A diagnostic test apparatus comprising: a shaft having a first end and a second end; a swab or a biopsy punch mounted on the first end of a shaft; and a cap for fitting over the first end of the shaft, said cap containing at least one diagnostic test reagent; wherein the shaft comprises at least one cap engagement element located proximate to the first end, said element extending radially outwardly of the swab or the biopsy punch for engagement with the cap to retain the cap on the shaft. Also provided are diagnostic caps for use in the test apparatus having a small internal volume and at least one diagnostic test reagent located in or on an absorbent plug inside the cap. Also provided are diagnostic caps for use in the test apparatus having a housing extending from a sample receiving port, said housing defining a lateral flow path for the sample. Also provided is a test system comprising a swab shaft, a biopsy punch shaft and a diagnostic cap in accordance with the invention, wherein the cap can be used interchangeably between the two different shafts.