Localized proton NMR spectroscopy is capable to measure in vivo brain metabolites in different diseases. Accuracy and precision of the quantification of metabolite concentrations is key, especially when comparing results from different patients, diseases, groups, sites or even species. Here we demonstrate that deterioration in SNR of the spectra does not only degrade the precision of obtained values but also could cause severe alterations of estimated absolute concentrations. E.g. aberrations of up to 100% or 8 mMol/L were found in Glx (Glutamate+Glutamine) concentrations.