What are the requirements for US citizens to return to the US regarding COVID testing?

US citizens returning to the US need to provide a COVID-19 test result. An ATK test is acceptable, but it must be performed at a medical center, and a written certificate is required. Tests are available at the airport, with results in approximately one hour for a fee of 550 baht. While ATK tests are generally accepted, some airlines may still require a PCR test, so it's advisable to check specific airline policies and requirements for departure and transit airports.