What are the diagnostic and treatment approaches for anemia not due to blood loss, including iron deficiency anemia, vitamin B12 deficiency, and anemia of chronic disease?

Differential Diagnosis of Anemia Excluding Blood Loss

Anemia not due to blood loss should be systematically classified using mean corpuscular volume (MCV) and reticulocyte count, which together identify whether the problem is decreased production or increased destruction, and guide targeted workup for iron deficiency, vitamin B12/folate deficiency, anemia of chronic disease, hemolysis, or bone marrow disorders. 1

Initial Diagnostic Approach

The minimum workup must include:

• Complete blood count with MCV and red cell distribution width (RDW) 1
• Reticulocyte count - distinguishes production defects (low/normal) from hemolysis (elevated) 1
• Serum ferritin and transferrin saturation 1
• C-reactive protein (CRP) - essential for interpreting ferritin in inflammatory states 1

A high RDW suggests iron deficiency even when MCV appears normal, particularly when microcytosis and macrocytosis coexist and neutralize each other 1, 2.

Microcytic Anemia (Low MCV) with Normal/Low Reticulocytes

Iron Deficiency Anemia

Diagnostic criteria depend critically on inflammatory status:

• Without inflammation: serum ferritin <30 μg/L confirms iron deficiency 1
• With inflammation: ferritin up to 100 μg/L may still indicate iron deficiency 1
• Supporting findings include low serum iron, elevated total iron-binding capacity, transferrin saturation <16%, and microcytic hypochromic red cells 1

Anemia of Chronic Disease (ACD)

Diagnostic criteria in the presence of inflammation:

• Serum ferritin >100 μg/L AND transferrin saturation <20% 1
• Ferritin 30-100 μg/L suggests combined iron deficiency and ACD 1

The pathophysiology involves inflammatory cytokines upregulating hepcidin, which blocks iron export from macrophages via ferroportin reduction, creating functional iron deficiency despite adequate stores 1, 2. Inflammatory cytokines also directly suppress erythropoietin production and bone marrow erythropoiesis 1.

Other Microcytic Causes

• Thalassemia - requires hemoglobin electrophoresis 1
• Lead poisoning (very rare) 1

Macrocytic Anemia (High MCV) with Normal/Low Reticulocytes

Vitamin B12 Deficiency

Critical diagnostic and treatment considerations:

Immediate treatment is mandatory before initiating folate therapy, as folate can mask B12 deficiency while allowing irreversible neurological damage to progress 1, 3. This includes subacute combined degeneration of the spinal cord 1, 3.

Diagnostic workup:

• Serum vitamin B12 level (deficiency if <200 pg/mL) 4
• Methylmalonic acid (MMA) and homocysteine - more sensitive than serum B12 and detect functional deficiency even with low-normal B12 levels 4
• Note: Neurological symptoms may precede hematological abnormalities in one-third of cases 4

Treatment protocols:

• With neurological involvement (sensory/motor/gait symptoms): Hydroxocobalamin 1 mg IM on alternate days until no further improvement, then 1 mg IM every 2 months, with urgent neurology/hematology consultation 1
• Without neurological involvement: Hydroxocobalamin 1 mg IM three times weekly for 2 weeks, then 1 mg IM every 2-3 months lifelong 1, 3

High-risk populations requiring surveillance:

• Extensive small bowel disease or ileal resection (terminal ileum is primary B12 absorption site) 1, 2
• Pernicious anemia (requires lifelong monthly injections) 3
• Vegans (no dietary B12 source) 3

Folate Deficiency

Only treat after excluding B12 deficiency 1. Oral folic acid 5 mg daily for minimum 4 months 1. Consider medications that impair folate (anticonvulsants, sulfasalazine, methotrexate) 1.

Other Macrocytic Causes

• Myelodysplastic syndrome (MDS) 1
• Medications: Azathioprine, hydroxyurea, methotrexate cause macrocytosis 1
• Hypothyroidism 1
• Alcoholism (isolated macrocytosis without anemia) 1

Normocytic Anemia with Normal/Low Reticulocytes

Anemia of Chronic Disease

Same diagnostic criteria as above (ferritin >100 μg/L, transferrin saturation <20%) 1. Optimize treatment of underlying disease before considering erythropoiesis-stimulating agents (ESAs) 1. If ESAs are used after optimizing disease therapy and IV iron, target hemoglobin should not exceed 12 g/dL 1.

Renal Anemia

Inappropriately low endogenous erythropoietin levels 1.

Bone Marrow Disorders

• Aplastic anemia, pure red cell aplasia 1
• Leukemias, MDS 1
• Bone marrow infiltration by malignancy 1

Elevated Reticulocyte Count (Any MCV)

Elevated reticulocytes exclude nutritional deficiencies and indicate either hemolysis or acute hemorrhage 1.

Hemolysis Workup

• Haptoglobin (decreased) 1
• Lactate dehydrogenase (elevated) 1
• Bilirubin (elevated, indirect) 1
• Direct antibody test (Coombs) 1

Hemolysis can present as falsely elevated MCV due to reticulocytosis 1.

Critical Pitfalls to Avoid

Mixed deficiencies neutralize MCV: Concurrent iron deficiency (microcytic) and B12/folate deficiency (macrocytic) may produce normal MCV, but elevated RDW reveals the underlying heterogeneity 1, 2.

Inflammation falsely elevates ferritin: Always measure CRP when interpreting ferritin; inflammatory states can mask true iron deficiency 1, 2.

Never give folate before excluding B12 deficiency: This prevents anemia but allows irreversible spinal cord damage 1, 3.

Functional B12 deficiency in chronic disease: Inflammatory cytokines may impair B12 transport and utilization despite normal serum levels; consider MMA/homocysteine if clinical suspicion is high 2.

Homocysteine interpretation requires renal function: Renal impairment elevates homocysteine independent of vitamin status 4.

Treatment Priorities Based on Morbidity/Mortality

1. B12 deficiency with neurological symptoms: Immediate IM hydroxocobalamin to prevent irreversible spinal cord damage 1, 3
2. Severe anemia (Hgb <7 g/dL): Consider transfusion, followed by IV iron supplementation 1
3. Iron deficiency: IV iron preferred over oral in inflammatory states due to hepcidin-mediated absorption block 1
4. Optimize underlying disease in ACD before ESAs 1