If you have a gastrointestinal disease that leads to significant malabsorption, your doctor may measure some of the levels of vitamins and minerals in your blood. Atrophic gastritis, for example, is a condition which specifically leads to the malabsorption of vitamin B12. This vitamin can be measured in your blood if atrophic gastritis is suspected.
If your routine laboratory tests (including a complete blood count and a chemistry panel) are normal, you do not need a measurement of your blood vitamin or mineral levels. If you are taking a medication such as a diuretic (water pill), your doctor is most likely monitoring the level of potassium in your blood already.
If you are found to be anemic and your doctor believes it is a result of a nutrient deficiency, he or she may measure the level of iron in your blood. Iron, in fact, is the only mineral for which blood levels correlate with the total amount of iron stored in your body. Other minerals such as sodium, potassium, phosphorous, calcium, and magnesium, are found predominantly in the cells of your body, so blood levels of these nutrients can, at times, be misleading.