To check for a vitamin B12 deficiency in adults, an initial blood test can assess the level of vitamin B12 in the serum. A level below 200 pg/mL indicates a vitamin B12 deficiency, and a level between 200 and 300 pg/mL is considered borderline low and may indicate a subclinical vitamin B12 deficiency, which means that a deficiency may be present but causes few to no symptoms.[1][2][3]
Relying solely on serum total vitamin B12 levels may not be enough to accurately diagnose a vitamin B12 deficiency, particularly when the results fall within the borderline deficiency range. For a more accurate diagnosis, it is often recommended to confirm the findings by analyzing additional vitamin B12 biomarkers. These may include elevated methylmalonic acid (MMA), low holotranscobalamin (holoTC), or elevated homocysteine (Hcy). [1][2][4]
Often, measuring both total serum vitamin B12 levels and MMA levels is the most effective clinical approach; this is because serum holoTC is not widely available, and MMA is more specific to determine vitamin B12 status than serum Hcy.[1][2][4] Another method, combined vitamin B12 (cB12), involves measuring all 4 biomarkers and pooling the results into one score. Although this is the most accurate diagnostic method, it is not the most practical or cost-efficient.[2]
Importantly, each of these tests has some strengths and limitations which should be accounted for (see table below),[5] and the method that is chosen will vary based on the individual, cost, time, and availability.
| Blood biomarker/test | Typical reference ranges | Typical cutoff values | Strengths | Limitations |
|---|---|---|---|---|
| Serum total vitamin B12 | 200–900 pg/mL (148–664 pmol/L) | Deficiency: <200 pg/mL (<148 pmol/L) Insufficiency: 200–300 pg/mL (148–221 pmol/L) | Widely available and standardized. | Unreliable as a stand-alone marker. |
| Serum holoTC | 40–150 pmol/L | Deficiency: <35–40 pmol/L | Better than Hcy and MMA in older adults; it helps detect vitamin B12 level changes early on. | People with genetic variations of the transcobalamin gene, macrocytosis (large red blood cells), or undergoing chemotherapy can have low holoTC levels but no vitamin B12 deficiency. |
| Serum MMA | 50–370 nmol/L | Deficiency: >370 nmol/L | Highly specific marker for B12 status; most sensitive marker in people with cancer. | Elevated in renal insufficiency; utility compromised in older people with kidney disease. |
| Plasma total Hcy | 4–10 µmol/L | Deficiency: >12–15 µmol/L | Useful first-line test in infants and toddlers; high sensitivity for vitamin B12 status changes. | Limited specificity; High levels can also be caused by folate deficiency, vitamin B6 deficiency, and/or renal dysfunction. |
| cB12 | –0.5 to 1.5 | Low: –1.5 to –0.5 Borderline deficiency: –2.5 to –1.5 Probable deficiency: < –2.5 | Better diagnostic capability than any single biomarker; accounts for age-related physiological changes. | Expensive and complex to measure all 4 biomarkers simultaneously for calculation. |
* holoTC: Represents the portion of vitamin B12 delivered to all tissues; Hcy: Biomarker that accumulates when vitamin B12 is deficient; MMA: Biomarker that accumulates when vitamin B12 is deficient; cB12: A blood test in which all 4 biomarkers are analyzed (i.e., total serum vitamin B12, holoTC, MMA, and Hcy), and the results are combined into one score
Reference: Joshua W M Adv Nutr. 2018.[2]; Luciana H et al Front Mol Biosci. 2016.[5]