What laboratory tests should be ordered when investigating anemia?

Initial Laboratory Workup for Anemia Investigation

When investigating anemia, order a complete blood count (CBC) with red cell indices, reticulocyte count, serum ferritin, transferrin saturation, and C-reactive protein as your essential first-line tests. 1, 2, 3

Core Laboratory Tests (Order These First)

Complete Blood Count with Indices

• Hemoglobin and hematocrit confirm the presence and severity of anemia, with lower values indicating more urgent need for investigation and higher likelihood of serious pathology 4, 1
• Mean corpuscular volume (MCV) is the critical parameter that guides your diagnostic pathway—it classifies anemia as microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL) 2, 3
• Red cell distribution width (RDW) detects mixed deficiencies or early iron deficiency even when MCV remains normal 3, 5
• Mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) provide sensitive indicators of iron deficiency in the absence of chronic disease or hemoglobinopathy 4, 3

Iron Studies (Essential for All Patients)

• Serum ferritin is the single most powerful test for iron deficiency, with levels <30 μg/L confirming iron deficiency in the absence of inflammation 4, 1, 2
• Transferrin saturation <16% suggests iron deficiency but must be interpreted alongside ferritin 1, 2
• Serum iron and total iron-binding capacity (TIBC) complete the iron panel 1, 6

Critical caveat: Ferritin is an acute phase reactant and can be falsely elevated with inflammation, malignancy, or liver disease—in these contexts, iron deficiency is unlikely if ferritin >100 μg/L 1, 3

Reticulocyte Count

• Reticulocyte count assesses bone marrow response to anemia 1, 2, 7
• Low or normal reticulocyte count indicates impaired erythropoiesis (production problem) 3
• Elevated reticulocyte count suggests increased red cell production in response to hemolysis or bleeding 3, 7

Inflammatory Marker

• C-reactive protein (CRP) helps interpret iron studies by identifying inflammation that may falsely elevate ferritin 1, 3

Additional Tests Based on Initial Results

For Microcytic Anemia (MCV <80 fL)

• Hemoglobin electrophoresis is mandatory when microcytosis and hypochromia are present in patients of appropriate ethnic background (Mediterranean, African, Asian descent) to rule out thalassemia and prevent unnecessary GI investigation 4, 1

For Normocytic or Macrocytic Anemia

• Vitamin B12 and folate levels rule out nutritional deficiencies that may mask microcytosis or cause macrocytosis 1, 2, 3
• Kidney function tests (creatinine, eGFR) evaluate for chronic kidney disease as a cause of normocytic anemia 1, 2
• Thyroid function tests (TSH) rule out hypothyroidism in normocytic anemia 1

For Suspected Hemolysis (Elevated Reticulocyte Count)

• Haptoglobin, lactate dehydrogenase (LDH), and indirect bilirubin confirm hemolytic process 3, 6
• Direct antiglobulin test (Coombs test) identifies immune-mediated hemolysis 8

For Chronic Kidney Disease Patients

• Serum bicarbonate assesses metabolic acidosis 1
• Calcium, phosphorus, and intact parathyroid hormone (PTH) in patients with GFR <30 mL/min/1.73 m² 1

What NOT to Order

• Faecal occult blood testing has no benefit in the investigation of anemia and should not be part of the initial laboratory workup 4
• Serum erythropoietin levels are usually not indicated in chronic kidney disease patients with normochromic, normocytic anemia 2

Practical Algorithm

1. Start with CBC + indices, reticulocyte count, iron studies (ferritin, transferrin saturation, iron, TIBC), and CRP 1, 2, 3

2. If MCV <80 fL: Check hemoglobin electrophoresis in appropriate ethnic populations before pursuing GI workup 4, 1

3. If MCV 80-100 fL: Add B12, folate, kidney function, and thyroid function tests 1, 2

4. If MCV >100 fL: Prioritize B12 and folate levels 2, 3

5. If reticulocyte count is elevated: Add hemolysis workup (haptoglobin, LDH, bilirubin, Coombs) 3, 6

6. If ferritin is equivocal (30-100 μg/L) with inflammation: Consider a trial of iron therapy—hemoglobin rise ≥10 g/L within 2 weeks strongly suggests iron deficiency 4

Critical Pitfalls to Avoid

• Never assume anemia is "normal aging"—always investigate the cause regardless of patient age 2
• Watch for combined deficiencies, especially in elderly patients and those with inflammatory bowel disease, where elevated RDW may be your only clue 3
• In men and postmenopausal women with confirmed iron deficiency anemia, GI investigation is mandatory to rule out malignancy unless there is obvious non-GI blood loss 4, 1
• All patients should be screened for coeliac disease when iron deficiency anemia is confirmed 4