The short, short version

DO

• use a rapid test to tell if the symptoms you have are covid
• use a rapid test to tell if you are or are not contagious RIGHT NOW (but that can change in as little as 6 hours)
• Use a PCR test to tell you whether or not you had covid 5 days ago

DO NOT

• use a rapid test to show you do not have COVID.  The negatives are not accurate, particularly when you have no symptoms.
• use a PCR test (or any other test) to test if you got it from exposure yesterday.  A PCR test will not be accurate until about 5 days after exposure.
• do not assume you are not contagious if you’re not feeling symptoms.  You’re contagious for probably 36 hours before you feel it.

The Short Version

A rapid test CAN tell you if you’re contagious at that moment.  It’s pretty reliable, but not so reliable that you should go breath on someone at risk.  But you can probably go hang out at that party if everything there tested negative on a RAT test right before coming.

A rapid test CAN tell you if the symptoms you have right now are COVID.  At any time, whether you have symptoms or not, if it tells you that you have COVID, you almost certainly have COVID.  

A rapid test CANNOT tell you you do NOT have COVID.  COVID could be incubating in your body and just not be enough for a RAT test to measure.

If you’ve been exposed but don’t have symptoms, you need a PCR or molecular test 5 days after exposure to know if you have COVID.  A PCR takes a while after exposure to have valid results, but the results are definitive, positive or negative.

Omicron usually takes about 2-4 days between exposure and symptoms.  (Delta and prior about 5 days, usually).

You are most contagious from about 24-36 hours before you have symptoms to about 48 hours after.

Slightly longer version

Rapid tests really measure the amount of virus in your nostrils.  This correlates pretty well to being contagious.  If you have symptoms, then you’re probably contagious (some vaccinated people can have symptoms and not be contagious).

They are only accurate if you have symptoms, and even then a negative should be retested several days later (their false negative rate is very high, false positive fairly low). If you do NOT have symptoms, a negative rapid test is NOT useful to determine if you have COVID. Notably, you CANNOT use a rapid test to determine if an exposure gave you COVID before you feel it coming on. 

As of 6/2022, expert belief is that a rapid test can determine if you are contagious at that moment (and that can change in 6 hours).  So, it’s good practice to test before going to e.g. a large gathering.

PCR (molecular) tests can show you if you have COVID before you have symptoms, but you do need 5 few days after exposure for the test to be valid. If you’re exposed in the morning and test that evening, the results are useless as the virus hasn’t had time to replicate enough to register.

By the time you get the results back for a valid PCR test, you’ve probably already been contagious for at least 24 hours.  Mask up and avoid high risk people at least (best to avoid all people) until you get the negative results back.

So, if you’ve had an exposure, CDC exposure guidelines are to get tested 5 to 7 days post exposure. (They day of exposure counts as day 0, not day 1.) However, unlike rapid tests, the results are highly accurate both negative and positive, assuming you’ve waited long enough.

A note on contagion: You are probably contagious about 36 hours before you show symptoms.  You are most contagious from this time to about 48 hours *after* you show symptoms.  IF you are fully vaccinated (including booster!), you are probably not very contagious 5 days after symptom onset, but may be *slightly* shedding virus (and therefore slightly contagious) up to 10 days after symptoms.  You can probably consider yourself “recovered” and no longer contagious after a rapid test comes back negative.

What’s The Difference Between COVID-19 Rapid and PCR Tests?

Updated: 6/15/2022