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Which test? (brief overview)

  • If you have symptoms, it’s best to get a PCR test to diagnose or rule-out COVID-19, including if you are vaccinated and/or if you have a negative rapid antigen test. A PCR test will pick up low levels of virus. Rapid antigen tests can also be done to pick up high levels of virus. A positive rapid antigen test accurately diagnoses COVID-19 infection but a negative rapid antigen result does not rule it out, so it’s important to wear masks and take precautions while waiting for the PCR test result.
  • If you are screening for infectiousness, a rapid antigen test can quickly identify infectiousness with high viral loads, regardless of vaccinations status, including in people who haven’t developed symptoms yet or who don’t develop symptoms. Rapid antigen tests are useful for screening for infectiousness 3-5 days after an exposure and for screening every 3-7 days.

Where to get free COVID-19 tests in the East Bay

COVID testing is supposed to be available without cost to you. You don’t need to have insurance or immigration papers. If you’re worried about getting billed or don’t have insurance or papers, we recommend getting tested at one of the county sites below. PCR tests using nose swab or using saliva (no swabs!) and rapid antigen tests are available.

COVID-19 testing at the Unidos en Salud site in the Mission, SF. (Creative Commons, Konstantin ‘KVentz’ Ventslavovich, 2020)
Community pop-up testing and vaccination at Serenity House in Oakland, July 2021.

Rapid antigen home tests

  • The rapid antigen home COVID-19 tests provide rapid results in 10-30 minutes for when people are at their most infectious stage. They are effective at picking up high viral loads and are less sensitive than the PCR tests but provide timely results and are cheaper and easier to use.
  • If you have an exposure, we recommend that you use the rapid antigen test to check for infectiousness 3-5 days after the exposure. The delta variant’s median incubation period is about 4 days before a person develops symptoms, and viral loads are likely to rise to infectious levels 1-2 days before symptoms begin.
  • Antigen tests are more accurate with repeat testing every 3-7 days after an exposure.
  • The BinaxNow, Ellume, Quidel QuickVue and Inteliswab COVID-19 antigen home tests are the most widely available; more are listed on the FDA website. These tests are done with a nasal swab, with results showing in 10-30 minutes on a card or little area of the device, like a home pregnancy test.
  • Pricing for some home rapid tests have been discounted starting in September with the new US COVID-19 plan. For example, the BinaxNow and Inteliswab antigen home self-tests can now be purchased at $14 for 2 tests (discounted from the retail price of $23.99) at pharmacies and online retailers, such as at Amazon, Kroger and Wal-Mart.
  • The CDC has guidance on self-testing and what people should do with test results.

Which test should I get? More details:

  • If you have symptoms, get a PCR test to diagnose or rule-out COVID-19, including if you are vaccinated and/or if you have a negative rapid antigen test.
    • Keep in mind that PCR test can take several days to get results, so stay home and isolate while you’re waiting for results.
    • Combining a PCR test and a rapid antigen test can both provide diagnosis as well as screen for current infectiousness.
    • A positive rapid antigen test result can quickly diagnose a COVID-19 infection with high viral loads. However, if a rapid antigen test is negative, you still need a PCR test to check for lower levels of virus and confirm a negative result.
    • Rapid antigen tests have the highest sensitivity for infections in people in their first week of symptoms.
    • If a PCR test is not available, repeat the antigen test every 3-7 days for 14 days to confirm the negative result. 
  • If you are screening for infectiousness and/or 3-5 days after an exposure, a rapid antigen test can quickly identify infectiousness with high viral loads, including in asymptomatic infection and in vaccinated people.
    • People with SARS-CoV-2 infections are typically infectious 1-2 days before symptoms start, and some people do not develop noticeable symptoms (they remain asymptomatic) even when they are infectious.
    • Rapid antigen tests have a 95% overall sensitivity for PCR cycle thresholds (Ct) below 25, which indicate higher viral loads.
    • For delta infections, viral loads are typically highest (PCR Ct <25) in the first 3 to 14 days after exposure.
    • After an exposure, test 3-5 days after the exposure and then repeat the rapid antigen test every 3-7 days for 14 days to confirm the negative result if you don’t get a PCR test. 
    • Screening every 3-7 days increases the sensitivity and chance the rapid antigen test will pick up someone who is infectious, even before symptoms and if they don’t develop symptoms. That way people know to stay home and curb the spread. 
  • Remember that a negative test result doesn’t mean you can stop other preventive measures. Getting vaccinated, wearing a mask, distancing and screening are all used in combination to get the delta variant under control.

The science behind COVID-19 PCR and antigen testing

This graphic shows when PCR and antigen tests work based on the original SARS-CoV-2 strain and is from Guglielmi, Nature 590, 2021, adapted from A. Crozier et al. BMJ 372, 2021, Mina et al. and Cevik et al. Delta data added in purple text above with references below. 

For the delta variant, emerging data indicates that the infectious stage likely begins sooner (2-3 days) and lasts longer (13-18 days). See references below. Rapid antigen testing will be most likely to pick up delta infections starting 3-5 days after exposure and up to about 14 days after exposure in unvaccinated people or about 11 days after exposure in vaccinated people.

Frequent rapid antigen testing every 3-7 days is more likely to catch a delta infection during the infectious period than less frequent PCR testing. Rapid antigen tests are also less likely than PCR test to pick up people who are no longer infectious but are still shedding viral particles. The more frequent the rapid antigen testing (2 times a week or more), the more likely you will detect infectiousness before a person becomes symptomatic or spreads the virus. Frequency of testing is balanced between willingness to test, cost ($10-15 per test in the US), availability and access. 

Rapid antigen screening every 3 or 7 days for delta infections

This table illustrates hypothetically how different frequencies of rapid antigen testing (every 3 or 7 days) might detect delta infections during the most highly infectious period (2-18 days after exposure). This table shows how we are more likely to catch infections earlier during the most infectious period with testing every 3-4 days (twice a week) rather than every 7 days (once a week). The green plus symbols (+) show days when infection is caught and the red “O” or exclamation marks (O or !) show days when infection might be missed.

  • The dashes (-) mark when the rapid antigen test is performed, is negative, and may be too soon to be in a period of infectiousness.
  • The question marks (?) mark when the rapid antigen test is performed, is negative, and we do not have enough data to know if it’s likely to be a day of high infectiousness. 
  • The red exclamation points (!) mark when the rapid antigen test is not performed because of the testing schedule and thus miss the infection during a likely infectious day. 
  • The red O’s (O) mark when the rapid antigen test is performed and may miss the infection (pre-symptomatic PCR Ct and viral loads not yet known).  
  • The green pluses (+) mark when the rapid antigen test is performed and more likely pick up the infection (viral loads expected to be high and PCR Ct expected to be <25).