Coming back to diagnosis
I have written here many times about what I see as the shortcomings of the diagnostic system we use in mental health.
It would be nice to imagine that there were some scientific way to determine diagnosis, but there is none. Absent biological or chemical tests to establish diagnoses, we fall back on consensus reality and struggle with the unevenness of such a standard. What we have in the DSM IV is an attempt to develop, by consensus, descriptions of all disorders thought to be reflective of mental illness of one kind or another. Categories have been expanded and elaborated in the years since the first edition was published; yet, all but the rarest of categories still depends on the subjective judgment of the examiner. Local custom, training of the examiner, examiner biases, insurance coverage, perceived stigma carried by various diagnoses, and funding sources can all influence the diagnosis made as much as the behavior and history of the patient.
That such factors as funding sources and examiner bias influence diagnosis goes against the image of the medical model as scientific. However, subjective and external factors often matter more than the symptoms displayed. In private practice, the fact that medical and insurance records cannot be guaranteed to be private, the tendency is to choose the least stigmatizing diagnosis possible. Occasionally a professional might apply a more serious diagnosis to someone they find irritating, in an unconscious attempt at retaliation. Or a facility has beds for patients with one kind of diagnosis but not another, so the effort is made to fit the patient where the space is. Or health insurance severely limits coverage for treatment for minor disorders but is more generous for ones that are more serious, resulting in the push to gain coverage, not strive for accuracy in diagnosis. All of these disturbances, in what we might like to believe are an orderly and scientifically based process, reflect variations in the consensus reality and its deviance from the ideal.
You really should read this post on CP&P and be sure to read the comments also as the guest poster, Tim Desmond gets at the issues really well.
I'm with Jung on this :
It is generally assumed in medical circles that the examination of a patient should lead to the diagnosis of his illness, so far as this is possible at all, and that with the establishment of the diagnosis an important decision has been arrived at as regards prognosis and therapy. Psychotherapy forms a startling exception to this rule: the diagnosis is a highly irrelevant affair since, apart from affixing a more or less lucky label to a neurotic condition, nothing is gained by it, least of all as regards prognosis and therapy. (Jung, CW 16, p.86)

