If a rapid strep test is negative, no back up culture is needed. It is now known that the rapid strep test for streptococcal pharyngitis has a sensitivity of 86%. This means that it will pick up 86% of active cases. Because these tests miss only about 1 in 6 cases of strep pharyngitis, the yield of backup cultures is small. It would cost $8 million to catch one case of rheumatic heart disease. And, to boot, adults are at low risk of getting strep throat anyway.