On-line measurement of urea in blood using optical spectroscopy in the visible range; Validation of the cell shrinkage hypothesis

The present work shows the development of a low-cost system to measure on-line blood urea concentration during hemodialysis procedures using optical spectroscopy in the visible range. Dissolved urea presents absorbance in the infrared, not showing any significant optical behavior in the visible range. Nevertheless, previous results obtained by the first author in Mexico displayed experimental evidence of a clear correlation between blood urea contents and optical absorbance in the red region obtained through the hemodialysis ducts. This paradox posed the necessity of identifying the underlying physic-chemical mechanism involved in the optical absorption of urea dissolved in blood in the visible margin. The proposed hypothesis considers that the cause of this dependence is the volume change of the red cells with the urea concentration due to the osmotic pressure and to the mechanisms of regulation of the cellular volume. A bigger concentration of urea would imply a reduction of the volume of the cells and, therefore, a decrease of the optical absorbance. To validate this hypothesis, we used a simplified cellular model based on a yeast cell suspension. Measurements were performed at different urea concentrations using three systems: a custom single wavelength absorbance meter, a fiber optic spectrophotometer and an electrical impedance spectroscopy measurement system as contrast technique.