diagnostic test

NOUN
15 LETTERS · STARTS WITH D

diagnostic test is a valid 15-letter word worth 18 points in Scrabble and 21 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
D2I1A1G2N1O1S1T1I1C3T1E1S1T1
2 + 1 + 1 + 2 + 1 + 1 + 1 + 1 + 1 + 3 + 0 + 1 + 1 + 1 + 1 =18 PTS
DOUBLE WORD → 36TRIPLE WORD → 54
MEANING

What does diagnostic test mean?