What is the appropriate work‑up for anemia?

Anemia Workup: Essential Laboratory Testing

Order a complete blood count with red cell indices, absolute reticulocyte count, serum ferritin, transferrin saturation, and vitamin B12/folate levels simultaneously at initial presentation—this panel provides the foundation for diagnosing the vast majority of anemias. 1, 2, 3

Diagnostic Thresholds

• Anemia is defined as hemoglobin <13 g/dL in men, <12 g/dL in non-pregnant women, and <11 g/dL in pregnant women 1, 2, 3
• For chronic kidney disease patients, initiate workup when hemoglobin drops below 12 g/dL in adult males and postmenopausal females, or below 11 g/dL in premenopausal females 1

Essential First-Line Laboratory Panel

Complete Blood Count with Indices

• Obtain hemoglobin, hematocrit, red blood cell count, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), red cell distribution width (RDW), white blood cell count with differential, and platelet count 1, 2, 3
• The MCV classifies anemia into microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL) categories, which directs subsequent testing 1, 2, 3
• Low MCH with elevated RDW is highly suggestive of iron deficiency, even when MCV remains normal 2

Reticulocyte Count

• Order an absolute reticulocyte count to assess bone marrow response—a low count indicates impaired erythropoiesis (production problem), while an elevated count suggests hemolysis or bleeding (destruction or loss problem) 1, 2, 3

Iron Studies Panel

• Measure serum ferritin, transferrin saturation (TSAT), and total iron-binding capacity simultaneously 1, 2, 3
• Ferritin <30 μg/L confirms iron deficiency in the absence of inflammation 4, 2
• Transferrin saturation <15-20% supports iron deficiency and is less affected by inflammation than ferritin 2

Inflammatory Marker

• Measure C-reactive protein (CRP) to interpret ferritin values, because ferritin rises as an acute-phase reactant during inflammation and may mask true iron deficiency 2

Nutritional Markers

• Measure vitamin B12 and folate levels to identify nutritional deficiencies, particularly in patients with macrocytic anemia 1, 2, 3

Classification-Based Diagnostic Approach

Microcytic Anemia (MCV <80 fL)

• The most common causes are iron deficiency anemia, thalassemia trait, anemia of chronic disease, or sideroblastic anemia 1, 2, 3
• If iron studies confirm deficiency (ferritin <30 μg/L or TSAT <15-20%), investigate the underlying cause including gastrointestinal bleeding in adult men and postmenopausal women, and menstrual losses in premenopausal women 2
• If iron studies are normal, obtain hemoglobin electrophoresis to evaluate for thalassemia trait, particularly in patients of Mediterranean, African, or Southeast Asian descent 2

Normocytic Anemia (MCV 80-100 fL)

• Common causes include acute blood loss, hemolysis, anemia of chronic disease, early iron deficiency, or chronic kidney disease 1, 2, 3
• If reticulocyte count is elevated, evaluate for hemolysis by measuring haptoglobin, lactate dehydrogenase (LDH), indirect bilirubin, and examining peripheral smear for schistocytes 4, 2
• If reticulocyte count is low or normal, consider anemia of chronic disease and assess for underlying inflammatory conditions 2

Macrocytic Anemia (MCV >100 fL)

• The primary causes are vitamin B12 deficiency, folate deficiency, medications (particularly thiopurines, methotrexate, anticonvulsants), alcohol use, or myelodysplastic syndrome 1, 2, 3
• Obtain a peripheral blood smear to look for hypersegmented neutrophils (indicating megaloblastic anemia) or oval macrocytes 2
• Review medication exposure, focusing on thiopurines, chemotherapy agents, anticonvulsants, and methotrexate, as these commonly induce macrocytosis 2
• If pancytopenia is present, refer for bone marrow examination to evaluate for aplastic anemia or myelodysplastic syndrome 2

Critical Interpretation Pitfalls

• Never assume anemia is "normal aging" in elderly patients—always investigate the underlying cause, as one-third have nutritional deficiency, one-third have anemia of chronic disease, and one-third have unexplained anemia requiring further evaluation 1, 5
• Ferritin interpretation requires clinical context, as it can be falsely elevated in inflammation, chronic disease, malignancy, or liver disease—always measure CRP concurrently 1, 2
• Watch for combined deficiencies, such as iron and B12 deficiency, especially in elderly patients and those with inflammatory bowel disease—mixed micro- and macrocytosis can neutralize the MCV, yielding a normal MCV but an elevated RDW 4, 2
• Applying uniform hemoglobin cut-offs leads to higher false-positive rates for iron deficiency in Black patients; therefore, incorporate additional iron status tests such as serum ferritin and transferrin saturation rather than relying solely on hemoglobin levels 2

Special Population: Chronic Kidney Disease

• Monitor hemoglobin at least every 3 months in patients with GFR <30 mL/min/1.73 m² 1, 3
• If GFR <30 mL/min/1.73 m², consider nephrology referral for evaluation of anemia of chronic kidney disease 3
• Treat identified iron deficiency first with oral or intravenous iron, and consider erythropoietin therapy only if anemia persists despite iron repletion 1

Hemolysis-Specific Workup

• If hemolysis is suspected based on elevated reticulocyte count, order haptoglobin (decreased in hemolysis), LDH (elevated), indirect bilirubin (elevated), and examine peripheral smear for schistocytes 4, 2
• Obtain a direct antiglobulin test (Coombs test) to evaluate for autoimmune hemolytic anemia 4
• Consider paroxysmal nocturnal hemoglobinuria screening, disseminated intravascular coagulation panel, and evaluation for viral/bacterial causes of hemolysis if initial tests are nondiagnostic 4