Does anyone have a vet that can (or uses a lab that can) do canine faecal occult blood testing WITHOUT the dog having to have been on a vegetarian diet?

In the USA it appears common place for dogs just to be tested whilst on kibble - they acknowledge that feeding raw can skew the results though. (This is based on a google search of several vet practices.)  However, when asked, my vet was insistent that dog had to be on egg/potato/rice only prior to testing and that kibble was not good enough.

Oldie is fed raw/home cooked mix and it has taken years to identify and eradicate the things that cause him issues so am loath to mess around with his diet too much if unnecessary.

The main no-nos for him are dairy, eggs and grain.

Any thoughts anyone please?

Tx.