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Hormonal
Vitamin B12 is essential for making healthy red blood cells and for keeping your nervous system working properly, and unlike many nutrients, your body stores several years' worth in the liver — which means a deficiency usually builds slowly and can go unnoticed for a long time before it's caught. This test is most often ordered after other blood work already points toward a specific pattern of anaemia, called megaloblastic anaemia, where red cells are unusually large. It's also checked when someone has nerve-related symptoms like tingling or numbness, since B12 deficiency is a well-known, treatable cause of exactly that pattern.
The test reports the concentration of vitamin B12 circulating in your blood, most often expressed in picograms per millilitre, with normal values generally falling between about 299 and 1054 pg/mL, though exact cutoffs vary slightly by laboratory. Because B12 is stored in the liver over years, a single low reading usually reflects a deficiency that has been building for some time, not a sudden, short-term dip. The result is generally interpreted alongside your red blood cell size from a CBC, since B12 deficiency has a distinctive effect on how red cells form.
B12 deficiency can come from several different directions: not getting enough from diet (more common in strict vegetarians and vegans, since B12 occurs naturally mainly in animal products), or the body failing to absorb it properly even when intake is fine — a problem more common in older adults and people with certain digestive conditions. Distinguishing between these two causes usually needs more than the B12 level alone, since the treatment differs depending on which one applies.
This test is most often ordered once other blood work already points toward a specific pattern, called megaloblastic anaemia, where red blood cells form unusually large — a distinctive change that low B12 (or, separately, low folate) tends to cause. Your doctor may also recommend the test directly if you have certain nervous system symptoms, since B12 plays a role in nerve health as well as red cell formation, without waiting for a CBC to raise the flag first.
Symptoms of low B12 can include numbness or tingling in the arms and legs, weakness, and loss of balance, though many people have no symptoms at all until levels are quite low, which is part of why the test is often ordered on the strength of a lab finding rather than symptoms alone. Most labs ask for 6 to 8 hours of fasting before this test, and certain medicines can affect the result, so it's worth telling your doctor about anything you're taking before the draw. The sample is a routine blood draw from a vein.
Most labs recommend not eating or drinking for 6 to 8 hours beforehand, since food intake and certain medicines can affect the result.
B12 deficiency causes a specific pattern — unusually large red blood cells — which a CBC can flag first. The B12 test then confirms whether that's actually the cause.
Strict vegetarians and vegans, since B12 occurs naturally mainly in animal products, and older adults, whose ability to absorb B12 from food tends to decline with age.
Yes — tingling or numbness in the hands and feet and balance problems are well-recognised symptoms of B12 deficiency, and they're a common reason doctors test for it directly.
It's commonly ordered alongside a folic acid test, since deficiencies in either can cause a similar pattern of anaemia, and telling them apart matters for treatment.
Yes, certain common medicines can lower B12 levels or interfere with the test itself, so it's worth telling your doctor everything you're currently taking before the draw.
Not always — in some cases levels can appear normal even with early functional deficiency, which is why doctors weigh the result alongside your symptoms rather than in isolation.
Treatment depends on the cause — dietary deficiency is often managed with supplements, while an absorption problem may need a different approach, such as injections. Your doctor decides based on the full picture, not the B12 level alone.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
