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.