The added value of including cognitive impairment in the diagnostic criteria for schizophrenia
Authors: SILVANA GALDERISI
Journal: World Psychiatry (2008)

Keefe's article provides convincing arguments in favor of the inclusion of cognitive impairment in the diagnostic criteria for schizophrenia. I would like to briefly discuss those arguments in the light of the criteria for assessing the clinical usefulness of diagnostic tests proposed by Bossuyt et al 1 and Boutros et al 2: a) demonstration by independent, well-conducted studies that a biological variable is deviant in a patient population as compared with healthy controls and shows good test-retest reliability; b) demonstration of the potential clinical usefulness of the test (the target patient population differs from groups of patients with disorders that commonly require differential diagnosis); c) evaluation of the sensitivity, specificity, positive and negative predictive value of the test (at this step the added diagnostic value of the test should be evaluated against the “gold standard”); d) standardization of the test in large multicenter, cross-cultural clinical trials and development of large normative databases that can eventually be used to examine single subject's data. No doubt that cognitive impairment in schizophrenia meets the first requirement: many independent studies reported deviant neuropsychological performance in groups of patients with schizophrenia as compared with healthy controls. The second requirement is only partially met, as distinct patterns of cognitive impairment have been identified in schizophrenia and mood disorders without psychotic features but, when the latter aspect comes into play, the discrimination becomes subtle and the quantification of the distance from normative data might be crucial. The third requirement is also partially met: cognitive impairment has a negative predictive value, that is missing in the present diagnostic criteria. The fourth step is “in fieri”: the Measurement And Treatment Research to Improve Cognition in Schizophrenia (MATRICS) represents an important advance toward test standardization and creation of normative databases 3. The inclusion of cognitive impairment in the diagnostic criteria could bring the added value of encouraging a more comprehensive approach to the management of the disorder. In which way should the diagnostic criteria be modified to include cognitive impairment? This issue is actually more difficult to address. Keefe's proposal conveys most of the present knowledge on the issue when recommending this wording: “a level of cognitive functioning suggesting a consistent severe impairment and/or significant decline from premorbid levels considering the patient's educational, familial, and socioeconomic background”. It may be useful to better clarify the meaning of the terms “consistent” (involving several cognitive domains or consistently found in different test sessions?) and “severe” (one standard deviation below the norm?). It would be also appropriate to specify that the decline from premorbid levels is not limited to the acute psychotic phase. For the assessment of all the above aspects, formal testing is more reliable than interview based instruments. The reluctance of clinicians often involves both formal testing and interview based instruments. Once an educational effort needs to be done, it is worth to point to the right direction. In summary, Keefe's arguments supporting the inclusion of cognitive impairment in the diagnostic criteria for schizophrenia are convincing and firmly rooted in research findings. The proposed implementation of the criterion is probably influenced by the awareness that clinicians are neither skilled nor inclined to spend their time in formal testing. However, also considering the efforts of many researchers to provide the field with validated and standardized assessment instruments, a more ambitious goal appears worth to be pursued.