Vitamin B12 (Serum or Plasma, Immunoassay)

Why Is This Test Done?

Your doctor may order this test if you have symptoms like fatigue, weakness, tingling in hands/feet, or unexplained anemia. It’s also used to check for pernicious anemia, vitamin B12 deficiency, or malabsorption issues (e.g., from celiac disease or atrophic gastritis).

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the draw if your lab uses a specific protocol. Bring any medications you’re currently taking, as some (like certain blood thinners) may affect results. Avoid smoking for 30 minutes before testing if instructed by your provider.

What Happens During the Test

A healthcare professional will draw blood from a vein in your arm. You may feel a small pinch, but it usually takes only a few minutes. The blood sample is sent to a lab for analysis using an immunoassay method to detect vitamin B12 levels.

What the Results Mean

Results are compared to a reference range, but exact values vary by lab. Low levels may indicate deficiency, while high levels could suggest conditions like certain cancers or kidney disease. Your doctor will interpret results in the context of your symptoms and medical history.