What this test measures
Vitamin B12 is required for two jobs: building red blood cells, and maintaining the myelin sheath that insulates nerves. When it runs short, both suffer — but on different timescales, and that is what makes this test worth understanding.
The standard test measures total B12 in the blood. That total includes B12 bound to haptocorrin, which the body cannot use, and a smaller fraction bound to transcobalamin, which is the part cells actually take up. Since the usable fraction is the minority of the total, a serum level sitting just inside the normal range can still leave cells short. This is the reason the test has a grey zone rather than a clean line.
B12 comes almost exclusively from animal foods. It is manufactured by bacteria, and humans obtain it through meat, fish, eggs and dairy. Plant foods contain essentially none in a usable form, and fermented foods and algae contain analogues that occupy the same receptors without doing the same work.
That single fact explains why deficiency is so common in India. A large share of the population eats vegetarian or near-vegetarian, dairy intake is often modest, and the result has been documented repeatedly: Indian studies of urban and rural adults have found low B12 in a very large proportion of those tested — figures well above what most Western populations report. Many of those people have no symptoms and no anaemia.
Diet is not the only route. Absorption requires stomach acid and intrinsic factor, so long-term acid-suppressing medication, H. pylori infection, autoimmune gastritis, coeliac disease and gut surgery all interfere. Metformin, taken by an enormous number of people in India, reduces absorption progressively over years.
What your result means
Read against the range on your own report — assays differ, and so do their lower limits.
Below 200 pg/mL is the usual definition of deficiency. It calls for a cause to be identified, not just a level to be raised.
Between 200 and 300 pg/mL is the indeterminate band, and it is where most confusion lives. Some people at 250 pg/mL have entirely normal cellular B12 activity; others have measurable deficiency. Symptoms, diet, medication and a homocysteine or MMA level decide it. A result in this band with tingling, poor concentration or a sore tongue is treated as a question worth pursuing.
Above 300 pg/mL is generally taken as adequate in someone without symptoms.
A high B12 is almost invariably supplementation, and is not itself a concern. Very high values in someone taking nothing are occasionally significant and warrant a look at liver and kidney function.
Some caveats worth carrying. Any recent supplement or injection makes the serum value unreadable for interpretation purposes — it reflects the dose, not your status. Folate supplementation can normalise the blood count while nerve involvement continues, which is why the two are measured together. Pregnancy lowers serum B12 without necessarily indicating deficiency. And in India, coexisting iron deficiency frequently cancels out the enlarged red cells that B12 deficiency would otherwise produce, leaving a normal-looking MCV and a falsely reassuring blood count.
The neurological symptoms are the ones to take seriously, because they can appear before any change in the blood count and recover less completely once established.
What each value means
Reference ranges differ between laboratories. Always read your result against the range printed on your own report.
Vitamin B12 (serum)
Reference range
200 – 900 pg/mL
- What it means
- Total B12 circulating in blood. Indian laboratories commonly print a range of about 190–900 pg/mL, though it varies by assay. Values below 200 pg/mL indicate deficiency; 200 to 300 pg/mL is an indeterminate band where symptoms can be present and further testing is often used. pg/mL and ng/L are the same unit written two ways.
- Why it matters
- 178 pg/mL sits below the usual lower limit. In India this is an ordinary result rather than an unusual one — surveys of urban vegetarian populations have found low B12 in a very large share of adults, because the vitamin is made by bacteria and reaches humans almost entirely through animal foods. Symptoms are gradual and easy to attribute elsewhere: tingling in the hands or feet, poor concentration, sore tongue, low mood, unsteadiness.
- Next step
- Have it read alongside a full blood count, ferritin and folate, and consider homocysteine if the value is borderline or the picture is unclear. The cause matters — diet, absorption, metformin and long-term acid suppression each lead somewhere different. Take no decision about supplementation without your doctor.
Interpret in context. One result doesn't tell the whole story.
Homocysteine
5 – 15 µmol/L
An amino acid that accumulates when B12 or folate is short, because both are needed to clear it. A raised homocysteine supports genuine deficiency when the serum B12 is borderline. It also rises with kidney impairment, low folate and certain genetic variants, so it is supportive rather than conclusive.
Methylmalonic acid (MMA)
70 – 380 nmol/L
The most specific available marker of B12 activity inside cells. It rises in B12 deficiency but not in folate deficiency, which is what makes it useful. It is expensive, offered by few Indian laboratories, and reserved for genuinely ambiguous cases. Reference limits differ between laboratories more than for most tests.
Serum folate
4 – 20 ng/mL
Measured alongside B12 because the two deficiencies produce a similar blood picture and similar neurological complaints, and because correcting folate without addressing a B12 deficiency can allow nerve damage to progress while the blood count improves.
Mean corpuscular volume (MCV)
80 – 100 fL
Average red cell size, from your routine blood count. B12 deficiency enlarges red cells and iron deficiency shrinks them. In India the two frequently occur together, and the MCV can land squarely in the normal range while both deficiencies are present — which is why a normal blood count is weak evidence against B12 deficiency here.
This is general education and cannot account for your history, your examination or your reports. If you need advice specific to your health, we’re always happy to see you in consultation.
What to do next
Find out why. Diet is the commonest explanation in India, but it is worth confirming rather than assuming. Long-term acid suppression, metformin, gut surgery, coeliac disease and autoimmune gastritis each point somewhere different, and the treatment differs accordingly — a dietary shortfall and a failure of absorption are not addressed the same way.
Have it read with company. A full blood count, ferritin and folate at minimum. Combined iron and B12 deficiency is common enough here that testing only one regularly misses the other.
Do not start B12 on your own, and do not stop anything either. Once supplementation begins, the serum level stops being interpretable for months, which can close off the chance of establishing what was actually happening. The dose, the route — oral or injection — and the duration depend on the cause and on whether nerves are involved. That decision belongs with your doctor.
Bring a full list of what you take. Multivitamins, energy tonics and fortified drinks frequently contain B12 and quietly explain both an unexpectedly high result and a deficiency that resolved without anyone knowing why.
Ask about timing for a recheck. Levels rise quickly with treatment but symptoms improve slowly, and nerve recovery is measured in months. Rechecking too early tends to produce a reassuring number and a disappointing week.
This information is educational and not a diagnosis.
Alitheau Learning Sessions
Understanding Blood Reports
- Why reference ranges differ between laboratories, and what that means for you
- How to read a liver function test, and why the pattern beats the worst number
- What HbA1c actually measures, and the common things that distort it
One result rarely tells the whole story
Have this read alongside everything else.
Conditions this result comes up in
Questions people ask
Below about 200 pg/mL is generally reported as deficient, and 200 to 300 pg/mL as borderline or indeterminate. The exact lower limit depends on the assay your laboratory uses, so read yours against the range on your own report. A borderline value with typical symptoms is often investigated further rather than dismissed.
Vitamin B12 is produced by bacteria and reaches humans almost entirely through animal foods — meat, fish, eggs and dairy. A large proportion of the Indian population eats vegetarian or near-vegetarian diets, and dairy intake is often too small to close the gap. Indian studies have repeatedly reported low B12 in a substantial share of adults, including in people with no symptoms at all.
It can. Serum B12 measures the total in circulation, and most of that is bound to a protein that cells cannot use. Someone can have an apparently adequate serum level with reduced activity inside the cells. Homocysteine and methylmalonic acid are the tests used to look at that, and they are why a normal-looking result with clear symptoms is sometimes investigated further.
Yes. Long-term metformin reduces B12 absorption, and the effect grows with dose and duration. Because metformin is so widely prescribed in India, periodic B12 checks are common practice for people taking it. This is a reason to discuss monitoring with your doctor, not a reason to change your medicine.
No. Fasting is only requested when B12 is drawn with glucose or a lipid profile. It is worth telling the laboratory if you have taken a B12 supplement or injection recently, since either raises the measured level for a considerable time and makes the result hard to interpret.