"To reach a diagnosis" means to come to a conclusion or make a determination about someone's illness or condition after examining and analyzing their symptoms and medical tests. Full definition
A vet may choose to do a physical examination and / or take X-rays, but these practices may not be totally effective in reaching a diagnosis. (articles.extension.org)
The practitioner will examine the tongue, smell the skin, check for mouth and ear odors and other physical and psychological clues to help reach a diagnosis. (floridawildvethospital.com)
This equipment not only allows us to reach a diagnosis faster, but at a very economical cost. (veterinarians.com)