Optical methods for determining the relationship between the biophysical constant of osmolarity and the parameters of aqueous biological solutions.

The early diagnosis of chronic kidney disease is critically important due to the asymptomatic progression of the condition in its initial stages and its increasing prevalence. This study substantiates the need to develop novel, accessible, and non-invasive diagnostic approaches based on optical technologies. A method based on refractometric measurement of urine refractive index is proposed, enabling the quantitative evaluation of key physicochemical parameters. Reliable linear relationships between urine osmolality, density, and temperature were established. The method demonstrates high measurement accuracy (±0.0001) and stability over a wide temperature range (288–303 K), making it a competitive and cost-effective alternative to traditional diagnostic techniques such as urinalysis strips, urinometry, and osmometry. Regression equations were developed to estimate osmolality based on density ($\mathrm{Os} = 33.128\rho - 33.107$) and density based on refractive index ($\rho = 2.4322n - 2.2444$). The results confirm the feasibility of using refractometry as a tool for rapid, non-invasive assessment of kidney function and early detection of CKD. The proposed approach simplifies routine clinical analysis and has the potential for implementation in both laboratory and point-of-care settings.