Survey of Reference Ranges and Clinical Measurements for Psychoactive Drugs in Serum

Abstract

Thirty-three members of the United Kingdom National External Quality Assessment Scheme for measurement of psychoactive drugs in serum responded to a questionnaire concerning 31 tricyclic, antipsychotic, and selective serotonin re-uptake inhibitor drugs. Considerable variation was found between laboratories in their reference ranges and in the observed range of measured drug concentrations in clinical samples. For the lower end of the reference range, the median ratio between the highest and lowest values reported was 6.8, while for the top end of the range, the median ratio was 3.6. Observed clinical values were significantly less than reference range values at the lower end of the range and significantly higher at upper-range values by median values of −57% and +47%, respectively. The median ratio between maximum and minimum measurements reported by laboratories was 10.0 for both the lowest non-zero measurements and the highest non-toxicological concentrations. The variation in clinical range values did not differ significantly from that in reference range values. The ratios take no account of local variation in patient groups and treatment regimens.