Widespread testing is crucial for tracking the spread of COVID-19. As of April 19, the United States alone had performed almost 997 million tests for COVID-19 since the outbreak of the pandemic, the highest number of any country. Collectively, the top five testing countries—the U.S., United Kingdom, India, Russia and Spain—have conducted more than 3 billion tests.

The diagnostic test of choice for COVID-19 is a polymerase chain reaction (PCR) test. It is a molecular test that analyzes an upper respiratory specimen, looking for genetic material (ribonucleic acid or RNA) of SARS-CoV-2, the virus that causes COVID-19. The PCR test amplifies small amounts of RNA from specimens into deoxyribonucleic acid (DNA), which is replicated until SARS-CoV-2 is detectable if present.