Abstract
Since 1974 laboratory performance in the estimation of oestrogens in urine from pregnant women has been assessed in about 150 laboratories by analysis, at intervals of three weeks, of samples of freeze-dried urine. An index of discrimination, Δ (= 2·7 SD derived from results reported on six pairs of duplicates), was calculated for each participant laboratory to provide a measure of between-batch precision. There has been a gradual improvement in laboratory performance assessed in this way. Initially, only 45 % of the participants were able to discriminate between two 24-hour urine samples, differing in oestrogen by at least 20 μmol, when analyses were performed on different days. In the final period ending March 1980, 61 % of participants achieved this standard.