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From , former About.com Guide

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Vitamin B-12 level

Overview Risks Results
What the risks are:
Risks associated with having blood drawn are slight:
  • excessive bleeding
  • fainting or feeling light-headed
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures to locate veins
Special considerations:
In testing for megaloblastic anemias of any sort, serum or red blood-cell folate levels are usually also obtained.

The blood test for levels of vitamin B-12 has become much more accurate within the past few years. Now, there are fewer false-normal results, because only biologically active B-12 is measured.

The cause of a vitamin B-12 deficiency is usually determined by the Schilling test.

Hemolysis of the blood sample (rupture of some of the red blood cells) may affect test results.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
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