We've addressed early questions about testing for Covid-19, but plenty remain. CNN Chief Medical Correspondent Dr. Sanjay Gupta explains the latest on the different types of coronavirus tests and how well they work.

You can listen on your favorite podcast app or read the transcript below.

Dr. Sanjay Gupta: When you were a kid, did you ever make one of those pinhole cameras? Where you cut a little hole into a piece of cardboard and then looked through it? Well, in some ways, that's kind of how we're looking at the coronavirus noawadays: through a tiny little window.

Part of the reason we haven't been able to get a bigger picture is because this is a new coronavirus, and we're learning as we go along.

We've also had inadequate testing across the nation, so the inability to know the true extent of this outbreak becomes a major barrier in terms of getting the country back to work. We need clearer vision, and so far, we haven't had that.

I'm Dr. Sanjay Gupta, CNN's chief medical correspondent. And this is "Coronavirus: Fact vs. Fiction."

Gupta: There are currently only two types of coronavirus tests available in the United States. Because I'm a health care worker who still takes care of patients in the hospital, I've had both of them.

Doctor: I'm going to give you a little poke over here. OK?

Gupta: OK.

Doctor: Poke. You doing OK?

Gupta: Yep.

Doctor: And just like that, we're all done.

Gupta: That's it. OK.

Doctor: Let's get you wrapped up.

Gupta: Early on, there were some significant delays in testing, and there was also the release of a flawed test, which really put us far behind.

Since then, there have been a lot of unauthorized, unvalidated tests, which have flooded the market.

The most common and most accurate test we have is called a PCR test -- a polymerase chain reaction test. Now that's the one that detects whether or not someone is currently infected with Covid-19. It involves a saliva test in some cases, or more commonly, a nasal swab. CNN's [Anchor] Brooke Baldwin referred to it as a "brain tickler."

Brooke Baldwin, CNN News Anchor: Oooooh!

Doctor: One more. ... All done.

Baldwin: [giggle]

Gupta: So that gets sent off to a lab, where the genetic material is extracted. And because there's such a small amount of that genetic material, it is then amplified. That's the polymerase chain reaction. If all goes well, results usually come back within a few hours, but it can take a few days if you have to send it to a lab somewhere.

Then there are the antibody tests. Those are the ones that can determine whether or not someone has had Covid-19 in the past and might have some immunity to it now. Those involve collecting a small blood sample, either through a needle in the vein or through a blood spot sample.

But here's the problem: Antibody tests have not been consistently accurate. There are a lot of bad tests out there, and it's still unclear how much those antibodies might protect you from the virus in the future.

When you're testing for the virus, the biggest problem would be having a false negative. Why? Because you would think that you don't have the virus, and then you might go back out into the community, into a nursing home, into a hospital -- and potentially infect people.

With the antibody test, what you're really hoping to avoid is a false positive. Then, someone might feel that they have the antibodies, thus feel that they are protected, go out into the community, to a hospital, to a nursing home and spread the virus.

So with the diagnostic virus test, you really have to reduce false negatives; with the antibody test, you really have to reduce false positives.

Kevin de León, senior fellow at University of Southern California's Schwarzenegger Institute for State and Global Policy: The promise of the immunology test to find out if we have the antibodies is huge.

Gupta: That's Kevin de León, senior fellow at the University of Southern California's Schwarzenegger Institute for State and Global Policy. The institute supports test sites across Los Angeles.

De León: This can influence policy makers at the local, at the state and federal level. That can actually inform us when it comes to social distancing. If I'm immune, and scientifically I've been proven to be immune, then I can reenter the workforce and I can play a bigger role in making sure we're safer.

Gupta: A company called Roche announced that it received emergency-use authorization -- EUA -- for an antibody test it claims is more accurate than most.

Roche says it has already started shipping its new test to leading labs around the world.

Here's Roche CEO Severin Schwan.

Severin Schwan, Roche CEO: It's really special because it's so accurate, it's almost near-perfect accuracy. And what that allows us is to really reliably test whether a person has been infected by the coronavirus or not, irrespective of whether you had symptoms or not.

Gupta: Now there's another kind of test that could be useful here. It's called an antigen test. Again, the test for the virus is the PCR or diagnostic test. The test for the antibody is called a serology test.

And now the antigen tests look for a protein on the surface of the virus. You may have already had one of these if you've ever had a test for strep throat or the flu.

Here's the problem: A reliable antigen test for the coronavirus isn't yet available in the United States, but the hope is that we'll soon have something that works kind of like an at-home pregnancy test, where a test strip would change color to tell if you might have the virus.

Frederick Nolte is a pathology professor and the head of coronavirus testing at the Medical University of South Carolina.

Frederick Nolte, Medical University of South Carolina professor and director of clinical laboratories: Antigen detection has been part of the diagnostic landscape for a number of years, and it has a number of appeals. It can be done relatively quickly, it's inexpensive, it can be deployed in a number of clinical settings outside of the laboratory, near to patients. But the chief concern with it has been the sensitivity.

Gupta: And a low sensitivity means a high false negative. And with high false negatives, people feel that they don't have the virus, and they go back out into the community and potentially continue the spread.

So "How available are PCR and antibody tests to the general public?" is probably the question I get more than any other.

Well, as of Monday, Johns Hopkins University's Covid Tracking Project was reporting over 7 million people in the United States have been tested. And they mean the diagnostic, or PCR test, in this case.

But again, the initial rollout of those tests was fraught with problems. And that caused major delays in the country's early response to the pandemic. Those problems are being addressed now, but there are still supply chain shortages.

The PCR test requires certain transport mediums, reagents and, yes, nasal swabs. And those things have been in short supply.

But just last week, the mayor of Los Angeles announced free diagnostic testing for all of the county's residents.

Eric Garcetti, mayor of Los Angeles: Because we know the 10 million residents in L.A. county need that. It's critical for us opening up in the future.

Gupta: That's Mayor Eric Garcetti on CNN.

Garcetti: And we wanted to be the first big city in America to take the advice of doctors around the country saying, "You have to find the silent spreaders. This is a silent killer," that people without symptoms who can spread this are a critical piece of knowledge in order to open up in the future.

Gupta: And in New York City, Mayor Bill de Blasio says the city will produce its own Covid-19 test kits, in partnership with a 3D printing company.

Bill de Blasio, mayor of New York City: We realized we had to find another source. The global market wasn't working. There weren't sources around this country that were reliable enough, so we decided we would make our own. And this has had to be put together very quickly. So we're really in uncharted territory creating these test kits in New York City.

Gupta: Again, it's these nasal swabs that have been in such short supply in so many places around the country.

Now there are also plenty of antibody tests floating around that haven't been reviewed or validated by the FDA. The agency said Monday that it was tightening its policy to keep unproven and even fraudulent tests from entering the market. It's been a big problem.

In one study of 12 antibody tests, four were shown to deliver false positive results more than 10% of the time. Remember, if you're testing for antibodies and you get a false positive, people may incorrectly assume they now have the antibodies and are protected and then go out into public and keep spreading. You really want to get that false positive rate under 2% -- as low as possible, really.

The future could lie in at-home testing. Antigen tests would be the easiest to mass-produce for home use. But again, like I said, we don't yet have a reliable antigen test for Covid-19.

White house coronavirus task force member Dr. Deborah Birx said this last month, on NBC's "Meet the Press."

Dr. Deborah Birx, coronavirus response coordinator, White House Coronavirus Task Force: We have to have a breakthrough innovation in testing. We have to be able to detect antigens, rather than constantly trying to detect the actual live virus or the viral particles itself. And to really move into antigen testing.

Gupta: If an antigen test is approved and mass-produced, it may serve as a valuable screening tool, but it's probably not going to replace the PCR saliva or swab tests when it comes to diagnosing Covid-19. The antigen test in this case would just be used to screen. The PCR test would still be the most accurate.

According to The Guardian, scientists working for the U.S. military have designed a PCR test that has the potential to detect the virus as early as just 24 hours after it's contracted. That could help stop infected people from spreading the virus before they even show symptoms. And keep in mind, a lot of people never show symptoms but can still spread the virus. It's another promising "maybe."

And remember this: Testing does need to go hand in hand with contact tracing. Once you find out who is infected, that person needs to be isolated. And then everyone who has had close contact with that person needs to be traced. And sometimes those people need to be quaratined as well. Test, trace and hopefully treat.

My colleague Anderson Cooper and I asked Bill Gates about this at CNN's town hall last week.

Bill Gates, philanthropist: If you want to try and open up, this contact-tracing thing has got to be taken seriously. South Korea took it seriously. It looks like there's an effort in New York that [former] Mayor Bloomberg is helping out with that will do some of that. That's a necessary part of reopening is to have the quick turnaround tests that actually low-income people can get access to, but the contact tracing to follow up so those people hear very quickly if they've been infected.

Gupta: Things are getting better, but it is clear that we're still not where we need to be. It's a brutal truth, but one that needs to be told.

There aren't enough PCR or diagnostic tests to meet demand, and the antibody tests haven't been as accurate as they need to be. And this is exactly why I keep advising people to behave like you have the virus. A lack of tests means a lack of knowledge.

The availability of quicker, easier and more accurate testing is crucial to get the country back on its feet.

We'll be back tomorrow. Thanks for listening.