This test card is used for quantitative determination of 25 hydroxyvitamin D3 in human serum, plasma or whole blood in vitro. The two most important forms of vitamin D are vitamin D3 (cholecalciferol) and vitamin D2 (ergot calciferol). In human body, vitamin D3 and D2 combine with vitamin D binding protein in plasma and transport to liver. They become 25 hydroxyvitamin d after 25 hydroxylation. More than 95% of 25 hydroxyvitamin D detected in serum is 25 hydroxyvitamin D3. Vitamin D deficiency is a common cause of secondary hyperparathyroidism. Increasing the content of parathyroid hormone, especially in the elderly without vitamin D, will lead to osteomalacia, increase bone turnover, reduce bone quality, and have the risk of fracture. The low content of 25 hydroxyvitamin D3 is also related to the low bone mineral density. Combined with other clinical data, the results can be used as an auxiliary means to determine bone metabolism.