Similar to food intake, nutritional supplements and medications become absorbed into the bloodstream and may affect the analytical accuracy of intended measurands. For example, if a physician is interested in determining vitamin levels or iron status in a patient, that patient should be instructed to refrain from taking vitamin and mineral supplements prior to the sample collection to avoid the probable false increase. Such false increases may mask true deficiencies, resulting in the omission of helpful treatments. However, in certain cases, physicians may intentionally choose to check vitamin levels shortly after supplementation to determine if sufficient uptake is occurring.
To avoid such potential pre-analytical errors, it is recommended to ask the patient if his/her doctor provided instructions to refrain from taking medications, vitamins, or other supplements prior to sample collection.