Diagnosis was the process of identifying an illness, malfunction, or other problem by examining the symptoms of a person or object. Practiced in medicine, it was typical for a medical doctor to perform a diagnosis on a patient, but one could perform a self-diagnosis. Alternatively, it was typical for an engineer to perform a diagnostic on computers or other technologies, but an artificial lifeform could perform its own self-diagnostic.
In 2375, after Seven of Nine ran a complete self-diagnostic and concluded she had Albright-Salzman syndrome, The Doctor stated that she instead had a mild case of sensory aphasia and warned her about the danger of self-diagnosis; patients always assumed the worst. (VOY: "Relativity")
In 2377, the Quarren Doctor Kadan made false diagnoses and manipulated the memory engrams of kidnapped crewmembers from the USS Voyager so that they could be used for the Quarren workforce. (VOY: "Workforce", "Workforce, Part II")