The problem of confirming the presence of an adulterant in a urine sample is solved by the use of a reagent capable of reacting with uric acid and non-urate markers in a urine sample. In one embodiment, a phosphotungtate reagent is used to react with the urine sample to create a blue coloration in the presence of uric acid or uric acid equivalents. A reduction or elimination of the blue coloration, resulting in a reduction in the light absorbance, of the urine sample can be used as an indicator of the historical presence of an adulterant. An Oxidant History test can also be generated using the phosphostungtate reagent, wherein the light absorbance resulting from the blue coloration is measured over time, with a measured reduction in the absorbance being an indication that an adulterant is or has been present in the urine sample and is oxidizing the uric acid and non-urate markers over time.

A method and analyzer for determining a measured value of a measured variable of process automation technology in a liquid or gaseous medium by means of an optical sensor includes taking a sample of the medium; mixing the sample with one or more reagents; supplying an emitter of the optical sensor with an exciter signal for producing sent light, wherein the sent light by interaction with the mixed sample is converted into received light as a function of the measured variable; producing a receiver signal by means of a receiver of the optical sensor from the converted, received light; and determining the measured value based on the receiver signal and a calibration function, which includes a term which takes aging of the reagents into consideration.