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This work describes the integration of thin-film optical filters in a lab-on-a-chip device for spectrophotometric analysis of biological fluids using white-light as illumination. Thin-film SiO2/TiO2 multilayers were optimized according to numerical simulations, so that the optical transmittance is tuned for specific wavelengths. These films were deposited on glass substrates by Ion Beam and then integrated in a previously developed lab-on-a-chip platform, which incorporates microfluidic and detection modules. The system operates with a white-light source that is filtered to a narrow spectral band, centred at the desired wavelength, allowing the selective measurement of the light intensity transmitted through the fluid. This intensity is quantified using a colorimetric detection system and then correlated with the biomolecule concentration. Several calcium concentrations in urine were successfully detected in the lab-on-a-chip using the filtering system.