A comparison of the utility of dimensional and categorical representationsof psychosis

Background. The usefulness of any diagnostic scheme is directly related to its ability to provide clinically useful information on need for care. In t his study, the clinical usefulness of dimensional and categorical represent ations of psychotic psychopathology were compared. Method, A total of 706 patients aged 16-65 years with chronic psychosis wer e recruited. Psychopathology was measured with the Comprehensive Psychopath ological Rating Scale (CPRS). Lifetime RDC, DSM-III-RI and ICD-10 diagnoses and ratings of lifetime psychopathology were made using OPCRIT. Other clin ical measures included: (i) need for care; (ii) quality of life; (iii) soci al disability; (iv) satisfaction with services; (v) abnormal movements; (vi ) brief neuropsychological screen; and (vii) over the last 2 years - illnes s course, symptom severity, employment, medication use, self-harm, time in hospital and living independently. Results. Principal component factor analysis of the 65 CPRS items on cross- sectional psychopathology yielded four dimensions of positive, negative, de pressive and manic symptoms. Regression models comparing the relative contr ibutions of dimensional and categorical representations of psychopathology with clinical measures consistently indicated strong and significant effect s of psychopathological dimensions over and above any effect of their categ orical counterparts, whereas the reverse did not hold. The effect of psycho pathological dimensions was mostly cumulative: high ratings on more than on e dimension increased the contribution to the clinical measures in a dose-r esponse fashion. Similar results were obtained with psychopathological dime nsions derived from lifetime psychopathology ratings using the OCCPI. Conclusions. A dimensional approach towards classification of psychotic ill ness offers important clinical advantages.