"To confirm the diagnosis" means to obtain further evidence or perform additional tests in order to establish or verify the accuracy of a medical or scientific identification of a condition or illness. Full definition
A blood test helps confirm diagnosis, and though the condition is permanent, it can be treated with a synthetic hormone replacement given twice daily. (dogcare.dailypuppy.com)
If the test confirms the diagnosis, breeding relatives need to have the test, too, so their status is known. (ashgi.org)
Your veterinarian will confirm the diagnosis by microscopic examination of a skin scraping. (ashgi.org)