diagnostic test vs diagnostic assay

diagnostic test and diagnostic assay both are nouns.

Word	Noun	Adjective	Verb	Adverb
diagnostic test	Yes	No	No	No
diagnostic assay	Yes	No	No	No

As nouns, diagnostic test and diagnostic assay are synonyms defined as:

diagnostic test and diagnostic assay: an assay conducted for diagnostic purposes

diagnostic test (noun)	diagnostic assay (noun)
an assay conducted for diagnostic purposes	an assay conducted for diagnostic purposes