Vitamin deficiencies are generally considered rare in the U.S., but a growing body of evidence suggests that certain deficiencies may be more prevalent than originally thought. For example, it has been estimated that 10% to 15% of adults over the age of sixty are affected by vitamin B12 deficiency, but most of these individuals lack any overt symptoms. Assessment of nutritional status has been performed by a variety of techniques, including microbiological, enzymatic, immunoassay, and chromatographic approaches. Interlaboratory comparisons have revealed discrepancies in the results obtained by laboratories using different assay methods or even when different laboratories use the same assay for a given analyte. Such variation adds significant complexity to the identification of "normal" ranges for nutritional analytes, and reduces the reliability of vitamin deficiency diagnoses. To improve the accuracy and reliability of nutritional status assessments, NIST is developing a series of serum-based Standard Reference Materials (SRMs).