Confirmation of synthetic glucocorticoids with liquid chromatography/mass spectrometry: organization and results of an international interlaboratory comparison test.

Within the framework of a European Union (EU) research project entitled "Food Safety Screening: Synthetic Glucocorticoids (QLK1-1999-00122)," an international interlaboratory ring test was organized to compare and evaluate different liquid chromatography/mass spectrometry (LC/MS) confirmatory methods that are applied in European monitoring programs for detecting the use of synthetic glucocorticoids. Liver and urine samples of bovines treated with synthetic glucocorticoids were collected and sent to the participants of the study for analysis. Participants received 3 liver and 3 urine samples and were free to use either their own LC/MS method or an LC/MS-based method developed during the EU research project. The residue concentrations in the samples were calculated as the mean of the concentrations reported by each laboratory. The mean dexamethasone concentration of liver sample L1 was calculated as 2.27 microg/kg [relative standard deviation (RSD) 43%, n = 9], which exceeds the maximum residue level (MRL) of 2 microg/kg. Three of the 9 laboratories (33%) reported concentration levels less than 2 microg/kg, resulting in obviously false compliant results. The overall mean concentration of flumethasone in liver sample L2 was calculated as 3.27 microg/kg (RSD 33%, n = 8). Applying a comparable limit for flumethasone of 2 microg/kg, 8 of the 9 laboratories would have obtained a correct noncompliant result. As for the blank liver sample, 1 participant found a false noncompliant result. The urine sample U1 contained prednisolone residues at a mean concentration of 1.58 microg/kg (RSD 43%, n = 9). Four out of 9 results were less than a theoretical minimum required performance level (MRPL) of 2 microg/kg. The calculated concentration of dexamethasone in urine sample U3 was 5.21 microg/kg (RSD 62%, n = 9). One of the 9 results was lower than 2 microg/kg. Urine sample U2 was correctly reported as blank by all participants.