Validity of a 'proxy' for the deficit syndrome derived from the Positive And Negative Syndrome Scale (PANSS)

Schizophrenia patients with the deficit syndrome (DS) may represent a homogeneous subgroup. To increase the practicability of diagnosing the DS, Kirkpatrick et al. [Kirkpatrick, B., Buchanan, RW., Breier, A. Carpenter, WT., 1993. Case identification and stability of the deficit syndrome of schizophrenia. Psychiatry Res. 47, 47-56.] proposed the use of a 'proxy' case identification tool using standardized symptom ratings instead of the Schedule for the Deficit Syndrome (SDS) which requires an independent clinical assessment. The Proxy for the Deficit Syndrome (PDS) is based on the extraction of symptoms that are essentially equivalent or overlap substantially with the restricted affect and diminished emotional range on the SDS. Kirkpatrick et al. [Kirkpatrick, B., Buchanan, RW., Breier, A. Carpenter, WT., 1993. Case identification and stability of the deficit syndrome of schizophrenia. Psychiatry Res. 47, 47-56.] reported good sensitivity and specificity in a comparison of SDS and PDS assessments among 100 chronic schizophrenia outpatients. The present investigation involves the comparison of the deficit syndrome as assessed by the "gold standard" Schedule for the Deficit Syndrome with the ratings of the same symptoms embodied in the "proxy instrument" the PANSS, within the same group of 156 inpatients. Forty-four patients were assessed by the SDS to have the deficit syndrome. Patients with and without the DS, as defined by the SDS, did not differ for age, education, age at illness onset and duration of illness. The two main 'proxy' measures PDS1 and PDS2 discriminated across the SDS groups. The direct dichotomous comparison of the actual SDS and the 'proxy' derived PDS groups demonstrated good specificity (78.6% and 79.5%) and moderate to very good sensitivity (61.4% and 86.4%) and there was a moderately low rate of false positive cases (21.4% and 20.5%). For the two main 'proxy' measures (PDS1 and PDS2) kappas were .38 and .59, representing poor to good agreement. In our sample of rigorously diagnosed schizophrenia inpatients, the use of a 'proxy' case identification tool for the deficit syndrome would appear to be a viable alternative in identifying a subgroup of schizophrenia patients with the deficit syndrome when the use of the actual SDS is not feasible. Further study is indicated before the PDS as extracted from the PANSS can be used in lieu of the SDS for identifying patients with this syndrome.