Anything diagnostic has to do with disease, especially figuring out which one a patient has.

If your doctor wants to run diagnostic tests, then she isn't exactly sure what's wrong with you — so the tests will help diagnose you (figure out what's wrong). The TV show House is all about diagnostic tests: it features illnesses that are unusual and hard to determine. But anything diagnostic is trying to figure out what's wrong. A writing test could be called diagnostic if it's trying to determine how good or bad your writing skills are.

Definitions of diagnostic
  1. adjective
    concerned with diagnosis; used for furthering diagnosis
    “a diagnostic reading test”
  2. adjective
    characteristic or indicative of a disease
    “a diagnostic sign of yellow fever”
    synonyms: symptomatic
    typical or distinctive
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