Is this in reference to a blood test?
Can you be a bit more specific as to the context?
OK.
It is a general term.
It simply means that the elevation in the results is enough to make the diagnosis.
It is a often used term in the setting of cardiac enzymes, where a "diagnostic elevation" of the enzymes (troponins, CK-MB) is high enough to diagnose a heart attack (myocardial infarction).
Thus, if the levels are high enough, then it is considered diagnostic, and the patient would be diagnosed with a heart attack.
Doctor (MD)
U.S. Physician/Surgeon in Neurosurgery