Scandinavian quality surveys of gentamicin assays 1978-1983

During 1978-1983 44 Scandinavian laboratories participated in 7 quality surveys of gentamicin serum assays. In each survey 2 test solutions of gentamicin were distributed. In the first 4 surveys very few non-microbiological assays were used while in the 7th survey almost half the laboratories had access to enzyme immunoassay (EMIT). The gentamicin survey mean (SM) concentrations, which did not differ significantly from the intended concentrations, were used as target values. In survey 1, 65% of the results of either of the 2 test solutions were outside the SM +/- 25% and 16% were outside SM +/- 50% ("highly misleading"). In surveys 5 and 7 the corresponding figures were 27 and 5% and 18 and 0%, respectively. The EMIT performed excellently in all surveys. Agar well diffusion assays using Diagnostic Sensitivity Test agar (Oxoid) performed better than other plate diffusion assays. Laboratories participating regularly performed better than others. It is concluded that the survey program, which continues, provides an external facility for unbiased control of commercially available as well as non-commercial assay techniques and that it has been instrumental in the improvement of gentamicin assay standard. It is stressed that the surveys cannot replace internal quality control, best performed by daily measurement and regular statistical evaluation of samples of known potency.