An embodiment provides a method for determining a concentration of an analyte in a fluid sample, including: introducing a fluid sample into a measurement chamber; operating a measurement device to introduce light of a first wavelength to the fluid sample; measuring, with a detector, absorbance of the light of the first wavelength with respect to the fluid sample; operating the measurement device to introduce light of a second wavelength to the fluid sample; measuring, with the detector, absorbance of the light of the second wavelength with respect to the fluid sample; determining, using a processor of the measurement device, an absorbance ratio of the fluid sample using both the measured absorbance of the light of the first wavelength and the measured absorbance of the light of the second wavelength; and providing, via an output device, a determined concentration value for the fluid sample that correlates to the absorbance ratio. Other aspects are described and claimed.