The Guildford trace elements external quality assessment scheme.

An international external quality assessment scheme for trace elements in biological fluids is described. Programmes for aluminium, copper, zinc, selenium, gold, lead, cadmium and mercury in serum, blood, urine, dialysis fluids and water are included with others under development. Preparation of reports is described and assessments of performance based on proximity to assigned values, differences in results between duplicate measurements and recovery of added analyte are given. Performance standards have been introduced to stimulate improvements in the quality of results.