This answer will only talk about psychiatric diagnosis or diagnoses of mental illnesses. A diagnosis is simply the identification of the nature of an illness, a disorder, or other problems by the examination of the symptoms presented, narrated, or exhibited.
In order to give a diagnosis, most mental health clinicians use the normative imposed by the DSM (The Diagnostic and Statistical Manual of Mental Disorders) produced by the American Psychiatric Association (APA). The DSM “contains descriptions, symptoms, and other criteria for diagnosing mental disorders. It provides a common language for clinicians to communicate about their patients and establishes consistent and reliable diagnoses that can be used in the research of mental disorders. It also provides a common language for researchers to study the criteria for potential future revisions and to aid in the development of medications and other interventions” (APA).
The last version of the DSM is the 5th edition is used by healthcare professionals in the United States –and many other countries — as the authoritative guide to the diagnosis of mental disorders.
There is also the International Classification of Diseases and Related Health Problems in its 10th revision (ICD-10) with a medical classification by the World Health Organization that includes and expands on mental disorders.
The DSM–5 and the ICD-10 should be thought of as companion publications. In America, most clinicians learn to apply the DSM as a guide to recognize symptoms and meet criteria before assigning a diagnosis.
Michael B. First, MD, Professor of Clinical Psychiatry at Columbia University says that before coming to a conclusion about a client, there are several steps that need to be followed before giving a diagnosis:
1: Rule Out Malingering and Factitious Disorder If the patient is not being honest with the report of symptoms, it is impossible to make an accurate diagnosis based only on symptoms. It’s important to make sure there is not an alter motive for the person to report symptoms just to get a diagnosis. In many cases, this is very hard to obtain and in many cases, the clinician has to accept the narration of symptoms as valid.
2: Rule Out Substance Etiology Virtually any psychiatric presentation can be caused by substance use. Many times, to have a clear assessment of the origins of the symptoms, the clinician needs to interview the client and check with their family members, look for signs of substance, and even have blood or urine tests.
3: Rule Out Disorder Due to a General Medical Condition Clinicians should consider direct medical examination for conditions that commonly account for psychiatric symptoms, such as depression resulting from thyroid dysfunction and many others. If the clinician is not a medical doctor, s[h]e may recommend the person to go have a medical examination to make sure that the origin of the disturbances is not a physiological disease.
4: Determining the Specific Primary Disorder The clinician should pinpoint the specific primary disorder using the DSM as a guide. Many diagnostic groupings in DSM-5 are organized around common presenting symptoms, and each disorder has a description and a list of symptoms that need to be checked.
5: Differentiate Adjustment Disorders From Residual Other or Unspecified Categories
6: Establish Boundary With No Mental Disorder Finally, clinicians should evaluate whether the patient’s symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning. Once all those steps are followed, the clinician is in a better place to conclude if the person meets the criteria for a particular diagnosis.