COMPARISON OF A RADIOIMMUNOASSAY (CHARM-II) TEST WITH HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY FOR DETECTION OF OXYTETRACYCLINE RESIDUES IN MILK SAMPLES FROM LACTATING CATTLE

A radioimmunoassay test for tetracyclines (Charm II) was compared with high-pressure liquid chromatography (HPLC) for detection of oxytetrac ycline (OTC) residues in milk samples from individual lactating cows. Oxytetracycline was administered by 1 of 3 routes (IV, IM, or intraute rine) to 21 lactating dairy cows. A total of 292 duplicate milk sample s were collected from milkings before and through 156 hours after OTC administration. Concentration of OTC in these samples was determined b y use of the Charm II test and an HPLC method with a lower limit of qu antitation, approximately 2 ng of OTC/ml. Samples were also classified with respect to presence of OTC residues relative to the FDA safe con centration (less than or equal to 30 ng/ml), using the Charm II (by co ntrol point determination) and HPLC methods. There was a significant ( P less than or equal to 0.05) difference between test methods in class ification of milk samples with respect to presence or absence of OTC a t the FDA safe concentration. A total of 48 of the 292 test results (1 6.4%) did not agree. Using the HPLC test results as the standard with which Charm II test results were compared, 47 false presumptive-violat ive test results and 1 false presumptive-nonviolative Charm II test re sult (a sample containing 31 ng of OTC/ml, as evaluated by HPLC) were obtained. The samples with false presumptive-violative Charm II result s contained < 30 ng of OTC/ml, as evaluated by HPLC. In some respects, the Charm II test performed appropriately as a screening test to dete ct OTC residues in milk samples from individual cows. However, the ten dency for the test to yield presumptive-violative test results at OTC concentrations lower than the FDA safe concentration (as evaluated by HPLC), suggests that caution should be exercised in using the test as the sole basis on which a decision is made to reject milk. As indicate d by the manufacturer, presumptive-violative Charm II test results sho uld be confirmed by additional testing. Although not specifically eval uated, the tendency for misclassification of milk samples as presumpti ve-violative by the Charm II test may or may not occur in commingled m ilk, compared with milk samples from individual cows.