Tests
1st tests to order
peripheral blood smear
Test
Macrocytic anemia and hypersegmented neutrophils are classically seen in folate and vitamin B12 (cobalamin) deficiencies. Macrocytes are seen in early folate deficiency, but they can also be present in other conditions.[Figure caption and citation for the preceding image starts]: Megaloblastic macrocytic anemia: A. Peripheral blood smear of a patient with megaloblastic anemia. B. Peripheral blood smear of healthy individualPhotomicrograph from Mark J. Koury, MD; used with permission [Citation ends].
Result
macrocytosis, anisocytosis, poikilocytosis, hypersegmented neutrophils
CBC
Test
Elevated mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) can precede anemia by a few weeks; thrombocytopenia and neutropenia are seen in advanced cases.[37] Can be seen in other conditions. Not useful to rule out folate deficiency.
Result
low hemoglobin, elevated MCV and MCH; increased MCV and MCH may be absent or less than expected in combined folate and iron deficiency; thrombocytopenia, neutropenia
reticulocyte count
Test
Low count indicates decreased production. Can be present in other deficiency states or marrow failure disorders.
Result
low corrected reticulocyte count
Investigations to avoid
red blood cell folate
Recommendations
Do not routinely use red blood cell (RBC) folate testing to diagnose folate deficiency.[53][56]
Rarely, the test may be used to help detect folate deficiency in the 5% of folate deficient patients with normal serum folate levels in which B12 deficiency has been excluded.[53]
Rationale
RBC folate testing is a more complex and expensive test than serum folate, and routine testing is not required as serum folate measurement alone is sufficient in the majority of cases.[53][56]
RBC folate may be normal in acute folate deficiency, and low in >50% of patients with vitamin B12 (cobalamin) deficiency, making this test less useful in routine clinical practice.
Tests to consider
serum folate
Test
Earliest indicator of folate deficiency. Performed as initial screening test. May not detect the 5% of patients who have folate deficiency but normal serum folate levels.[51][54]
In some patients, such as those with macrocytic anemia, empiric treatment with folate supplementation may be considered instead of serum folate testing.[56]
Result
low
serum vitamin B12
Test
Extremely important test.
Underlying vitamin B12 (cobalamin) deficiency should be ruled out before implementing therapy with folic acid, because such therapy may resolve the hematologic manifestations of vitamin B12 deficiency but allow the neurologic complications of untreated vitamin B12 deficiency to progress.
Vitamin B12 deficiency and folate deficiency can coexist in certain patients.
Result
normal
serum LDH
Test
Sign of ineffective erythropoiesis, present in advanced anemia.
Not useful to rule in or rule out folate deficiency.
Result
elevated
serum unconjugated bilirubin
Test
Sign of ineffective erythropoiesis, present in advanced anemia.
Not useful to rule in or rule out folate deficiency.
Result
elevated
serum iron panel
Test
Sign of ineffective erythropoiesis.
Not useful to rule in or rule out folate deficiency.
Result
elevated serum iron, ferritin, transferrin
plasma or serum methylmalonic acid
plasma homocysteine
Test
Useful in borderline folate levels, suspected combined deficiency states, and folate-deficient patients with normal serum folate.[51]
Elevated in >95% of patients with vitamin B12 (cobalamin) deficiency.
Sensitivity is only 86% in detecting clinical folate deficiency states.[57]
Expensive.
Level affected by renal function.
Result
elevated
bone marrow aspirate/biopsy
Test
Classic findings in both folate and vitamin B12 (cobalamin) deficiencies.
Not required to confirm diagnosis.[Figure caption and citation for the preceding image starts]: Megaloblastic marrow cellsPhotomicrograph from Mark J. Koury, MD; used with permission [Citation ends].
Result
megaloblastic changes, erythroid hyperplasia, abnormal nuclear appearance
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