Vitamin B12

Measured in pg/mL

A measure of total B12 in your blood, including forms your cells can't readily use — so it's an imperfect read on whether B12 is actually reaching your tissues.

What it is

Vitamin B12 (cobalamin) matters for nerve function and making red blood cells, and this test measures the total amount circulating in your blood. The limitation is that most of that total is bound to a carrier protein that doesn't deliver B12 to cells, so a serum B12 can look reassuring while cells are still short — and it can also read high for reasons unrelated to your B12 status, such as liver or blood conditions. When the picture is unclear, clinicians sometimes look at related markers like methylmalonic acid (MMA) or homocysteine, which reflect whether B12 is doing its job, and read B12 alongside folate and a blood count.

Typical range

  • Adults180–914 pg/mL

Source · Reference-lab convention (Mayo; ARUP concordant) (2026)

Reference ranges vary between laboratories, and what is normal for you depends on your own history. Your own lab’s range is the one printed on your report.

Worth asking your clinician

  • Given how I've been feeling, is my serum B12 a reliable read, or would markers like MMA or homocysteine add useful context?
  • Could anything I'm taking or any part of my health history be affecting this number in either direction?
  • How does this sit alongside my folate result and my blood count?