Diagnostic Testing
Vitamin B12 Deficiency: Symptoms, Testing, and What Levels Mean
True B12 deficiency affects roughly 2 to 4 percent of US adults, and marginal levels are more common. Here are the symptoms, who is at risk, and how blood testing confirms it.
Reviewed by the clinical team at Zest Regen Health · November 7, 2026 · 7 min read
Content managed by Phil Kim, Lucky Consulting Group
Vitamin B12 deficiency affects roughly 2 to 4 percent of United States adults, and another 12 percent or so have marginal levels below 300 pg/mL, yet its symptoms, which include fatigue, tingling in the hands and feet, and memory trouble, overlap heavily with thyroid, iron, and other conditions. At Zest Regen Health in Columbia, Maryland, B12 is measured as part of a diagnostic evaluation when symptoms and risk factors point to it, and borderline results are confirmed with functional markers rather than treated on a guess.
What does vitamin B12 do in the body?
Vitamin B12 supports red blood cell formation, nerve function, and DNA synthesis, and it plays a role in cellular energy metabolism. The body stores large amounts of B12, mostly in the liver, which is why a deficiency develops slowly, often over years, and why early symptoms can be subtle or absent.
How common is B12 deficiency?
Using NHANES data from 2007 to 2018, the NIH Office of Dietary Supplements estimates that about 3.6 percent of US adults age 19 and older have B12 deficiency, defined as a serum level below 200 pg/mL (148 pmol/L), with a similar rate, about 3.7 percent, in adults over 60. The American Academy of Family Physicians cites a range of roughly 2 to 3 percent. Marginal status is more common: about 12.5 percent of adults have serum B12 below 300 pg/mL, and the share rises with age because stomach acid and absorption decline over time.
Common symptoms
- Fatigue, weakness, or feeling short of breath with activity
- Numbness, tingling, or pins and needles in the hands or feet
- Memory problems, brain fog, or mood changes
- A sore, smooth, or red tongue, or mouth ulcers
- Balance problems or unsteadiness
Each of these can also come from other causes. Fatigue alone is not a reason to assume B12 deficiency, which is exactly why testing is interpreted in the context of a full evaluation rather than in isolation.
Who is at higher risk?
- Adults over 60, as stomach acid and absorption decline with age
- People taking long-term acid-suppressing medications (proton pump inhibitors) or metformin
- Strict vegetarians and vegans who do not supplement
- Anyone with a history of gastric bypass or bowel surgery
- People with autoimmune conditions that affect absorption, such as pernicious anemia
How B12 is tested
The usual first test is serum B12, also called total cobalamin. Levels below roughly 200 pg/mL are commonly considered deficient, levels between 200 and 300 pg/mL are borderline, and levels above 300 pg/mL make deficiency unlikely in most situations. Because serum B12 alone can miss some cases and over-flag others, two functional markers help confirm the picture: methylmalonic acid (MMA), which rises as B12 stores deplete and is more sensitive than serum B12 alone, and holotranscobalamin, which measures the metabolically active fraction of B12. A low or borderline serum B12 together with an elevated MMA is the standard confirmatory pattern.
| Serum B12 level | How it is often read |
|---|---|
| Below 200 pg/mL (148 pmol/L) | Generally considered deficient; confirmatory testing is typically appropriate |
| 200 to 300 pg/mL (148 to 221 pmol/L) | Borderline; MMA or active B12 testing helps decide whether true deficiency exists |
| Above 300 pg/mL (221 pmol/L) | Deficiency is unlikely, though risk factors and symptoms still matter |
What a low level does and does not mean
A low B12 result can explain fatigue and nerve symptoms and is worth correcting, but it does not guarantee every symptom will resolve, and many people with low-normal levels have no symptoms at all. Treatment, which may be oral or injectable depending on the cause, depends on why the level is low: diet, absorption, and medication-related causes are managed differently. That distinction is a physician conversation, not a supplement choice.
Why B12 testing belongs in a diagnostic evaluation
At Zest Regen Health, B12 is one marker among many in a diagnostic wellness panel that may also include a complete blood count, iron studies, thyroid testing, vitamin D, and metabolic markers. The goal is to find the actual driver of fatigue rather than chase a single number, and the care team reviews results in plain language before any supplement or treatment is recommended.
See how diagnostic wellness testing works at the Columbia clinic
Read about iron and ferritin testing for fatigue
Read about vitamin D blood testing and what levels mean
Meet our physicians, who evaluate patients at Zest Regen Health
Next step
If fatigue, tingling, or memory concerns have you wondering whether a vitamin level is involved, a free 15-minute consultation at the Columbia clinic can help you decide whether testing is worthwhile and which tests would actually answer the question. The team explains what a panel can and cannot tell you before anything is ordered.
Talk with our care team about this topic
A member of our clinical team can answer your questions at the Columbia, Maryland clinic. If Diagnostic Wellness Testing may be appropriate for you, the first step is a consultation.
Care at Zest Regen Health is physician-guided. Meet our physicians, who evaluate patients at the Columbia, Maryland clinic.