Vitamin B7 (biotin) is a water-soluble vitamin that serves as a cofactor for several essential enzymes involved in metabolism. This test measures biotin levels in the blood to evaluate nutritional status and diagnose deficiency. Biotin is required for the activity of carboxylase enzymes that are crucial for fatty acid synthesis, gluconeogenesis, and amino acid metabolism. It also plays a role in histone modification and gene regulation. Biotin deficiency can cause a characteristic red scaly rash around the eyes, nose, mouth, and genital area, as well as hair loss, neurological symptoms, and impaired immune function. While deficiency is rare in healthy individuals, it can occur in those consuming raw egg whites (which contain avidin that binds biotin), patients on total parenteral nutrition, or those with certain genetic disorders. This test is essential for evaluating unexplained skin rashes, hair loss, and neurological symptoms.