The vast majority of measurements of vitamin D levels in the blood serum of patients at present, both in domestic and foreign practice, are reduced to the determination of the total concentration of calcidiol [25(OH)D] in the patient’s blood serum [25(OH)D2 + 25(OH)D3]. This is because the half-life of 25(OH)D occurs over a long period of time (two to three weeks). 25(OH)D analyses are well-established and quite economical. The biologically active metabolite of vitamin D calcitriol [1.25 (OH) 2D], is measured much less frequently because the half-life of 1.25 (OH) 2D occurs within ~four hours, and the cost of the test is quite high. These observations have led some authors to question the validity of the assessment of vitamin D levels 25(OH)D serum measurement. Our analysis of domestic and foreign literature, as well as the results of statistical studies, allow us to say that in the data set we studied, containing blood serum analyses of 67,254 inhabitants of European countries, the level of calcitriol is directly related to the level of calcidiol, and this relationship is very stable (p<0.00001). Moreover, in the case of cholecalciferol, ergocalciferol or calcidiol, the increase in the level of 25(OH)D is accompanied by an increase in 1.25 (OH) 2D, but the increase in 1.25 (OH) 2D is significantly slower than the increase in 25(OH)D, and the degree of association between these indicators is not as high (p=0.04). Thus, in most cases, the serum 25(OH)D concentration can be considered an adequate marker of vitamin D levels, however, in a number of cases, especially in the analysis of patients with a confirmed diagnosis of COVID-19, the level of the active vitamin D metabolite calcitriol 1,25(OH)2Dshould be determined much more often, along with the level of calcidiol 25(OH)D, the level of the active metabolite of vitamin D — calcitriol 1.25 (OH) 2D. Vitamin D deficiency among the population of the Russian Federation is extremely common, which is a generally recognized risk factor for insufficient activity of the immune system, a risk factor for the severity of the course and clinical consequences of a large number of diseases, so testing of vitamin D levels in blood serum should be mandatory as part of regular health checkups, especially in risk groups and especially in winter and spring.