Laboratory tests Because levels of serum 25 (OH) D reflect body stores of vitamin D and correlate with symptoms and signs of vitamin D deficiency better than levels of other vitamin D metabolites, the best way to diagnose vitamin D deficiency is generally considered to be 25 (OH) D (D2 + D3) levels Target 25 (OH) D levels are > 20 to 24 ng / mL (about 50 to 60 nmol / L) for maximal bone health; whether higher levels have other benefits remains uncertain, and higher absorption of calcium may increase risk of coronary artery disease.