Vitamin B12
What it measures, and what changes the number.
Vitamin B12 (cobalamin) is needed for making red blood cells and for nerve function. This test measures the total amount of B12 in serum, including both the portion available to cells and the portion bound to carrier proteins that cells cannot readily use.
It is usually ordered with folate, since the two vitamins work together and affect red cells in similar ways, and read alongside the blood count, especially MCV and hemoglobin. Ferritin is often checked at the same time when red cell changes are being looked into.
Most labs use immunoassays, and results can differ between platforms. Recent supplements or injections raise the measured level, and certain antibodies in the blood can interfere with some assays. Fasting is sometimes requested. Spanish labs may print it as Vitamina B12.
Reference ranges labs publish
Each lab states an interval for its own method and the population it measured. They differ, they are not interchangeable, and none of them is a verdict on a single result.
- Everyone (not split by sex or age): 180 to 914 ng/L · Immunoenzymatic assay · Source: Mayo Clinic Laboratories, B12 (Vitamin B12 Assay, Serum) (read 2026-09-24)
- Everyone (not split by sex or age): 232 to 1245 pg/mL · ECLIA (immunoassay) · Source: Labcorp, 001503 (Vitamin B12) (read 2026-09-24)
Often read alongside: Folate, MCV, Hemoglobin, Ferritin, Red blood cells