Serum 25-OH Vitamin D test measures the level of 25-hydroxyvitamin D in the blood, which is the best indicator of vitamin D status. Vitamin D is crucial for bone health, as it helps the body absorb calcium. It also plays a role in immune function and inflammation reduction. This test is used to check if there is too much or too little vitamin D, which can indicate bone disorders, nutritional deficiencies, or other health issues. Adequate levels of vitamin D are important for maintaining bone strength and overall health, making this test essential in preventive health care.