Is the Defined Daily Dose system a reliable tool for standardizing antipsychotic dosages?

The present study was carried out to establish whether the Defined Daily Doses (DDDs) system could be reliably applied to standardize antipsychotic dosages. Initially, the relationship between antipsychotic doses expressed as DDDs, chlorpromazine equivalents (CPZEs) and percentages of the British National Formulary (BNF) maximum recommended daily dose were investigated by calculating Spearman's rank correlation coefficients. Second, factors associated with antipsychotic dose, expressed as DDDs, CPZEs and percentages of the BNF maximum recommended daily dose, were investigated by means of linear regression analysis. The study sample consisted of 277 patients with schizophrenia. The relationship between antipsychotic daily doses expressed as multiples of DDDs and CPZEs revealed a significant correlation (Spearman's rho=0.779, P<0.001). Similarly, the relationship between antipsychotic daily doses expressed as multiples of DDDs and percentages of the BNF maximum recommended daily dose revealed a significant correlation (Spearman's rho=0.869, P<0.001). Linear regression analyses highlighted a high degree of coherence between antipsychotic doses expressed as DDDs, CPZEs and percentages of the BNF maximum recommended daily dose. In conclusion, this study found that the DDD system is a reliable tool for standardizing antipsychotic doses in drug utilization research.