If a rape kit is tampered with, it will not be admissible in court. In fact, when any evidence is tampered with, it is not admissible in court.
There are many ways that a rape kit could be tampered with. For example, the chain of custody of that evidence may not be known. If for example, the individual that collected the evidence, did not immediately bring it the lab, the whereabouts of that kit may be unknown for some time.
In addition ,a rape kit may have had evidence removed form the kit, including DNA or other material. When any evidence is removed, it is likely that the entire kit would be inadmissible in court.
In order for evidence to be admissible in court, it must have a valid chain of custody and it must not have been tampered with in any way.