What is the appropriate workup for anemia?

Anemia Workup

Begin with a complete blood count (CBC) with red cell indices, reticulocyte count, and iron studies (ferritin, transferrin saturation, serum iron, TIBC) as the minimum initial laboratory evaluation for any patient with suspected anemia. 1

Initial Laboratory Assessment

Essential First-Line Tests

• CBC with differential – Obtain hemoglobin, mean corpuscular volume (MCV), red cell distribution width (RDW), and evaluate all three cell lines; abnormalities in two or more lineages require hematology referral for possible bone marrow pathology 1
• Reticulocyte count – Differentiates decreased production (low reticulocyte index <1.0-2.0) from increased destruction or blood loss (high index >2.0) 1, 2
• Iron studies – Measure serum ferritin, transferrin saturation, serum iron, and total iron-binding capacity together; ferritin alone is insufficient because inflammation falsely elevates it 1
• Inflammatory markers – Check C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) to identify inflammatory states that alter iron study interpretation 1

Interpretation by MCV Classification

Microcytic anemia (MCV <80 fL):

• Most commonly caused by iron deficiency; other causes include thalassemia, anemia of chronic disease, and sideroblastic anemia 2, 3
• Ferritin <30 μg/L confirms iron deficiency in the absence of inflammation 1
• With inflammation present, ferritin up to 100 μg/L may still represent iron deficiency 1
• Transferrin saturation <16% indicates absolute iron deficiency 1

Normocytic anemia (MCV 80-100 fL):

• Typically caused by hemorrhage, hemolysis, bone marrow failure, anemia of chronic inflammation, or renal insufficiency 1, 2
• Low reticulocyte count suggests decreased production from early nutritional deficiencies, chronic kidney disease, medication-induced suppression, or bone marrow failure 2
• High reticulocyte count indicates acute blood loss or hemolysis 2
• Elevated RDW in normocytic anemia strongly suggests underlying iron deficiency or mixed deficiency state 1, 2

Macrocytic anemia (MCV >100 fL):

• Most commonly caused by vitamin B12 or folate deficiency, alcoholism, certain medications (thiopurines, antiretrovirals, hydroxyurea), or myelodysplastic syndrome 1, 2, 3

Directed Investigation Based on Initial Findings

When Iron Deficiency is Identified

• Perform stool guaiac testing immediately to evaluate for gastrointestinal bleeding 1
• Screen for celiac disease serologically or via small bowel biopsy at gastroscopy; celiac disease is found in 3-5% of iron deficiency anemia cases 1
• Urinalysis or urine microscopy to exclude urinary blood loss 1
• In men and postmenopausal women, gastroscopy and colonoscopy should be first-line GI investigations; 60-70% will have an identifiable GI source 1, 3
• Review dietary iron intake, menstrual blood losses in premenopausal women, chronic NSAID use, long-term proton pump inhibitor therapy, and history of GI surgery 1

When Reticulocyte Count is Low

• Review all medications for bone marrow suppressants (NSAIDs, antibiotics, chemotherapy agents) 2
• Check vitamin B12 and folate levels; combined deficiencies can produce normal MCV despite true deficiency 1, 2
• Assess renal function (serum creatinine, estimated GFR); anemia develops when GFR falls below 20-35 mL/min/1.73 m² and is likely due to erythropoietin deficiency when creatinine ≥2 mg/dL 1
• Measure thyroid function; hypothyroidism causes normochromic, normocytic anemia mimicking erythropoietin deficiency 1
• Consider riboflavin deficiency, which presents with normochromic, normocytic anemia and marrow aplasia 2

When Reticulocyte Count is High

• Investigate for hemolysis – Measure indirect and direct bilirubin, haptoglobin, lactate dehydrogenase, and direct antiglobulin test (Coombs test) 1, 2
• Look for clinical signs including jaundice, hepatosplenomegaly, and decreased haptoglobin 2
• Evaluate for acute hemorrhage if hemolysis markers are negative 2

When Anemia of Chronic Disease is Suspected

• Characterized by ferritin >100 μg/L and transferrin saturation <20% 1, 2
• Inflammatory cytokines suppress endogenous erythropoietin production and directly inhibit erythropoiesis 1
• Do not confuse with normocytic anemia from other causes; inflammatory markers (CRP, ESR) must be elevated 2
• Identify and treat the underlying inflammatory, infectious, or malignant condition driving the anemia 2

Special Populations and Contexts

Chronic Kidney Disease

• Up to 25-37.5% of CKD patients have concurrent iron deficiency despite normocytic anemia 2, 4
• Erythropoietin deficiency is the primary driver; measurement of serum erythropoietin levels is usually not indicated 1
• Transferrin saturation is more reliable than ferritin in CKD because it is less affected by inflammation 4
• Hypochromic red blood cells >2.5% (when available) signal functional iron deficiency 1

Inflammatory Bowel Disease

• Minimum workup includes CBC with RDW and MCV, reticulocyte count, differential blood count, serum ferritin, transferrin saturation, and CRP 1
• Patients with extensive small bowel resection or ileal Crohn's disease require more frequent assessment for vitamin B12 or folate deficiency 1, 2
• Monitor hemoglobin every 6 months for mild disease and more frequently during active inflammation 1

Premenopausal Women

• Threshold for investigation should be lower if GI symptoms, family history of GI pathology, or anemia persists despite menstrual blood loss explanation 1

Indications for Bone Marrow Examination

Reserve bone marrow aspiration and biopsy for specific scenarios: 2

• Unexplained pancytopenia or abnormalities affecting multiple hematologic lineages
• Concern for infiltrative process (malignancy, myelodysplastic syndrome)
• Dysplastic features or blasts on peripheral blood smear
• Failure to identify cause after comprehensive noninvasive workup
• Progressive anemia despite treatment of identified causes

Common Pitfalls to Avoid

• Do not rely on ferritin alone in inflammatory conditions; always incorporate transferrin saturation because ferritin behaves as an acute-phase reactant 1, 4
• Do not assume anemia of chronic disease without measuring iron studies; concurrent iron deficiency is common and treatable 2, 4
• Do not overlook combined deficiency states (iron plus B12/folate) that produce normal MCV despite true deficiencies 1, 2
• Do not neglect medication review; many drugs cause bone marrow suppression or hemolysis 2
• Do not skip GI evaluation in unexplained iron deficiency; 60-70% of referred patients have identifiable GI bleeding sources 1, 3