B12 testing — what to ask your doctor

If you’ve been vegan for more than a year and have never supplemented, or if you have any symptoms that could be B12-related, you should be tested. Here’s what to ask for, what the numbers mean, and why the “standard” B12 test alone is often not enough.

The four relevant tests

1. Serum B12 (total cobalamin) — the standard, widely available, cheap test.
2. Methylmalonic acid (MMA) — rises when intracellular B12 is functionally low. The most specific marker of true deficiency.
3. Homocysteine — rises when methionine synthase (the B12-dependent enzyme) is impaired. Elevated homocysteine also independently raises cardiovascular risk.
4. Holotranscobalamin (holoTC, or “active B12”) — measures the biologically active fraction of circulating B12. Increasingly available; arguably the best early indicator.

What each number means

Serum B12

• Under 200 pg/mL — deficient. Supplement and retest in 3 months.
• 200–300 pg/mL — borderline. Confirm with MMA or holoTC.
• 300–500 pg/mL — adequate to comfortable.
• Over 500 pg/mL — optimal for many vegans.

Serum B12 is imperfect because it measures both the active fraction and inactive B12 analogues. A “normal” serum B12 can mask functional deficiency, especially in people who consume algae, spirulina, or some fermented products. Hence the secondary tests.

MMA

• Normal: 0.07–0.27 µmol/L (lab-specific — check the reference range on your report).
• Elevated MMA with low-normal or low serum B12 confirms B12 deficiency at the cellular level.
• MMA is a “downstream” marker — it rises when B12-dependent enzymes aren’t functioning, regardless of what serum B12 says.

Homocysteine

• Optimal: below 10 µmol/L
• Borderline: 10–15 µmol/L
• Elevated: above 15 µmol/L

Homocysteine can also rise from folate deficiency, renal impairment, or certain genetic polymorphisms, so it isn’t B12-specific. Interpret in combination with MMA.

HoloTC

• Under 35 pmol/L: possible deficiency; follow up with MMA.
• 35–50 pmol/L: borderline.
• Over 50 pmol/L: adequate.

HoloTC reflects active B12 available for tissue use. It drops earlier than serum B12 in developing deficiency, making it a better screening test — if available.

How to ask for the right tests

Many doctors will default to a generic “CBC plus B12” panel. If you want a complete picture, say:

“I’ve been on a plant-based diet for X years. I’d like to check my B12 status. Please order serum B12, MMA, and homocysteine. If holoTC is available at this lab, I’d like that instead of or in addition to serum B12.”

Some U.S. labs code these as:

• Vitamin B12 — CPT 82607
• MMA, serum — CPT 83921
• Homocysteine — CPT 83090
• HoloTC — CPT 82608 (availability varies)

Insurance coverage varies. If out-of-pocket, expect roughly $20–50 for serum B12, $60–120 for MMA, $40–80 for homocysteine. Direct-to-consumer services like Quest and Labcorp offer these without a doctor’s referral in many U.S. states.

Timing matters

• Don’t supplement for at least 3–4 days before testing if you want an accurate serum B12 reading. Recent supplementation artificially elevates the number.
• MMA is less sensitive to recent supplementation; it reflects cellular status over weeks.
• Fasting isn’t required for B12 but often is for the associated metabolic panel — follow the lab’s instructions.

Interpreting the combined picture

Serum B12	MMA	HoloTC	Interpretation
Normal	Normal	Normal	Adequate
Low	High	Low	True deficiency — supplement and retest
Low-normal	Normal	Normal	Likely adequate at cellular level
Normal	High	Low	Functional deficiency despite “normal” serum
High (>1000)	Variable	Variable	Usually supplementation artifact; sometimes indicates other conditions

A high serum B12 is rarely concerning on its own in supplementing vegans — it reflects the supplement, not disease. However, persistently extreme serum B12 without supplementation may indicate liver disease or certain cancers and warrants follow-up.

What to do with the results

• Adequate: keep supplementing per B12 dosage for adults. Retest every 2–3 years or sooner if symptoms appear.
• Borderline: start or increase supplementation; retest in 3 months.
• Deficient: discuss with your doctor. Oral high-dose therapy may suffice; clinical deficiency often warrants injections for 1–8 weeks followed by oral maintenance.

Common misconceptions

• “Normal serum B12 means I’m fine.” Not necessarily. Functional deficiency is possible with normal serum B12 but elevated MMA.
• “I should stop my supplement before testing so I get a ‘true’ number.” Stop for 3–4 days, not longer. You don’t want to induce deficiency to measure it.
• “A cheap at-home fingerprick kit is enough.” Some are acceptable for serum B12 screening; none currently measure MMA or holoTC reliably.

The punchline

If you’re tested once, ask for serum B12 plus MMA at minimum. That combination catches nearly all deficiency cases that a single serum B12 would miss. Everything else is nuance.

For the full picture, see Vitamin B12. For what deficiency feels like if you haven’t tested, see B12 deficiency symptoms.