External quality assessment of assays of lead in blood.

The operation and results of the United Kingdom External Quality Assessment Scheme for Lead in Blood, which currently has about 140 participants within and outside the U.K. are described. The choice of specimens, scheme design, and scoring of performance are discussed, as is the validity of the consensus values used as a basis for the scoring. There has been a continued improvement in performance as assessed by this program, some of which appears to have been stimulated by the introduction of Variance Index scoring. The relative performance of the methods for assaying lead in blood is also examined.