What is the appropriate work‑up for macrocytic anemia in a patient with chronic kidney disease stage 3b and normal vitamin B12, folate, and thyroid function?

Work-Up for Macrocytic Anemia in CKD Stage 3b with Normal B12, Folate, and Thyroid

In your patient with CKD stage 3b and macrocytosis despite normal B12 and folate, immediately obtain a reticulocyte count, comprehensive iron studies (serum iron, TIBC, transferrin saturation, and ferritin), and perform a peripheral blood smear examination to distinguish between erythropoietin deficiency, functional iron deficiency, and medication effects. 1

Initial Laboratory Assessment

Reticulocyte Count

• Obtain a reticulocyte count corrected for the degree of anemia (reticulocyte index). 1 A low or inappropriately normal reticulocyte count in the setting of anemia indicates inadequate bone marrow response, which in CKD patients replete with B12 and folate most commonly reflects either insufficient erythropoietin production or inflammation. 1, 2

• The reticulocyte count helps differentiate between production defects (typical of CKD-related anemia) versus hemolysis or blood loss (which would show elevated reticulocytes). 1

Comprehensive Iron Studies

• Measure serum iron, total iron-binding capacity (TIBC), transferrin saturation (TSAT), and serum ferritin. 1 Iron deficiency is present in 25-37.5% of CKD patients presenting with anemia. 1

• Transferrin saturation is more reliable than ferritin alone in CKD because ferritin is an acute-phase reactant that becomes falsely elevated during inflammation. 1, 3 In non-dialysis CKD patients not on erythropoiesis-stimulating agents, ferritin <25 ng/mL in males or <11 ng/mL in females predicts true iron deficiency. 1, 3

• If ferritin is elevated (>100 ng/mL) but TSAT is low (<20%), this suggests functional iron deficiency due to inflammation-induced hepcidin elevation. 3 Consider measuring C-reactive protein to assess the inflammatory contribution to elevated ferritin. 1, 3

Peripheral Blood Smear

• Review the peripheral smear for hypersegmented neutrophils (suggesting megaloblastic process despite normal B12/folate levels), macro-ovalocytes, hypochromic cells, or abnormalities in other cell lines. 1

• Abnormalities in two or more cell lines (white cells, red cells, platelets) warrant hematology referral for possible bone marrow pathology. 1

Evaluate for Occult Blood Loss

• Perform stool guaiac testing for occult gastrointestinal bleeding. 1 The presence of iron deficiency in a CKD patient not on erythropoiesis-stimulating agents requires a search for blood loss, which is usually gastrointestinal. 1

• Your patient's elevated MCV with normal B12/folate may mask concurrent microcytosis from iron deficiency—the two abnormalities can neutralize each other, resulting in a falsely normal or elevated MCV. 1 An elevated red cell distribution width (RDW) supports this possibility. 1

Assess for Medication-Related Macrocytosis

• Review all medications for agents that can cause macrocytosis independent of B12/folate deficiency. 1 Common culprits include:

  • Hydroxyurea 1
  • Azathioprine or 6-mercaptopurine 1
  • Diphenytoin 1
  • Methotrexate
  • Zidovudine

• Specifically inquire about alcohol use, as alcoholism is the most common cause of macrocytosis in general populations (36.5% in one series) and can occur with normal B12/folate levels. 4

Evaluate for Inflammation

• Measure C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR). 1, 3 Chronic inflammation in CKD impairs erythropoiesis through multiple mechanisms: inhibition of erythropoietin production, direct impairment of early erythroblast growth, and stimulation of hepatic hepcidin release causing functional iron deficiency. 2, 3

Consider Erythropoietin Deficiency as Primary Mechanism

• Macrocytosis in CKD can result from erythropoietin deficiency itself or from treatment with erythropoiesis-stimulating agents (ESAs), which shift larger immature reticulocytes into circulation. 1 Even without ESA therapy, the anemia of CKD is typically normocytic but can be macrocytic. 1

• With eGFR 37 mL/min (CKD stage 3b), erythropoietin deficiency is the fundamental driver of anemia, as failing kidneys cannot produce adequate amounts of this hormone. 2, 3

Algorithmic Approach

1. If reticulocyte count is low and iron studies show TSAT <20% or ferritin <25 ng/mL (males) or <11 ng/mL (females): Diagnose iron deficiency and search for occult GI bleeding. 1

2. If reticulocyte count is low, iron studies are adequate (TSAT >20%, ferritin >100 ng/mL), and peripheral smear shows no megaloblastic features: Diagnose erythropoietin-deficient anemia of CKD. 1, 2

3. If ferritin is elevated (>100 ng/mL) but TSAT is low (<20%): Diagnose functional iron deficiency due to inflammation; measure CRP to confirm. 1, 3

4. If peripheral smear shows hypersegmented neutrophils or macro-ovalocytes despite normal B12/folate: Consider repeating B12 with methylmalonic acid and homocysteine levels, as tissue deficiency can occur with borderline-normal serum levels. 1

5. If no clear cause is identified: Review medications for macrocytosis-inducing agents, assess alcohol use, and consider hematology referral for bone marrow evaluation if anemia is severe or progressive. 1, 4

Common Pitfalls

• Do not assume macrocytosis in CKD is solely due to B12/folate deficiency based on MCV alone. 1 Macrocytosis can reflect iron excess, ESA therapy effects, medication effects, or alcohol use. 1

• Do not rely on ferritin alone to assess iron status in CKD. 1, 3 Inflammation falsely elevates ferritin; always interpret ferritin alongside TSAT and consider CRP measurement. 1, 3

• Do not overlook coexisting iron deficiency masked by macrocytosis. 1 Check RDW and review the peripheral smear for hypochromic cells. 1

• Do not delay evaluation for occult GI bleeding when iron deficiency is present. 1 Iron deficiency in CKD without menstrual losses mandates GI investigation. 1

• Do not forget that up to 20% of macrocytosis cases remain unexplained even after thorough evaluation. 4 In such cases, repeat CBC in 3 months as you planned, and monitor for progression or development of new findings. 4