Detection of colistin in spiked and incurred milk samples by LC- and ELISA-technique

A liquid chromatography (LC) method for the determination of the polypeptide antibiotic colistin (COL) was developed and applied to incurred milk samples from cows treated by a drug containing COL and ampicillin. At maximum residue limit (MRL) concentration (50 μg kg−1) the recovery rate was 106.1% and the coefficient of variation 13.6%. Limits of detection and quantification were 9.3 and 14.3 μg kg−1, respectively. There was no indication that the analytical results were influenced either by elevated somatic cell count or by ampicillin. COL-ELISA proved to be a reliable screening method. All spiked and incurred samples with COL-concentrations ≥MRL-concentration were evaluated as positive by ELISA. Neither with ELISA nor with the LC-method concentrations ≥MRL were detected in samples of untreated cows. By ELISA-screening no car tanker milk sample (n = 416) was suspicious to contain COL.