Reproducibility of mean values of duplicate specimens in the Basic Ligand Assay Survey.

Three paired samples were distributed at three--six-month intervals in the Basic Ligand Survey during 1976. The stabilities of mean values obtained by widely used commercial kit procedures and by all methods for eight analytes are reported. Significant improvements in accuracy and long-term precision were identified in assays for digoxin and thyroxine during 1976. However, a number of assays yielded unstable mean values that affected both within--laboratory maintenance of quality control and medical usefulness of the results.