After a probable diagnosis of megaloblastic anemia is established, specific laboratory tests are needed to determine the cause. Table 5 displays the specific tests and results to determine Vitamin B12 deficiency versus Folate deficiency. Once again, the general causes of megaloblastic anemia include:
- B12 or folate deficiencies
- Pernicious anemia (autoimmune form of B12 deficiency)
- Other causes not due to B12 or folate deficiency (usually certain drugs)
Table 5. Test Results in B12 and Folate Deficiency. | Vitamin B12 Deficiency | Folate Deficiency |
| Decreased Vitamin B12 levels | Decreased Folate levels |
| Increased MMA* | Normal MMA |
| Increased Homocysteine | Increased Homocysteine |
*serum methylmalonic acid (MMA)
MMA and homocysteine are very important for a definitive diagnosis of B12 deficiency because there are certain diseases and situations (e.g., certain cancers, hematologic malignancies, and kidney disease) where a person has normal or even elevated B12 levels yet has an actual functional defect—that is, the B12 is there but not functioning properly. These tests are useful because Vitamin B12 is required for methylmalonic acid (MMA) to be converted to succinyl-CoA, and in combination with folic acid, for homocysteine (HC) to be converted to methionine. Inadequate B12 levels or faulty function of B12 can cause accumulation of MMA and HC. Thus, MMA and HC are elevated.
MMA and HC are further useful for the differential diagnosis of B12 and folate deficiency; in B12 deficiency, both are elevated, whereas in folate deficiency, usually only HC is elevated. Another test, holotranscobalamin (holoTC), is sometimes used in cases that are difficult to diagnose. HoloTC is the active form of B12 and is decreased in B12 deficiencies.
If a diagnosis of B12 or folate deficiency is established, the next step is to determine the reason. Possible reasons/causes for low B12 are lack of dietary intake, increased need in growth or pregnancy, diseases of the GI tract resulting in malabsorption of B12, certain drugs, and the autoimmune disease known as pernicious anemia (covered on the next page). Folate deficiency can be caused by a lack of dietary intake, increased need, malabsorption, etc.
