Testing for PKD only has to be done for those cats or kittens to be used for breeding that don't already come from PKD  clear lines:-

The Selkirk Rex Registration Policy states

Selkirk Rex and Selkirk Rex Variants may be registered on the Active Register, at the breeder's request, only if

1.  They have themselves been screened clear of PKD under a recognised scheme, (FAB/PKD scheme in the UK). Official confirmation of the screening result is required

2. They are from parents deemed clear because of the results from the screening of cats (as in A.1) on every line of the pedigree in previous generations. Pedigree line in this case refers to both parents, or all 4 grandparents, 8 great-grandparents, 16 great-great-grandparents and so on, or any combination of the same inclusive of both the sire's and dam's ancestors

The Selkirk Rex Cat Club, with reference to the Joint Rex BAC,  has devised a form, similar to that used by other BAC's requiring independent testing of uniquely recognised cats, to enable dna testing with recognised laboratories across the world.

 In the UK, the only recognised laboratory is the Feline Advisory Bureau at Langford. Now,  however, by using this form  other recognised laboratories in the world,  that have schemes for the dna testing for PKD - (UCL Davies, Animalsdna  etc),  can  be used, providing the sample is taken  by a veterinary practice on a uniquely identified cat (identified by microchip).  The cat needs to be identified by both registered name and microchip number on the form.

 Please note when using this form the vet should both take and send off the samples for testing.