Extending the detection limit of the TDx fluorescence polarization immunoassay for benzoylecgonine in urine.

This modified calibration method decreases from 300 to 27 micrograms/L the limit of detection for the cocaine metabolite (hydrolysis product), benzoylecgonine, by the Abbott Laboratories TDx fluorescence polarization immunoassay. For this determination we used 30 controls prepared from a single urine pool known to be negative for cocaine metabolite. Assay of 80 controls prepared from 20 different patients' urine samples yielded a limit of detection of 44 micrograms/L. To test these limits of detection, we analyzed 90 patients' urine samples known to be negative for cocaine metabolite and 74 patients' samples known to be positive for cocaine metabolite, using the TDx with our revised calibration. Results for two of the known negative samples and 96% of the samples containing cocaine in the 50 to 100 micrograms/L range fell above the 44 micrograms/L limit. The TDx showed excellent calibration stability. For 28 days during the test, the instrument was not recalibrated. During this period the day-to-day analysis of 50 micrograms/L controls produced a mean TDx response of 0.485 (SD 0.007) with a coefficient of variation of 1.5%.