Anemia: Which Blood Tests Do You Need?

Anemia — defined as a hemoglobin below 13.5 g/dL in men or below 12.0 g/dL in women — is one of the most common blood disorders worldwide. But "anemia" is a finding, not a diagnosis. The critical step is determining why you're anemic, because treatment depends entirely on the cause. The right blood tests make that determination possible.

Complete Blood Count (CBC): The starting point for any anemia workup. Key values include:

• Hemoglobin (Hgb) — the oxygen-carrying protein in red blood cells. Low hemoglobin confirms anemia. Normal: 13.5–17.5 g/dL (men), 12.0–15.5 g/dL (women).
• Hematocrit (Hct) — the percentage of blood volume composed of red blood cells. Normal: 38.3–48.6% (men), 35.5–44.9% (women).
• MCV (Mean Corpuscular Volume) — the average size of your red blood cells. This is the most important value for classifying anemia. Low MCV (below 80 fL) = microcytic anemia (usually iron deficiency). High MCV (above 100 fL) = macrocytic anemia (usually B12 or folate deficiency). Normal MCV = normocytic anemia (chronic disease, acute blood loss, or hemolysis).
• RDW (Red Cell Distribution Width) — measures variation in red blood cell size. Elevated RDW with low MCV strongly suggests iron deficiency (as opposed to thalassemia, which has low MCV but normal RDW).

Iron Studies: Serum iron, TIBC (Total Iron-Binding Capacity), and transferrin saturation tell you how much iron is available for red blood cell production right now. Low serum iron + high TIBC + low transferrin saturation (below 20%) = iron deficiency.

Ferritin: The single best indicator of total body iron stores. A ferritin below 30 ng/mL is diagnostic of iron deficiency in most clinical contexts. Below 15 ng/mL is nearly 100% specific for iron deficiency. However, ferritin is also an acute phase reactant — it rises with inflammation, infection, and liver disease, potentially masking true iron deficiency.

Anemia has dozens of causes, but they fall into three major categories based on the MCV (red blood cell size) from your CBC:

Microcytic Anemia (MCV below 80 fL) — small red blood cells:

• Iron deficiency — by far the most common cause worldwide. Results from blood loss (heavy periods, GI bleeding), inadequate dietary intake, or malabsorption (celiac disease, gastric bypass). Iron studies show low ferritin, low serum iron, high TIBC.
• Thalassemia trait — a genetic condition common in Mediterranean, African, and Southeast Asian populations. Causes mild, lifelong microcytic anemia that doesn't respond to iron supplementation. Distinguished from iron deficiency by normal or elevated iron stores and normal RDW.
• Anemia of chronic disease — sometimes microcytic but more often normocytic. Iron is trapped in storage (normal or high ferritin) due to inflammatory signaling (hepcidin).

Macrocytic Anemia (MCV above 100 fL) — large red blood cells:

• B12 deficiency — causes megaloblastic anemia. Common in vegans/vegetarians, older adults (reduced absorption), pernicious anemia (autoimmune destruction of intrinsic factor), and metformin users. Can cause irreversible neurological damage if untreated.
• Folate deficiency — similar blood picture to B12 deficiency but without neurological symptoms. Caused by inadequate dietary intake, alcoholism, certain medications (methotrexate), and pregnancy (increased demand).
• Hypothyroidism, liver disease, and alcohol use can also cause macrocytosis independent of B12/folate status.

Normocytic Anemia (MCV 80–100 fL) — normal-sized red blood cells:

• Anemia of chronic disease — the second most common cause of anemia globally. Associated with chronic infections, autoimmune diseases, cancer, and kidney disease.
• Acute blood loss — MCV is initially normal because you're losing mature red blood cells.
• Hemolytic anemia — red blood cells are being destroyed faster than produced. Reticulocyte count will be elevated.

These are the two most common nutritional anemias, and distinguishing them is critical because the treatments are completely different — and giving the wrong treatment can mask the other deficiency.

Iron Deficiency Anemia:

• CBC findings: Low hemoglobin, low MCV (microcytic), high RDW, low reticulocyte count
• Iron studies: Low ferritin (below 30 ng/mL), low serum iron, high TIBC, low transferrin saturation (below 20%)
• Symptoms: Fatigue, weakness, pale skin, brittle nails (koilonychia/spoon nails), restless legs, pica (craving ice or dirt), cold intolerance
• Common causes: Heavy menstrual bleeding (most common cause in premenopausal women), GI blood loss (ulcers, colon polyps, cancer — GI workup is essential in men and postmenopausal women with unexplained iron deficiency), celiac disease, pregnancy
• Treatment: Oral iron (ferrous sulfate 325 mg, taken with vitamin C on an empty stomach, every other day for best absorption). Response: reticulocyte count peaks at 7–10 days, hemoglobin rises 1 g/dL every 2–3 weeks. Continue supplementation for 3–6 months after hemoglobin normalizes to fully replenish stores.

B12 Deficiency Anemia:

• CBC findings: Low hemoglobin, high MCV (macrocytic), often with hypersegmented neutrophils on peripheral smear
• B12 level: Below 200 pg/mL is deficient. 200–400 pg/mL is borderline — check methylmalonic acid (MMA) for confirmation. MMA is elevated in true B12 deficiency but normal in folate deficiency.
• Symptoms: Fatigue, glossitis (smooth, beefy-red tongue), paresthesias (tingling in hands/feet), balance problems, cognitive changes. Neurological symptoms can occur without anemia and can become permanent if untreated.
• Common causes: Pernicious anemia, vegan/vegetarian diet, metformin use, gastric bypass, Crohn's disease affecting the ileum
• Treatment: IM B12 injections (1000 µg daily for 1 week, then weekly for 1 month, then monthly) or high-dose oral B12 (1000–2000 µg daily). Response: reticulocyte count peaks at 5–7 days.

Critical warning: Never treat a macrocytic anemia with folate alone without checking B12 first. Folate can correct the anemia caused by B12 deficiency but will not treat the neurological damage, which will progress silently.

Monitoring your response to treatment is just as important as the initial diagnosis. Here's what to expect and when to recheck:

Iron deficiency treatment monitoring:

• 2 weeks: Reticulocyte count should peak (indicating your bone marrow is responding). If it doesn't, consider malabsorption, incorrect diagnosis, or ongoing blood loss.
• 4–6 weeks: Recheck CBC. Hemoglobin should rise by approximately 1 g/dL every 2–3 weeks. If hemoglobin isn't improving, reassess compliance, absorption, and ongoing losses.
• 3 months: Recheck CBC and ferritin. Hemoglobin should be normalizing. Ferritin tells you whether iron stores are being replenished.
• 6 months: Final check. Ferritin should be above 50 ng/mL (ideally 50–100 ng/mL) before stopping supplementation. Stopping too early is the most common reason for recurrence.

B12 deficiency treatment monitoring:

• 1 week: Reticulocyte count should peak if the bone marrow is responding.
• 6–8 weeks: Recheck CBC and B12 level. Hemoglobin should be improving, and MCV should be trending toward normal.
• 3–4 months: CBC should be fully normalized. If neurological symptoms were present, they may take 6–12 months to improve (and may not fully resolve if diagnosis was delayed).
• Ongoing: If the cause is pernicious anemia or another permanent absorption issue, B12 supplementation is lifelong. Recheck B12 annually.

When to escalate: If hemoglobin fails to improve after 4–6 weeks of appropriate treatment, or if anemia recurs after treatment, consult your doctor. Possible reasons include ongoing blood loss (may need endoscopy/colonoscopy), coexisting deficiencies (combined iron + B12 deficiency), or an alternative diagnosis (myelodysplastic syndrome, chronic kidney disease).