Quality control program of steroid receptor assays: an international study

Lyophilized calf uterine cytosol standards were prepared for quality control of estrogen receptor (ER) determination, and lyophilized cytosols and tissue powders were used for quality control of progesterone receptor (PR) analysis. Two series of four samples were analyzed either for ER or PR contents, twice within one month, by 7 laboratories in 5 countries. Coefficient of variation (CV) of the between-laboratory averages assayed in a single run of ER-positive (ER+) and PR-positive (PR+) standards varied from 29.6 to 61.8% and from 32.4 to 76.2%, respectively. All laboratories, with the exception of a single value, could recognize samples of low, medium, an high ER level, as well as a negative sample. Most laboratories evaluated properly also the level of PR samples. The average between-laboratory CV values of protein determination in the relevant standards were 23%.