In many "rape" situations there is no question that intercourse has occurred and no question about who the parties involved are.  Often times the "victim" and "perp" agree on identities and that intercourse has occurred.  The presence or absence of criminal action hangs not on physical fact but on consent or lack thereof.  In such situations, rape kits provide no material evidence.  While DA's are compelled to turn over exculpatory evidence (not that they actually do anyway), they have no particular requirements on how they investigate a case.  Nowhere in the constitution are rape kits mentioned.  Unless legislation has been passed to require a rape kit for every sexual transgression, there is no mandate that one be obtained.   While we as a society should ensure that solid investigation into every rape allegation is conducted, each situation is different and rape kits are not always material in any way.  I suggest that conviction hungry DA's already process rape kits when material.