Vitamin B12, also known as cobalamin, is necessary for red blood cell formation, neurological function, and DNA synthesis. When the body does not have enough B12, a deficiency can develop, potentially leading to a range of health concerns if it goes unrecognized. Blood tests serve as the primary method for accurately diagnosing a B12 deficiency.
Recognizing the Need for Testing
Persistent fatigue, general weakness, or nerve-related issues like tingling sensations, numbness in the hands or feet, and difficulty with balance can suggest a B12 deficiency. Cognitive changes such as memory problems and mood disturbances may also indicate a deficiency. These symptoms often develop gradually, making early recognition challenging.
Beyond symptoms, several factors can increase an individual’s risk for developing a B12 deficiency. People following strict vegan or vegetarian diets are at higher risk because B12 is primarily found in animal products. Older adults often face absorption challenges, making them more susceptible to deficiency.
Conditions affecting the digestive system, such as Crohn’s disease or celiac disease, can impair B12 absorption. Gastric surgeries, including weight-loss procedures, can also impact the body’s ability to absorb B12. Long-term use of certain medications, such as metformin, commonly prescribed for diabetes, and proton pump inhibitors, used to reduce stomach acid, can interfere with B12 absorption.
Key Blood Tests for Diagnosis
Diagnosing a B12 deficiency typically begins with a serum vitamin B12 level, which measures circulating B12 and provides an initial snapshot of the body’s B12 status. While a low serum B12 level often indicates a deficiency, results can sometimes be borderline or appear normal even when a functional deficiency exists at the cellular level.
When serum B12 levels are borderline, or if symptoms persist despite a seemingly normal result, healthcare providers may order secondary or confirmatory tests. One such test is the methylmalonic acid (MMA) test (code: 83890). Elevated MMA levels are a more sensitive indicator of B12 deficiency because B12 is required to convert MMA into succinyl-CoA; without enough B12, MMA accumulates.
Another confirmatory test is the homocysteine test (code: 83090). High levels of homocysteine can also suggest a B12 deficiency, as B12 is involved in the metabolism of homocysteine. However, elevated homocysteine can also be due to folate deficiency, making it less specific for B12 alone compared to MMA. The holotranscobalamin (HoloTC) test measures the metabolically active form of B12, providing an earlier indicator of B12 status than total serum B12. Low HoloTC levels can signal an early deficiency before total B12 levels drop significantly.
Interpreting Your Results
Interpreting the results of B12 blood tests involves considering the levels of each marker in relation to established reference ranges. For serum B12, levels below approximately 200 picograms per milliliter (pg/mL) generally indicate a deficiency, while levels between 200 and 300 pg/mL are often considered borderline. Normal ranges typically fall above 300 pg/mL, though these figures can vary slightly between different laboratories.
Elevated levels of methylmalonic acid (MMA) are a specific sign of B12 deficiency, even if serum B12 levels are at the lower end of the normal range or borderline. An MMA level above approximately 0.4 micromoles per liter (µmol/L) often confirms a functional B12 deficiency.
High homocysteine levels, particularly when found alongside low or borderline B12 and elevated MMA, further support a diagnosis of B12 deficiency. A homocysteine level above 15 micromoles per liter (µmol/L) is generally considered elevated. If holotranscobalamin (HoloTC) levels are measured, a low result, typically below 50 picomoles per liter (pmol/L), can suggest an early B12 deficiency, indicating reduced availability of the active form of the vitamin.
Laboratory results should always be discussed with a healthcare professional. They can interpret the full panel of tests in the context of an individual’s symptoms, medical history, and risk factors. Self-interpretation of lab values can be misleading, and a doctor’s guidance is essential for accurate diagnosis and appropriate next steps.
Addressing a Diagnosed Deficiency
Once a B12 deficiency is diagnosed, treatment focuses on replenishing stores and addressing the underlying cause. For severe deficiencies or those caused by absorption issues, vitamin B12 injections deliver B12 directly into the bloodstream, bypassing any absorption problems in the digestive tract. Individuals with milder deficiencies or those due to dietary factors may be prescribed oral B12 supplements, which can be effective even with mild malabsorption.
Treatment plans are tailored to individual needs and deficiency severity. Managing the root cause may involve dietary changes for vegans or vegetarians, managing underlying gastrointestinal conditions, or adjusting medications that interfere with B12 absorption. All treatment decisions should be made in consultation with a healthcare provider to ensure effectiveness and safety.