Patients with combined deficiency of folate and iron may not demonstrate the erythrocyte macrocytosis otherwise typical of folate deficiency anemia. In these patients, however, the red cell distribution width (RDW) will typically be elevated.
A non-fasting specimen results in falsely elevated results.
Patients taking folate may have misleading results.
Folates other than (N)-5-methyltetrahydrofolate and folic acid antagonists (such as methotrexate) may, under some circumstances, be present in serum and will also be measured by this method.
The analytic variability (CV) of both serum and red blood cell folate assays is considerable. Homocysteine and methylmalonic acid levels are alternate assays for folate deficiency.
Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedures, may have circulating anti-animal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.