Patellas are generally checked by vets -- it isn't a very hard thing to look for. But what people say on a website, and what was actually done -- or if done at all -- or if the vet even seriously looks for anything (a lot of vets for large scale pet breeders make lots of money off them and are happy to say little or just rubberstamp 'passes' for health results)... But patellas are one of the few health tests that a vet rather than specialist does. OFA simply takes the vet's exam data to add the info to their database -- so technically there's no difference between a vet's results and the fact that they are on OFA except the vet/owner filled out the form and sent to OFA. Lots of good breeders BTW don't use OFA -- and some crappy ones do and will point to their results being listed there as 'proof' that they are a responsible breeder. Some feel OFA isn't meticulous and health focused enough (Rod may well dive in here to explain why...
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Karlin is quite accurate. OFA has to rely upon what the examining veterinarians report. In the case of patellas, unlike in many instances, the examining vet does not have to be a board certified specialist to submit a report to OFA. (There is another exception discussed below.) Boarded specialists usually don't take their work lightly. They have higher standards to meet, and they don't like to be second-guessed by some other specialist.
Examining patellas should be taken very seriously by the general practice vet who, presumably, is conducting a full body examination. But I can understand the motivation of an unethical vet with a close, profitable relationship with a breeder, to be less than thorough in feeling each puppy's knees.
In the US, more and more health clinics, sponsored by kennel clubs and breed clubs and usually held at dog shows, are offering patella exams. Even though those vets likewise are general practice vets, when you go to a patella clinic, you and the vet both know that an accurate exam is important. Also, all cavalier owners should take their dogs to vets for annual exams, and tell the vets that patella exams are important to them.
(The other non-specialist exception I can think of is, amazingly, heart murmur exams. OFA accepts non-specialist reports of heart auscultation exams. The fact that a non-boarded vet has submitted the report is noted by OFA, but you need to know how to read an OFA line item to realize that a lot of cavalier breeding stock claimed to be OFA-heart-clear were really only examined by a general practice vet. That is unacceptable under the MVD breeding protocol, but that fact hardly bothers a vast majority of cavalier breeders in the US.)