To Diagnose
する verb
診断 is a diagnosis. Make this into a verb, and you have to diagnose.
しんだんする
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
その医者が私に癌だと診断するほんの少し前に、ある看護婦さんが私の事をちらっと振り返って見たんです。
Right before the doctor diagnosed me with cancer, a nurse glanced over her shoulder at me.