Endogenous and synthetic steroids in bovine urine--preparation of in-house reference material, stability studies and results of a proficiency test.

ABSTRACT

Within the framework of the German National Residue Control Plan a specific number of samples of animal origin have to be analysed for natural and synthetic steroids each year. As a measure of external quality control of the methods applied in routine analysis a proficiency test was carried out. To this end, in-house reference material containing incurred residues of 17α- and 17ß-nortestosterone and 17α- and 17ß-estradiol as well as fortified residues of 17α-methyltestosterone and 17α-trenbolone in bovine urine were produced. Before sending the proficiency test material to the participants, the homogeneity of all samples was tested and confirmed. Furthermore extensive short- and long-term stability studies were carried out. The statistical evaluation of the proficiency test was performed by applying robust statistics as described in standard DIN 38402. Based on the target value and standard deviation z-scores were calculated as standardised measure of the laboratory performance. The evaluation of the proficiency test showed that nine laboratories submitted quantitative results within the tolerance limits for all analytes. Taking into account the individual decision limits, there were no false negative results. In overall evaluation, 11 of 12 laboratories participated successfully.