The anemia workup algorithm boils down to one core principle: let the MCV guide you. When a patient’s hemoglobin comes back low (below 13 g/dL in men, below 12 g/dL in women), your next move is to look at the mean corpuscular volume and sort the anemia into one of three buckets — microcytic, normocytic, or macrocytic. From there, each branch has a predictable set of labs that will get you to a diagnosis in the vast majority of cases.
Each of those branches draws on a familiar set of tests, and knowing what belongs in a comprehensive anemia panel helps you order the right studies the first time.
That said, the algorithm isn’t a flowchart you follow blindly. Clinical context matters enormously. A 22-year-old woman with heavy periods and a hemoglobin of 10.5 g/dL is a fundamentally different problem than a 68-year-old man with the same lab value. This guide walks through the systematic approach I use in practice, with specific lab cutoffs and the decision points that actually matter.
Step 1: Confirm Anemia and Assess Severity
Before diving into the workup, confirm you’re actually dealing with anemia and not hemodilution (common in pregnancy, heart failure, or aggressive IV fluid resuscitation). A complete blood count (CBC) is your starting point.
| Severity | Hemoglobin (g/dL) | Clinical Implication |
|---|---|---|
| Mild | 10.0 – lower limit of normal | Outpatient workup appropriate |
| Moderate | 7.0 – 10.0 | Expedited workup; consider symptoms |
| Severe | Below 7.0 | Transfusion threshold; inpatient evaluation often needed |
Severity dictates urgency. A hemoglobin of 6.2 g/dL with active chest pain is a transfusion-now situation, not a send-labs-and-wait scenario.
Step 2: Classify by MCV
The mean corpuscular volume (MCV) splits anemia into three categories, and this single value narrows your differential dramatically.
- Microcytic (MCV < 80 fL): Iron deficiency, thalassemia trait, anemia of chronic disease (sometimes), sideroblastic anemia, lead poisoning
- Normocytic (MCV 80–100 fL): Anemia of chronic disease/inflammation, acute blood loss, chronic kidney disease, mixed deficiency states, early iron deficiency, bone marrow infiltration
- Macrocytic (MCV > 100 fL): B12 deficiency, folate deficiency, myelodysplastic syndrome, alcohol use, hypothyroidism, medications (methotrexate, hydroxyurea, valproic acid)
One caveat: patients with combined iron and B12 deficiency can present with a misleadingly normal MCV. If something doesn’t add up clinically, check for both regardless of the MCV.
Step 3: Order the Right Second-Tier Labs
This is where the workup branches based on your MCV classification. Here’s exactly what to order and what the results mean.
For Microcytic Anemia
Start with an iron studies panel — serum iron, ferritin, total iron-binding capacity (TIBC), and transferrin saturation.
| Lab | Iron Deficiency | Anemia of Chronic Disease | Thalassemia Trait |
|---|---|---|---|
| Ferritin | < 30 ng/mL (diagnostic) | Normal to elevated (> 100) | Normal |
| TIBC | Elevated (> 400 µg/dL) | Low to normal | Normal |
| Transferrin Sat | < 20% | < 20% | Normal |
| Serum Iron | Low | Low | Normal |
| RDW | Elevated (> 15%) | Normal | Normal |
A ferritin below 30 ng/mL is essentially diagnostic of iron deficiency — you can stop there. However, ferritin is an acute-phase reactant, so in patients with concurrent inflammation (elevated CRP), a ferritin below 100 ng/mL with a transferrin saturation under 20% still strongly suggests iron deficiency.
If iron studies are normal and MCV is very low (60–70 fL range) with a normal RDW, think thalassemia trait. A hemoglobin electrophoresis will confirm beta-thalassemia trait (elevated HbA2 above 3.5%).
For Normocytic Anemia
Check the reticulocyte count first. This tells you whether the bone marrow is responding appropriately.
- Elevated reticulocyte count (> 2%, or corrected reticulocyte index > 2): The marrow is working hard. Think hemolysis or acute blood loss. Order LDH, haptoglobin, indirect bilirubin, direct Coombs test, and a peripheral blood smear.
- Low or normal reticulocyte count: The marrow isn’t keeping up. Consider anemia of chronic disease, chronic kidney disease (check BUN/creatinine, EPO level), or bone marrow pathology.
A haptoglobin below 25 mg/dL with elevated LDH is highly specific for hemolysis. The direct Coombs test then sorts autoimmune from non-immune hemolytic causes.
For Macrocytic Anemia
Check vitamin B12 and folate levels. If B12 is borderline (200–400 pg/mL), order methylmalonic acid (elevated in true B12 deficiency) and homocysteine (elevated in both B12 and folate deficiency) to clarify.
If B12 and folate are normal, review medications and alcohol history. A peripheral smear showing hypersegmented neutrophils (≥5 lobes) points to megaloblastic anemia even when serum levels are borderline. When macrocytosis is unexplained — especially in patients over 60 with cytopenias — consider myelodysplastic syndrome and refer for bone marrow biopsy.
Step 4: Identify the Root Cause
Finding iron deficiency isn’t the end of the workup — it’s the beginning of asking why. In premenopausal women, heavy menstrual bleeding is the most common explanation. But in men and postmenopausal women, iron deficiency anemia demands GI evaluation.
Current guidelines from the American Gastroenterological Association recommend bidirectional endoscopy (upper and lower) for unexplained iron deficiency anemia in men and postmenopausal women, particularly those over 50. GI malignancies are found in roughly 6–10% of these patients.
Similarly, if you diagnose B12 deficiency, determine the mechanism — pernicious anemia (check anti-intrinsic factor antibodies), malabsorption (celiac disease, post-gastric bypass), or dietary insufficiency (vegan/vegetarian diet without supplementation).
Step 5: Treat and Monitor
- Iron deficiency: Oral ferrous sulfate 325 mg (65 mg elemental iron) every other day is now preferred over daily dosing — better absorption, fewer side effects. Recheck CBC and ferritin at 8 weeks. Switch to IV iron (ferric carboxymaltose, iron sucrose) if oral therapy fails or is not tolerated.
- B12 deficiency: Intramuscular cyanocobalamin 1000 mcg daily for 7 days, then weekly for 4 weeks, then monthly. High-dose oral B12 (1000–2000 mcg daily) is equally effective for most patients without neurologic involvement.
- Folate deficiency: Folic acid 1 mg daily. Always rule out concurrent B12 deficiency first — folate supplementation can mask B12 deficiency and allow neurologic damage to progress.
- Anemia of chronic disease: Treat the underlying condition. Erythropoiesis-stimulating agents (ESAs) are reserved for CKD-related anemia and select chemotherapy patients.
Frequently Asked Questions
What’s the single most useful lab in an anemia workup?
After the CBC, it’s the reticulocyte count. It tells you whether the bone marrow is responding to anemia or failing to compensate, which fundamentally changes your differential. Yet it’s one of the most commonly forgotten labs in initial workups.
When should I refer to hematology?
Refer when anemia is unexplained after standard workup, when you suspect a bone marrow disorder (pancytopenia, circulating blasts on smear), when hemolytic anemia doesn’t have a clear cause, or when anemia is refractory to appropriate treatment after 8–12 weeks.
Can a patient have more than one type of anemia at the same time?
Absolutely — and it’s common. A patient with alcoholic liver disease might have macrocytic anemia from folate deficiency and microcytic anemia from GI blood loss. The MCV may look deceptively normal. An elevated RDW (red cell distribution width) is a clue that mixed pathology is present.
What ferritin level actually rules out iron deficiency?
A ferritin above 100 ng/mL in a non-inflamed patient effectively rules out iron deficiency (negative likelihood ratio of 0.13). Below 30 ng/mL confirms it. The gray zone — 30 to 100 — is where transferrin saturation and clinical context become critical.
How quickly should hemoglobin improve with iron therapy?
Expect a hemoglobin rise of about 1 g/dL every 2–3 weeks with adequate iron replacement. If you’re not seeing improvement by 4–6 weeks, reassess compliance, consider malabsorption, or rule out ongoing blood loss.
Key Takeaways for Clinicians
- Let the MCV drive your initial differential — microcytic, normocytic, and macrocytic anemias have distinct workup pathways.
- A ferritin below 30 ng/mL is diagnostic of iron deficiency. In inflamed patients, use a threshold of 100 ng/mL with low transferrin saturation.
- Always check a reticulocyte count in normocytic anemia — it splits the differential between marrow failure and peripheral destruction.
- Iron deficiency in men and postmenopausal women requires GI evaluation to rule out malignancy.
- Finding the type of anemia is step one. Finding the cause is what actually helps the patient.
