Low diagnostic reliability, the need to meet criteria of two disorders, and its status as residual diagnosis in clinical practice led us to hypothesize that schizoaffective disorder (SAD) is characterized by considerable heterogeneity, particularly in comparison with schizophrenia (SZ) and bipolar disorder (BD). As this has not been investigated the aim of this study is to test whether heterogeneity is larger in SAD than in SZ and BD.
Systematic search for studies simultaneously comparing all three diagnoses regarding demographic, clinical, psychometric (clinical rating scales and IQ tests), and biological parameters; comparison of heterogeneity as measured by standard deviation (SD).
Standard deviation of SAD samples (N = 47) was smaller than in both differential diagnoses. SDs were 7% higher in BD than in SAD (SZ: 2% higher); in studies employing DSM-IIIR/–IV pooled SD was 4% higher in BD (8% lower in SZ). Differences between diagnoses were limited to the comparison of SAD and BD, and became smaller when only psychotic BD was considered.
Heterogeneity of SZ and BD is not smaller than that of SAD. SAD seems not to be more diverse than other functional psychoses. Results are preliminary because of the novelty of the approach and to the small number of studies.