The quality of the clinical-chemistry analysis of blood serum is greatly affected by interfering substances contained in the sample. At present, skilled laboratory personnel select specimens which can be processed by the analyser. Selection is performed by visual inspection of the serum colour. To reduce laboratory labour costs and risk of infections, we developed an inexpensive, simple and compact solid-state colorimeter to determine concentrations of interferents. This sensor can be integrated into modern clinical-chemistry analysers. The determination of concentrations of interferents is based on the measurement of the extinction coefficients of the serum sample at four different wavelengths. In this paper, this instrument and its performance are presented.