a test called a throat culture is the main way to know if a person has strep throat. this test is correct 90 to 95 percent of the time. there is another test called a rapid strep test, or radt. the rapid strep test is faster than a throat culture but correctly finds the illness only 70 percent of the time. both tests can show when a person does not have strep throat. they can show this correctly 98 percent of the time.