Diagnostic testing for suspected vitamin B12 or folate deficiency

Some clinicians may choose an alternate testing algorithm depending on patient factors. UpToDate topics on vitamin B12 and folate deficiency discuss the presenting findings, diagnostic approach, differential diagnosis, and treatment in more detail, as well as additional post-diagnostic testing for the underlying causes of these deficiencies. Refer to laboratory-specific lower limits of normal.

Typical values for vitamin B12 are as follows:
- Deficient: <200 pg/mL
- Borderline: 200 to 300 pg/mL
- Normal: >300 pg/mL
Typical values for folate are as follows:
- Deficient: <2 ng/mL
- Borderline: 2 to 4 ng/mL
- Normal: >4 ng/mL

MMA: methylmalonic acid; RBC: red blood cell.
* Folate testing may be omitted if diet and gastrointestinal anatomy and function are normal. If dietary folate deficiency is suspected in a patient who has recently received a normal meal, RBC folate should be measured instead of serum folate. If one level is deficient and the other is borderline, then it may be necessary to follow more than one diagnostic path (eg, if folate is deficient and vitamin B12 is borderline, then folate deficiency may be confirmed but MMA and homocysteine testing may be required to determine vitamin B12 status). Another alternative in this setting would be to administer both vitamins.
¶ Additional testing may be appropriate. Examples include testing of MMA and homocysteine levels to further support a diagnosis of vitamin B12 deficiency; testing for autoantibodies to intrinsic factor if there is vitamin B12 deficiency not attributable to a known gastrointestinal condition; or screening endoscopy for malignancy in individuals with newly diagnosed pernicious anemia.

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Diagnostic testing for suspected vitamin B12 or folate deficiency