Vitamin D and vitamin B12 reports: what the tests measure and their units

Why these two tests are so common

Vitamin D and vitamin B12 are among the most frequently requested tests in India, often as part of a health check package. Both are important for the body, and low levels are common, but the tests are also easy to over-read. A result printed in red does not by itself mean you need a high-dose supplement or an injection, and a number inside the range does not always mean there is no deficiency.

This guide explains what each test measures, what units you may meet, why interpretations differ, and what a doctor considers before acting on the result.

What the vitamin D test measures

The vitamin D test on your report is almost always 25-hydroxyvitamin D, written 25(OH)D. It is the main circulating form, made from vitamin D that the skin produces with sunlight and from vitamin D in food and supplements. It is a better indicator of the body’s stores than the active hormone form, which the body tightly regulates.

Vitamin D helps the gut absorb calcium and supports bone strength. Deficiency over a long period can cause weak or soft bones, bone pain and muscle weakness, but many people with low values have no symptoms. Common causes of low levels include little skin exposure to midday sunlight, darker skin (which makes less vitamin D for the same exposure), covering clothing, indoor lifestyles, air pollution, obesity and some medical conditions. Indian studies have repeatedly found low vitamin D to be common despite the plentiful sunshine.

What the vitamin B12 test measures

Vitamin B12 (cobalamin) is needed to make red blood cells and DNA and to keep nerves working properly. The usual test is serum vitamin B12, the total amount in the blood. B12 is found naturally mainly in animal foods such as meat, fish, eggs and dairy, so people who follow vegetarian or vegan diets are more likely to run low, as are older adults, people with certain stomach or bowel conditions, and people taking some long-term medicines, including metformin and acid-reducing tablets (proton pump inhibitors and similar drugs).

Low B12 can cause anaemia with large red cells (a high MCV on a blood count), tiredness, tingling or numbness in the hands and feet, balance problems or memory changes, but symptoms develop slowly and can be mild.

Units and conversions

Both tests exist in two sets of units, and mixing them up is a common source of confusion.

For example, a vitamin D of 20 ng/mL is about 50 nmol/L, and 30 ng/mL is about 75 nmol/L. A B12 of 300 pg/mL is about 221 pmol/L, and 200 pg/mL is about 148 pmol/L. If you compare your result with an article from the UK or Europe, check which unit it uses, because the number is 2.5 times larger for vitamin D in the other unit.

Why cut-offs disagree

This is the main reason the same result can be called “deficient” on one report and “adequate” on another.

For vitamin D, authoritative bodies describe the thresholds differently. The US National Institutes of Health’s Office of Dietary Supplements, summarising the Institute of Medicine, describes below 12 ng/mL (30 nmol/L) as a level at which deficiency is a risk and 20 ng/mL (50 nmol/L) or more as enough for most people. Many laboratories, following other societies, print 30 ng/mL (75 nmol/L) as the lower limit of “sufficient”, and call 20 to 30 “insufficient”. Such differences are the result of expert opinion about where benefit begins, and they are not a measurement error.

For vitamin B12, many laboratories print a normal range of about 200 to 900 pg/mL (roughly 150 to 665 pmol/L). Values below about 200 pg/mL are commonly regarded as low. Values from about 200 to 300 pg/mL are often called borderline, because some people in this band still have a functional deficiency. In that case doctors may add a second test, such as methylmalonic acid (MMA) or homocysteine, which rise when B12 is not working properly in the cells.

The test method also matters. Different instruments give different B12 values for the same sample, and a high-dose supplement taken shortly before testing can push the level up temporarily.

Factors that change the result

• Season and sunlight: vitamin D tends to be higher at the end of the sunny months and lower afterwards.
• Supplements: a recent vitamin D or B12 supplement, including multivitamins, raises the level.
• Biotin supplements can interfere with some immunoassay methods used for these tests. Tell the laboratory what you take.
• Fasting is not normally required for vitamin D. Some laboratories ask for fasting before a B12 test, so follow the instructions you were given.
• Pregnancy, liver or kidney disease and some medicines can alter both tests.

Because the result depends on what you have taken and when, a recent change of diet or supplements should be mentioned to your doctor.

What doctors consider next

A low result is the start of a question: why is it low, and what should be done? Doctors look at symptoms, diet, medicines, other blood results and, for B12, the blood count. They may repeat a test, check calcium, kidney function or parathyroid hormone for vitamin D, or do further tests for B12. Treatment, if needed, varies widely by the level of deficiency, the cause and the person, so this guide does not give amounts. Please avoid taking high doses from the internet, from a friend’s prescription or in several products at once: too much vitamin D can raise blood calcium to harmful levels, and some B12 forms do not suit everyone.

If treatment is started, doctors usually repeat the test after some weeks or months to check that it is working, and often adjust the plan.

Food, sunlight and lifestyle in everyday terms

Doctors usually discuss the cause as well as the number. For vitamin D, the main natural source is the skin’s response to sunlight, and the amount made depends on the time of day, season, skin colour, how much skin is uncovered, sunscreen use and age. Food sources are limited: oily fish, egg yolk, some fortified products and, in some countries, fortified milk or oils. For vitamin B12, the main sources are meat, fish, eggs and dairy products, and some foods are fortified. People who eat no animal products at all generally need a reliable source, and doctors can advise on this.

None of this means you should change your diet or supplements by yourself on the basis of one report. It is a reason to talk about your habits with your doctor, so that the plan fits your life, your culture and your health conditions, and so that the test can be repeated at a sensible time to see if it has worked.

Do you need these tests at all?

Vitamin D and B12 are often included in health-check packages for people with no symptoms. Authorities differ on whether routine testing of well people is useful, and some guidelines advise testing only people with symptoms or specific risks, such as long-term metformin use, bowel or stomach surgery, certain bone conditions or kidney disease. If a package includes these tests, it is reasonable to ask your doctor what you would do differently depending on the result.

A worked example (example values)

Imagine example reports: vitamin D 14 ng/mL and vitamin B12 240 pg/mL, with a laboratory range for vitamin D of 30 to 100 ng/mL and for B12 of 211 to 911 pg/mL.

Vitamin D at 14 ng/mL equals about 35 nmol/L, which is low on most scales, including the stricter ones. A doctor would likely discuss a plan and ask about diet, sun exposure and bone symptoms.

The B12 value of 240 pg/mL (about 177 pmol/L) is inside this laboratory’s range, so it is not flagged. Yet it is in the borderline band. If the person were vegetarian, had tingling in the feet or a high MCV on their blood count, the doctor might order an MMA or homocysteine test or consider a trial of treatment. Without those clues the doctor might simply recheck later. That is the difference between reading a flag and interpreting a result.

Questions worth asking your doctor

• Is my level low enough to treat, and by which cut-off?
• What may be causing it in my case?
• Do I need any follow-up tests, such as calcium, kidney function or MMA?
• What should I take, in what form, and for how long?
• When should I be retested, and should I fast for it?

Sources

• MedlinePlus: Vitamin D Test — what the 25-hydroxy test shows
• MedlinePlus: Vitamin B Test — B12 and folate testing
• NHS: Vitamin B12 or folate deficiency anaemia — causes, symptoms and diagnosis
• NHS: Vitamin D — sources, deficiency and safe intakes
• NIH Office of Dietary Supplements: Vitamin D fact sheet for consumers — blood level thresholds and ng/mL to nmol/L conversion
• Testing.com: Vitamin B12 test — results and borderline levels

A note on this guide

This guide is general information about vitamin D and B12 reports. It is not medical advice and does not recommend any supplement or dose. Confirm what your own results mean, and what to take, with your doctor.