That depends on whom you ask. As part of your regular blood test, your doctor can order a test for 25-hydroxyvitamin D (25-OHD).
The problem is not with the test. The problem is how to interpret the results. An expert committee convened by the Institute of Medicine in November 2010 concluded that "the cut-point values used to define deficiency, or as some have suggested, 'insufficiency,' have not been established systematically using data from studies of good quality."
Even so, most experts agree that anyone with a 25-OHD level of less than 15 ng/mL or 37.5 nmol/L (depending on the units reported by a lab) needs more vitamin D. A 2002 study found that 42% of African-American women of childbearing age had vitamin D levels below 15 ng/mL.
The IOM committee says that people are at risk of vitamin D deficiency at 25-OHD levels below 30 nmol/L (12 ng/mL), and that some people -- but not everyone -- may be at risk of vitamin D deficiency at 25-OHD levels from 30 nmol/L up to 50 nmol/L (12-20 ng/mL).
The Vitamin D Council considers the ideal 25-OHD level to be between 40 ng/mL and 70 ng/mL. But the IOM says there is no evidence of increased benefit at levels above 30 ng/mL, and that "there may be reason for concern" at levels above 50 ng/mL.
"There is a critical public health and clinical practice need for consensus cut-points for serum 25-OHD," the IOM committee states.