Different biochemical methods of assessment of dietary status can provide data that may reflect a person's recent dietary intake or long-term intake, or it may not reflect intake at all. Many variables can affect nutrient levels in body fluids. These include activity level, stress, age, gender, and the status of other nutrients. There is no one definitive value that correctly identifies an actual deficiency state and it is not always clear if a deficiency state is due to that nutrient or some other metabolic alteration that can cause an abnormal level. A good test should be sensitive to changes in status of a nutrient and be minimally influenced by other factors.
This section describes various biochemical tests of selected nutrients. Clinical nutrition assessment, as done in hospitalized settings, is described here, although it is performed using a combination of methods.