You feel the symptoms—the fatigue, the brain fog, the tingling—and you know something is wrong. The next step is getting a definitive diagnosis, but navigating medical testing can be confusing. To be your own best advocate, you need to know exactly what to ask for.
Simply asking your doctor to “check your B12” might not be enough. Let’s walk through the specific blood tests that give a complete picture of your B12 status, so you can have an informed and productive conversation with your healthcare provider.
The Starting Point: The Standard Serum B12 Test
This is the most common test doctors order. It measures the total amount of vitamin B12 circulating in your bloodstream.
- What it is: A blood draw to measure
Serum Cobalamin. - The Limitation: This test can be misleading. It measures all B12 in the blood, including both the “active” form your body can use and the “inactive” form it can’t. It’s possible to have symptoms of a deficiency even if your serum B12 level falls within the “normal” lab range, a topic we cover in depth separately.
For bariatric patients, a “low-normal” result on a serum B12 test should never be considered acceptable. Your clinical team should be aiming for levels in the upper-end of the normal range or higher.
The Gold Standard: Functional Tests for a Clearer Answer
If your symptoms persist despite “normal” serum B12 levels, or if you want a more accurate picture from the start, you should request functional B12 tests. These don’t just measure the amount of B12 in your blood; they measure whether your body is actually using it at the cellular level.
The two most important functional tests are:
- Methylmalonic Acid (MMA): This is the most sensitive and specific test for B12 deficiency. When your cells don’t have enough active B12, levels of MMA build up in the blood and urine. A high MMA level is a direct indicator of a problem.
- Homocysteine: Like MMA, homocysteine is an amino acid that requires B12 to be metabolized. High levels can indicate a deficiency in B12 (or folate). While less specific than MMA, an elevated homocysteine level is another strong clue.
Comparing the Tests: Your Doctor’s Visit Toolkit
| Test Name | What It Measures | What a “Bad” Result Means | Why It’s Useful |
|---|---|---|---|
| Serum B12 (Cobalamin) | Total B12 in the blood. | Low or “low-normal” levels. | A good starting point, but can be insufficient on its own. |
| Methylmalonic Acid (MMA) | A substance that builds up when B12 is deficient at the cellular level. | High levels. | The most sensitive test. It confirms a functional deficiency, even if serum B12 is normal. |
| Homocysteine | An amino acid that builds up when B12 or folate is low. | High levels. | A strong supporting indicator of a B12 or folate deficiency. |
Your Action Plan:
When you see your doctor, go in prepared. Say, “Given my bariatric surgery, I’m concerned about a functional B12 deficiency. In addition to a serum B12, I would like to have my Methylmalonic Acid (MMA) and Homocysteine levels tested to get a complete picture.”
