A diagnostic indicator is a sign or symptom that provides positive evidence for a potential diagnosis. To say that Y is a diagnostic indicator for X is to say that Y is correlated with X in a meaningful way, but does not mean that Y implies X.
In order to clear up any ambiguity, many clinicians prefer the term diagnostic evidence; however, "provides diagnostic evidence" is a less snappy and satisfying term than "is a diagnostic indicator", so the latter is not going to leave the vernacular anytime soon.
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