A serum vitamin B12 level below 200 pg/mL (148 pmol/L) is generally considered low by most laboratories, though some use a cutoff of 250 pg/mL (185 pmol/L). The exact number that flags as “low” on your lab report depends on which lab processed your blood, because there is no single universal standard. What makes this tricky is that a significant number of people with levels above 200 pg/mL can still be functionally deficient, meaning their bodies aren’t getting enough usable B12 even though the number looks acceptable.
Standard Cutoffs for B12 Levels
Most labs in the United States define a normal serum B12 as roughly 200 to 900 pg/mL, with anything below 200 pg/mL flagged as subnormal. Some labs set the lower boundary at 250 pg/mL instead. In the UK, guidelines use a slightly different unit (pmol/L) and place the deficiency threshold at around 148 pmol/L, which is equivalent to about 200 pg/mL.
Here’s how the ranges generally break down:
- Below 200 pg/mL (148 pmol/L): Considered deficient by most labs. Treatment is typically recommended.
- 200 to 300 pg/mL (148 to 220 pmol/L): A gray zone, sometimes called borderline or marginally deficient. Additional testing is often needed.
- Above 300 pg/mL (220 pmol/L): Generally considered sufficient for most people.
These numbers measure the total amount of B12 circulating in your blood, but they don’t tell the whole story about how much your cells are actually using. That’s why a “normal” result doesn’t always mean you’re in the clear, and why doctors sometimes order follow-up tests.
The Borderline Range: 200 to 400 pg/mL
The range between roughly 200 and 400 pg/mL is where things get complicated. Your lab report may say “normal,” but the NIH recommends that anyone with a level between 150 and 399 pg/mL have a follow-up test for a substance called methylmalonic acid (MMA) if deficiency is suspected. MMA builds up in the blood when your body doesn’t have enough B12 to run certain chemical reactions, so it acts as a more sensitive signal of true deficiency. A normal MMA level is usually under 0.40 µmol/L.
Homocysteine is another marker that rises when B12 is low. Levels above 12 µmol/L can suggest a problem, though high homocysteine can also reflect low folate rather than low B12 specifically. When both MMA and homocysteine are elevated alongside a borderline B12 level, the case for genuine deficiency is strong. When MMA is normal, a borderline B12 number is less likely to be causing problems.
Why Your Level Might Read Lower Than Expected
Several common situations can push B12 levels down without necessarily meaning you have a true deficiency. Oral contraceptives and hormone replacement therapy are known to lower measured B12 in the blood, and UK clinical guidelines note these results are generally not clinically significant on their own. Pregnancy causes a natural decline as well. In one study of healthy pregnant women, average B12 dropped from about 374 pg/mL in the first trimester to 305 pg/mL in the third trimester, and the proportion of women with levels under 200 pg/mL jumped from about 3% to nearly 18%.
Two widely prescribed medication types deserve attention. Metformin, the most common drug for type 2 diabetes, reduces B12 levels in 10 to 30% of people who take it. Proton pump inhibitors (PPIs), used for acid reflux and ulcers, can also lower B12 because stomach acid is needed to release the vitamin from food. Taking both medications together compounds the risk. If you’ve been on either for more than a year and notice symptoms, testing is reasonable.
Symptoms Don’t Always Match the Number
B12 deficiency can show up as blood-related problems, nerve problems, or both, and the two don’t always travel together. Some people develop anemia (specifically a type where red blood cells are abnormally large) while their nervous system is fine. Others develop tingling, numbness, balance problems, or cognitive changes with a completely normal blood count. This disconnect is one reason a single B12 number can be misleading.
Neurological symptoms can appear at B12 levels that some labs would still call “normal.” There is no clean threshold below which nerve damage starts and above which it doesn’t. What is clear is that untreated deficiency, particularly the kind caused by an absorption problem rather than a dietary gap, can lead to irreversible nerve damage over time. People often notice that nerve-related symptoms take longer to improve with treatment than blood-related ones, and in some cases they persist even after blood counts return to normal.
B12 Levels in Older Adults
Deficiency becomes far more common with age, largely because the stomach produces less acid and less of a protein called intrinsic factor, both of which are essential for absorbing B12 from food. In one study of adults over 60 in an Indian community, 63% had B12 levels below 110 pmol/L (roughly 150 pg/mL), and another 33% fell in the borderline range. Researchers have suggested that routine screening makes sense for adults 75 and older, with levels below 150 pmol/L (about 200 pg/mL) treated as deficient and those between 150 and 250 pmol/L (200 to 340 pg/mL) warranting additional testing.
The challenge in older adults is that symptoms of B12 deficiency, such as memory problems, unsteady walking, and fatigue, overlap heavily with normal aging and other conditions like dementia. This makes the lab number especially important in this age group, even when symptoms seem vague or attributable to something else.
When Absorption Is the Problem
If your B12 is genuinely low, figuring out why matters as much as the number itself. Dietary insufficiency is the most straightforward cause: B12 is found almost exclusively in animal products, so vegans and strict vegetarians are at inherent risk. But many people with low B12 eat plenty of meat and dairy, and their issue is absorption.
The most well-known absorption problem is pernicious anemia, an autoimmune condition where the body attacks the stomach cells that produce intrinsic factor. Without intrinsic factor, B12 can’t cross from the gut into the bloodstream no matter how much you consume. A blood test for anti-intrinsic factor antibodies catches about 90% of pernicious anemia cases, but a negative result doesn’t fully rule it out. Other absorption issues include celiac disease, Crohn’s disease, and any surgery that removed part of the stomach or small intestine.
The distinction matters for treatment. A dietary shortfall can often be corrected with oral supplements or fortified foods. An absorption problem typically requires higher-dose oral supplementation or injections that bypass the gut entirely, often on an ongoing basis. Once you’re receiving B12 replacement, retesting the B12 level itself is generally unnecessary. Instead, your doctor will monitor your blood counts and symptom improvement to gauge whether treatment is working.