diagnostician
A medical professional, typically a physician, who specializes in determining the nature of a disease or condition. Often used in clinical or hospital contexts.
He works as a diagnostician at the university hospital.
The diagnostician examined the patient's symptoms in detail.
診断 is the act or process of diagnosis, whereas 診断医 is the person who performs it.
prognostician
Compound of 診断 (diagnosis) and 医 (doctor). The term is transparently derived from its components.