How do doctors determine vitamin or nutrients deficiency?

History of patient: The doctor will want to know what kind of diet is consumed by the person in question. True deficiency is very rare in the first world. If there are symptoms, and each deficiency will produce recognizable symptoms, then the level of that nutrient or vitamin can be measured with a blood sample. If all fits together, and again i emphasize how rare it is, then supplementation is given.

Various methods: This can be surprisingly difficult. Fat-soluble vits like vit d can be reliably measured by blood levels, as can b12, but it's hard to assess levels of most b vits ; vit c as they vary day to day. Minerals are hard to measure because they're mostly in cells, not blood. Serum Amino Acids ; fatty acids in red blood cell membranes give good info.Some of the best tests are functional tests-see comment:.

Blood tests: Usually vitamin or other nutrient deficiency is diagnosed with a blood test specific to the nutrient.