What laboratory tests should be ordered to evaluate anemia?

Laboratory Evaluation of Anemia

Order a complete blood count with differential, reticulocyte count, and iron studies (ferritin and transferrin saturation) as your core initial panel for any patient with suspected anemia. 1

Essential First-Line Tests

• Hemoglobin measurement is superior to hematocrit because it demonstrates better inter-laboratory reproducibility, lower coefficient of variation, and remains unaffected by sample storage time or serum glucose levels. 1

• Complete blood count with differential—including white blood cell and platelet counts—must be obtained to assess overall bone marrow function; abnormalities in two or more cell lines warrant immediate hematology referral for possible marrow pathology. 1

• Mean corpuscular volume (MCV) directs your diagnostic pathway algorithmically:

  • Low MCV (<80 fL): pursue iron deficiency, thalassemia, or hemoglobin synthesis disorders 1
  • Normal MCV (80-100 fL): evaluate for anemia of chronic disease, hemolysis, or marrow disorders 1
  • High MCV (>100 fL): order vitamin B12, folate, and thyroid function tests 1

Reticulocyte Count: Assessing Marrow Response

• Absolute reticulocyte count or reticulocyte index (adjusted for anemia severity) differentiates production failure from increased red cell loss or destruction. 1

• Low reticulocyte count indicates inadequate marrow output from iron deficiency, intrinsic red cell production defects, insufficient erythropoietin (especially in chronic kidney disease), or inflammatory inhibition. 1

• Elevated reticulocyte count signals active hemolysis or acute blood loss—immediately add lactate dehydrogenase, haptoglobin, bilirubin, and peripheral smear to your workup. 1

Iron Studies: Interpretation Framework

• Serum ferritin serves as your surrogate marker for tissue iron stores and is essential in the initial iron panel. 1

• Ferritin thresholds vary by clinical context:

  • General population: <45 ng/mL indicates iron deficiency when clinical context supports it 1
  • Non-dialysis CKD males: <25 ng/mL predicts insufficient iron stores 1
  • Non-dialysis CKD females: <11 ng/mL predicts insufficient iron stores 1

• Transferrin saturation (TSAT) reflects iron immediately available for erythropoiesis and is less affected by inflammation, making it more reliable than ferritin in chronic kidney disease and inflammatory conditions. 1

Critical Pitfall: Ferritin as an Acute-Phase Reactant

• Never interpret ferritin in isolation in patients with inflammation, chronic kidney disease, or chronic disease—ferritin can be falsely elevated in these settings, masking true iron deficiency. 1

• In inflammatory states, prioritize TSAT over ferritin as your primary marker of iron availability for erythropoiesis. 1

Context-Specific Additional Testing

• Peripheral blood smear provides morphologic clues when CBC and reticulocyte data are inconclusive, identifying specific anemia subtypes. 1, 2

• For iron deficiency without obvious blood loss (excluding menstrual loss):

  • Begin with non-invasive testing for Helicobacter pylori and celiac disease 1
  • Proceed to bidirectional endoscopy (upper and lower) in males and post-menopausal females 1
  • In non-dialysis CKD patients receiving erythropoiesis-stimulating agents, actively investigate occult gastrointestinal bleeding 1

• For macrocytic anemia (high MCV): order vitamin B12, folate, and thyroid function tests. 1

• For microcytic anemia unresponsive to iron therapy: obtain hemoglobin electrophoresis, especially in individuals of Mediterranean, African, or Southeast Asian ancestry to evaluate for thalassemia. 1

Key Clinical Pitfalls to Avoid

• A normal or low reticulocyte count in the presence of anemia is always abnormal—it indicates inadequate marrow response from erythropoietin deficiency, iron unavailability, or marrow suppression by inflammation or pathology. 1

• In chronic kidney disease, do not automatically attribute anemia to CKD alone—the anemia of CKD is typically normochromic and normocytic, and comprehensive evaluation is required to exclude other etiologies. 1

• When combining ferritin and TSAT in CKD or inflammatory conditions, interpret both together—relying on ferritin alone will misclassify iron status in these populations. 1