The anemia differential diagnosis is the stepwise process of working out why hemoglobin is low. In practice it rests on three questions: how big are the red cells (the MCV), is the bone marrow responding (the reticulocyte count), and what do the smear, iron studies, and vitamin levels show? Answering those in order narrows dozens of possible causes to a short, testable list.
Anemia is a finding, not a diagnosis. The same low hemoglobin can come from a heavy period, a hidden ulcer, a vitamin deficiency, an inherited hemoglobin disorder, or a failing marrow, as in aplastic anemia. Below is the framework I use at the bedside and teach to students.
Step One: Confirm Anemia and Its Severity
Anemia means a reduced hemoglobin concentration or red cell mass, which lowers the blood’s capacity to carry oxygen. The widely used World Health Organization thresholds are a hemoglobin below 13 g/dL in adult men and below 12 g/dL in non-pregnant adult women. Reference ranges vary slightly by laboratory, altitude, and ancestry.
Before chasing a cause, ask two practical questions. Is this new or long-standing? Comparing with old blood counts often answers it. And is the patient stable? Rapid bleeding or severe symptoms take priority over a full workup. It also helps to check whether other cell lines are affected, since low white cells or platelets alongside anemia point toward marrow disease or other hematological conditions rather than a simple deficiency.
Step Two: Sort by Red Cell Size (MCV)
The mean corpuscular volume (MCV) on a complete blood count is the single most useful branching point. Normal adult MCV is roughly 80–100 fL.
| Category | MCV | Main causes to consider |
|---|---|---|
| Microcytic | Below 80 fL | Iron deficiency, thalassemia trait, anemia of chronic disease (later stage), sideroblastic anemia, lead poisoning |
| Normocytic | 80–100 fL | Acute blood loss, hemolysis, chronic kidney disease, anemia of chronic disease, marrow failure or infiltration, early iron deficiency |
| Macrocytic | Above 100 fL | Vitamin B12 or folate deficiency, alcohol, liver disease, hypothyroidism, certain drugs, myelodysplastic syndromes, brisk reticulocytosis |
Microcytic Anemia
Small cells mean the body cannot make enough hemoglobin, usually because of too little iron or a defect in globin chains. Iron deficiency is the most common cause worldwide and, in adults, should always prompt a search for blood loss, particularly from the gut or heavy menstrual bleeding.
Thalassemia trait is the key mimic. It typically produces a very low MCV with a normal or high red cell count and normal iron stores. Treating it with iron does nothing and can cause harm, which is why iron studies matter before prescribing.
Macrocytic Anemia
Large cells split into megaloblastic causes, where DNA synthesis is impaired, and non-megaloblastic causes. Vitamin B12 and folate deficiency are the classic megaloblastic causes, and the smear often shows hypersegmented neutrophils. Drugs such as methotrexate and hydroxyurea do the same.
Non-megaloblastic macrocytosis is seen with alcohol use, liver disease, and hypothyroidism. In older adults with unexplained macrocytosis and other low counts, a myelodysplastic syndrome needs to be excluded.
Step Three: Is the Marrow Responding?
The reticulocyte count measures young red cells released from the marrow in the last day or two. It separates problems of production from problems of loss or destruction, and is especially useful in normocytic anemia.
- Low reticulocytes (hypoproliferative): the marrow is not keeping up. Causes include iron, B12, or folate deficiency, kidney disease (low erythropoietin), anemia of chronic disease, and primary marrow problems.
- High reticulocytes: the marrow is working hard to replace cells lost to bleeding or destroyed by hemolysis.
Understanding the composition and function of bone marrow helps explain these patterns. Infiltration by leukemia, lymphoma, myeloma, or fibrosis, and other bone marrow disorders, can crowd out normal red cell production and usually cause abnormalities in several cell lines.
Recognizing Hemolysis
When red cells are destroyed early, a characteristic pattern appears: raised reticulocytes, raised lactate dehydrogenase (LDH), raised indirect (unconjugated) bilirubin, and low or absent haptoglobin. The direct antiglobulin (Coombs) test then separates autoimmune hemolysis from non-immune causes such as hereditary spherocytosis, G6PD deficiency, sickle cell disease, and mechanical destruction.
Key Laboratory Tests in the Workup
Iron studies are the most frequently misread part of the workup, because inflammation changes them. The table below shows the typical textbook patterns.
| Test | Iron deficiency | Anemia of chronic disease | Thalassemia trait |
|---|---|---|---|
| Ferritin | Low | Normal or high | Normal |
| Serum iron | Low | Low | Normal |
| TIBC / transferrin | High | Low or normal | Normal |
| Transferrin saturation | Low | Low or normal | Normal |
| Red cell count | Low | Low | Normal or high |
Ferritin is an acute-phase protein, so infection, inflammation, or liver disease can lift it into the normal range even when iron stores are depleted. In those settings, transferrin saturation and the clinical picture carry more weight.
Other useful tests include the peripheral blood smear (target cells, spherocytes, schistocytes, sickle cells, blasts), B12 and folate levels, kidney and liver function, thyroid tests, hemoglobin electrophoresis for suspected hemoglobinopathies, and bone marrow examination when the cause remains unclear or marrow disease is suspected.
Clinical Clues From History and Examination
The lab pathway works best when guided by the patient’s story. Symptoms such as fatigue, breathlessness, palpitations, and pallor are common to all anemias, so the distinguishing clues come from elsewhere.
- Diet and absorption: vegan diet, bariatric surgery, or celiac disease suggest iron or B12 deficiency.
- Bleeding history: heavy periods, black stools, or regular anti-inflammatory use point toward iron loss.
- Family history and ancestry: relevant for thalassemia, sickle cell disease, and hereditary spherocytosis.
- Jaundice or dark urine: suggests hemolysis.
- Numbness, tingling, or balance problems: typical of B12 deficiency.
- Bruising, infections, or enlarged spleen or lymph nodes: raise concern for marrow or blood cancers.
When to See a Doctor
Seek prompt care if you have anemia with chest pain, fainting, severe breathlessness, black or bloody stools, or vomiting blood. Unexplained anemia in a man or a postmenopausal woman always deserves investigation for hidden blood loss. Anemia combined with fevers, unusual bruising, or weight loss also needs urgent assessment.
Once the cause is known, treatment is usually straightforward. Our guide to anemia management covers iron replacement, vitamin therapy, transfusion, and treating underlying disease.
Frequently Asked Questions
What is the first test in an anemia workup?
A complete blood count with red cell indices, especially the MCV, is the starting point. A reticulocyte count and peripheral smear are usually added early. Together they point toward the right follow-up tests.
Can you have iron deficiency with a normal ferritin?
Yes, particularly when inflammation, infection, or liver disease is present, because ferritin rises in those states. Doctors then rely on transferrin saturation, other markers, and the clinical context. Iron deficiency and anemia of chronic disease can also coexist.
How do doctors tell iron deficiency from thalassemia trait?
Both cause small red cells, but thalassemia trait usually has a normal or high red cell count and normal iron studies. Hemoglobin electrophoresis or genetic testing confirms the trait. This distinction prevents unnecessary iron treatment.
When is a bone marrow biopsy needed?
It is reserved for anemia that remains unexplained after blood tests, or when other cell lines are abnormal, blasts appear on the smear, or marrow infiltration is suspected. Most anemias are diagnosed without it.
For more on every type of anemia, see our complete anemia guide.
