Abstract <p>Wearable devices that continuously monitor the body’s physiological signals make it possible to diagnose and treat diseases, as well as improve the effectiveness of physical activity. When measuring quantitative parameters using near-infrared (IR) spectroscopy, a problem arises related to indefiniteness of the optical length of the optical pathway. Without knowing optical length, it is impossible to obtain precise values; hence, the quantitative indicators will be the relative concentrations of hemoglobin in terms of relative units. In this study, a comparative assessment and analysis of the results of measuring hemoglobin oxygenation using the NIRS4 device and the developed algorithm are conducted. The experiments conducted to determine the oxygenation index are described. It is found that the measured values for each hemoglobin fraction correspond to known values, and the calculation of the oxygenated hemoglobin concentration using the developed method gives changes in the concentration of the same order as the typical data for other methods and devices. The results of the study can be used for early diagnosis of the biochemical activity of the body. This presents new opportunities for the development of methods for diagnosing and monitoring a person’s health status based on indicators of the hemoglobin concentration in the blood.</p>