One chart shows how many coronavirus tests per capita have been completed in 6 countries. The US has finally caught up.

• Governments worldwide are still ramping up coronavirus testing.
• Currently, Italy has performed the greatest number of coronavirus tests per million people: nearly 21,000.
• The US and South Korea are not far behind, having each done more than 10,000 tests per million residents.
• A comparison of testing per capita in six countries shows the US is no longer woefully behind other nations. But even more testing is still needed.
• Visit Business Insider's homepage for more stories.

Nearly 4 million Americans have now been tested for the coronavirus.

The US's testing ramp up has led to one of the highest rates of testing per capita in the world: more than 11,800 tests per 1 million residents. Italy tops the list, however, with a rate nearly double that in the US.

The UK and Turkey, meanwhile, have performed about 7,600 and 7,500 tests per capita, respectively.

Not every country reports testing using the same metric, however, which can make the rates difficult to compare. The UK reports it as "people tested," the US reports "total test results," and Japan reports it as "persons." South Korea, meanwhile, reports their testing total as "cases," Turkey as number of "tests," and Italy as number of "swabs."

In total, all these tests have confirmed 2.5 million coronavirus infections worldwide. At least 168,500 people have died from COVID-19.

Testing per capita now looks different than it did in March

Six weeks ago, the US was woefully behind South Korea's tests-per-capita rate. On March 8, South Korea's total number of tests done per million citizens was roughly 700 times the US's, despite the fact that the two countries announced their first coronavirus cases on the same day.

In the US, faulty kits and delays in the widespread rollout of tests have hampered public-health authorities' ability to accurately determine how many Americans have gotten the virus.

In early March, the US had done fewer COVID-19 tests per capita than many other countries with large coronavirus outbreaks — just five tests per million Americans, according to the Centers for Disease Control and Prevention.

Testing influences what we know about the coronavirus' death rate

The charts above compare the number of diagnostic tests, which determine whether someone has an active COVID-19 infection, across countries. Such tests involve taking samples of mucus and saliva and running a test in a lab to see whether those samples contain the coronavirus' genomic sequence.

Antibody tests, by contrast, can help determine whether a person already had COVID-19 by examining their blood for coronavirus antibodies.

Read more: Tests that can tell if you're immune to the coronavirus are on the way. Here are the companies racing to bring them to the US healthcare system.

Without adequate numbers of both test types, it's challenging for public-health officials to grasp the scope of the outbreak's spread or determine how deadly it is.

More widespread testing could change the known COVID-19 death rate, a basic calculation that divides the number of reported deaths by the number of confirmed cases. Experts expect that as reported case counts increase, countries' death rates will drop.

But even with the notable increase in US testing over the past six weeks, many infected Americans are likely being missed in the official case counts.

That's because many people who likely have mild cases COVID-19 aren't being tested, and 25% and 50% of people infected with the virus show no symptoms, so probably aren't included in the total, either.

To fully "remobilize the economy" in the US, a new report from Harvard University experts suggests, the US needs to be testing 20 million people per day by mid-summer. Currently, the US is testing fewer than 200,000 people per day.

Andy Kiersz contributed reporting to this story.

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