly, making tests positive as soon as symptoms begin. “Testing for all three viruses immediately can yield accurate results for flu and RSV, but may be too early for COVID,” Larremore added.

Implications for isolation policies

The study suggests that a “test to exit” strategy – where individuals test again before ending isolation – could prevent more infections with less inconvenience than the standard five-day isolation policy.

“The five-day isolation policy often made people isolate longer than necessary,” noted Middleton. “Test-to-exit effectively releases non-infectious individuals while retaining those with high viral loads.”

Larremore’s previous research was instrumental in informing how COVID-19 vaccines were distributed early in the pandemic and for helping to convince policymakers to prioritize rapid testing.

The experts hope that their new model can help companies develop better tests, clinicians provide better advice, and policymakers offer swift, data-driven guidance in future pandemics.

The study is published in the journal Science Advances.

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