Fig. 2.

Rifampin dilution and spiking studies. Opiate concentrations were measured with Cobas OPI2 assay. (A) The apparent opiates results in diluted urine samples of the patient. The concentration of rifampin in each of the diluted urine samples was calculated using the neat rifampin concentration (52390 ng/mL) and the associated dilution factors. B) The apparent opiates results in urine specimens containing various spiked rifampin. Additionally, to achieve a positive result using the OPI2 from Roche required 2521 ng/mL of Rifampin.