External quality assessment of urinary pregnancy oestrogen assay: further experience in the United Kingdom.

Four years' experience with external quality assessment of urinary pregnancy oestrogen and creatinine assays using a revised scheme design, incorporating frequent distributions, individualised reports and running scores based on variance index scores, is described. During this period there was a rapid and substantial improvement in interlaboratory agreement, which appears attributable in part to these improvements in scheme design. The derivation and use of a new index, the SDBIS (standard deviation of the bias index score), which assesses variability of bias, are described. Investigations showed that urine-based calibration materials could improve between-laboratory agreement; there were only minor differences in performance between method groups.