This test card is used for quantitative determination of 25 hydroxyvitamin D in human serum, plasma or whole blood in vitro. Vitamin D is a kind of sterol hormone, which is produced by skin light. Two important forms of vitamin D include vitamin D3 (cholecalciferol) and vitamin D2 (ergocalcitol). In human body, vitamin binding protein combines with vitamin D3 and vitamin D2, and transports them to liver, where both forms are hydroxylated to produce vitamin D, namely 25 hydroxy Basic vitamin D. 25 hydroxyvitamin D is a kind of metabolite, which can be detected in blood. Because it is the main storage form of vitamin D in human body, it can promote intestinal absorption of calcium and regulate calcium balance. Vitamin D is the main element to maintain bone health. Severe deficiency of vitamin D in childhood will lead to skeletal deformity, rickets. The decrease of 25 hydroxyvitamin D is related to the decrease of bone mineral density. Combined with other clinical data, the results are helpful to evaluate bone metabolism.