Method for calibrating an infrared spectroscopy apparatus to produce spectra to determine the concentration of a component C1 in one fluid F1 of an animal, includes: a) obtaining samples of fluid F1 and of a different fluid from each animal belonging to a group of representative animals, fluid F2 containing detectable amounts of at least one component C2 which is directly or indirectly related to a metabolic pathway of component C1 b) measuring the concentration of component C1 in the samples of fluid F1 c) producing the complete IR absorption spectrum of the samples of fluid F2 d) identifying in the absorption spectra of fluid F2 samples the spectral ranges that correlate with the concentration of component C1 in fluid F1 samples and e) calculating, on the basis of at least one of the correlating spectral ranges, a predictive mathematical model of the concentration of component C1 in fluid F1.
