diagnostic assay vs diagnostic test

diagnostic assay and diagnostic test both are nouns.

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

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

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

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