What labs are recommended in the workup of anemia beyond iron studies (Iron deficiency studies)?

Anemia Workup: Essential Labs Beyond Iron Studies

When evaluating anemia, after confirming iron deficiency is not the cause (or in addition to iron studies), you must obtain a complete blood count with red cell indices, reticulocyte count, and peripheral blood smear as the foundation, then add vitamin B12, folate, renal function tests (creatinine/GFR), and inflammatory markers (CRP) based on the clinical context. 1

Core Laboratory Panel (Minimum Requirements)

Complete Blood Count with Indices

• Red cell distribution width (RDW): Elevated RDW (>14%) suggests iron deficiency or mixed nutritional deficiencies 1, 2
• Mean corpuscular volume (MCV): Categorizes anemia as microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL) to narrow differential diagnosis 1
• Mean corpuscular hemoglobin (MCH): More reliable than MCV for detecting iron deficiency as it's less affected by storage conditions and counting machines 1

Reticulocyte Count

• Corrected reticulocyte index (RI): Distinguishes between production defects (low RI <1.0) versus destruction/loss (high RI >2.0) 1
• Low reticulocyte count with adequate iron, folate, and B12 suggests erythropoietin deficiency or bone marrow suppression 1
• High reticulocyte count indicates hemolysis or acute blood loss requiring different workup 1

Peripheral Blood Smear

• Provides morphologic clues that laboratory indices may miss 3
• Identifies hypochromic cells, target cells, schistocytes, spherocytes, or other abnormalities guiding diagnosis 1

Nutritional Deficiency Assessment

Vitamin B12 and Folate

• Measure when MCV >100 fL (macrocytic anemia) 1
• Also check in patients with extensive small bowel resection, ileal Crohn's disease, or ileal-anal pouch at least annually 1
• Low levels confirm megaloblastic anemia requiring specific replacement therapy 1

Important Caveat

Macrocytosis can also result from thiopurine medications (azathioprine, 6-mercaptopurine), alcohol abuse, hypothyroidism, or reticulocytosis—not just vitamin deficiency 1

Renal Function Assessment

Creatinine and GFR

• Essential when anemia is normocytic with low reticulocyte count 1
• GFR <30 mL/min/1.73 m² strongly suggests erythropoietin deficiency as primary cause 1
• Anemia can develop with serum creatinine as low as 2.0 mg/dL, particularly in patients with reduced muscle mass 1

Inflammatory Markers

C-Reactive Protein (CRP)

• Mandatory in all anemia workups to identify anemia of chronic disease/inflammation 1
• Elevated CRP indicates inflammatory state where ferritin interpretation requires higher thresholds (>100 μg/L for iron deficiency diagnosis) 1, 2
• Helps distinguish between true iron deficiency and functional iron deficiency from inflammation 1

Additional Tests for Specific Clinical Scenarios

When Hemolysis is Suspected (High Reticulocyte Count)

• Haptoglobin: Low levels indicate intravascular hemolysis 1
• Lactate dehydrogenase (LDH): Elevated in hemolysis 1
• Indirect bilirubin: Elevated in hemolysis 1
• Direct Coombs test: Identifies autoimmune hemolytic anemia 1

When Chronic Disease is Present

• Serum calcium and phosphorus: Check every 3 months if GFR <30 mL/min/1.73 m² 1
• Intact parathyroid hormone (iPTH): At least once if GFR <30 mL/min/1.73 m², then every 3 months if abnormal 1
• Serum bicarbonate: Monitor for metabolic acidosis every 3 months if GFR <30 mL/min/1.73 m² 1

When Thalassemia is Suspected

• Hemoglobin electrophoresis: Recommended when microcytosis is present with normal iron studies, especially in appropriate ethnic backgrounds 1
• Prevents unnecessary gastrointestinal investigation for microcytic anemia not due to iron deficiency 1

Algorithmic Approach to Testing Sequence

1. Start with CBC, reticulocyte count, and peripheral smear 1, 3

2. If MCV <80 fL (microcytic): Iron studies are primary, but if normal, obtain hemoglobin electrophoresis for thalassemia 1

3. If MCV 80-100 fL (normocytic):

   • Check reticulocyte count first 1
   • If low: Check creatinine/GFR, vitamin B12, folate, CRP 1
   • If high: Check haptoglobin, LDH, indirect bilirubin, Coombs test 1

4. If MCV >100 fL (macrocytic): Check vitamin B12 and folate levels immediately 1

5. In all cases: Obtain CRP to assess for inflammation affecting interpretation 1

Critical Pitfalls to Avoid

• Do not skip the reticulocyte count: This single test distinguishes production defects from destruction/loss and fundamentally changes the diagnostic pathway 1, 3

• Do not ignore inflammatory markers: CRP elevation changes ferritin interpretation thresholds and may reveal anemia of chronic disease as primary or contributing cause 1, 2

• Do not assume dietary deficiency in men or postmenopausal women: Approximately 62% have gastrointestinal bleeding requiring endoscopic evaluation 2

• Do not overlook medication effects: Thiopurines, hydroxyurea, and diphenytoin cause macrocytosis independent of vitamin deficiency 1

• Do not forget renal function: Normocytic anemia with low reticulocyte count in the setting of elevated creatinine indicates erythropoietin deficiency, not nutritional deficiency 1