Why could you get an inconclusive result on a whooping cough test?

Time: Whooping cough occurs in three stages- the first causes runny nose and sneezing and people usually don't know they have it- this is the time it is easiest to get a positive result. Once the cough starts- the second phase, it is more difficult to get the bacteria from a nose sample and so that would lead to an inconclusive result. Doctor's can generally diagnose by just hearing the cough