Comparing diagnostic stability in two first episode follow up studies with in a single geographical area between 1992-2007
Williams, Katherine C.
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Background: Over the last 21 years, there have been two cohort studies of first episode psychoses with baseline diagnosis and a follow up diagnosis conducted in the same catchment area, the city of the Nottingham in the UK, between 1992 and 2007. This study considers how the diagnostic stability of psychotic disorders has changed over time. Methods: The SIN (Schizophrenia in Nottingham) study commenced in 1992-1994 and followed 167 subjects after three years, until 1997. The AESOP (Aetiology of Schizophrenia and Other Psychoses) study subjects (N=203) were recruited in Nottingham between 1997 and 1999, and followed at eight years, to 2007. Both the SIN and AESOP studies used the same diagnostic methodology (SCAN assessments, ICD-10 diagnoses and diagnostic consensus meetings of senior clinicians), excluded organic psychoses and included subjects aged 16-65. Positive Predictive Value (PPV) was calculated and used as the measure of diagnostic stability in each study. For the purpose of this comparison 7 diagnostic groupings were used schizophrenia schizoaffective disorder, depressive illness, mania/bipolar disorder, acute and transient psychotic disorder and drug induced psychosis. Results: In the SIN study, after three year follow up, greatest diagnostic stability is seen in bipolar disorder/mania followed by schizophrenia, drug induced psychosis and then depressive disorder. The lowest diagnostic stability was seen in schizoaffective disorder, followed by acute and transient psychotic disorder and then delusional disorder. In the AESOP study after 8 year follow up, greatest diagnostic stability is seen in drug-induced psychosis followed by bipolar disorder/mania, schizophrenia and depressive psychosis. The lowest diagnostic stability was seen in acute and transient psychotic disorder, followed by delusional disorder and then schizoaffective disorder. The diagnostic stability (PPV) of drug-induced psychosis increased in the AESOP study relative to the SIN study (80% cf 69%). Whereas, the stability of a diagnosis of an acute and transient disorder decreased in AESOP compared to SIN (13% cf 37%). A paired t-test showed no significant difference in diagnostic stability between the two studies (p=0.84). A Mann Whitney U test showed no significant difference between the rankings of diagnostic stability in the two studies (p=0.95). Discussion: This comparison has demonstrated that the diagnostic stability is consistent in two studies in one geographical area for the disorders shown to have high stability such as schizophrenia and bipolar affective disorder. We must conclude that drug induced psychoses have become a stable diagnosis over time, when seen in the context of a first episode of psychosis. Those people with a delusional disorder, schizoaffective disorder or acute and transient psychotic disorder at first presentation are more likely to change diagnostic categories over time than remain the same.