When Should You Test for Vitamin D?

A vitamin D blood test will show very different results depending on what time of year it is taken. Tests in September will generally indicate the maximum or peak level. A test in March will measure the lowest level or trough. The challenge is to maintain adequate levels throughout the year which experts suggest is 125nmol (50ng) rather than the bottom of the range which is 75nmol (30ng). In northern regions (north of Boston) this necessitates supplements as high as 5,000 IU daily during the winter months.