Given laboratory results showing mild anemia with low vitamin B12, low‑normal ferritin, elevated total iron‑binding capacity, low mean corpuscular hemoglobin concentration, and elevated red‑cell distribution width, what is the most likely diagnosis and what treatment recommendations are indicated?

Combined Iron and Vitamin B12 Deficiency Anemia

This patient has combined iron and vitamin B12 deficiency anemia requiring immediate supplementation of both nutrients.

Diagnosis

The laboratory findings clearly indicate a mixed anemia with dual nutritional deficiencies:

Iron Deficiency Evidence

• Ferritin 21.2 ng/mL (low-normal, concerning in context)
• TIBC 412.6 µg/dL (elevated, indicating iron depletion)
• Low MCHC 30.1 g/dL (hypochromia)
• Elevated RDW 15.8% (anisocytosis from iron deficiency)

While ferritin is technically above the absolute deficiency threshold of <15 µg/L, a level of 21.2 ng/mL represents depleted iron stores. British Society of Gastroenterology guidelines define iron deficiency as ferritin <30 µg/L in the absence of inflammation 1. The elevated TIBC confirms true iron deficiency rather than anemia of chronic disease.

Vitamin B12 Deficiency Evidence

• Vitamin B12 169 pg/mL (below normal range of 200-600)
• Hemoglobin 11.3 g/dL (anemia in females)
• Normal MCV 90.6 fL (macrocytosis masked by concurrent iron deficiency)

The laboratory note explicitly warns that 5-10% of patients with B12 levels between 200-400 pg/mL experience neuropsychiatric and hematologic abnormalities 1. At 169 pg/mL, this patient is definitively deficient.

Critical Diagnostic Insight

Vitamin B12 deficiency masks iron deficiency 2. Research demonstrates that cobalamin deficiency causes ineffective erythropoiesis, preventing iron utilization by erythroblasts. This artificially maintains serum iron and ferritin levels until B12 is repleted, at which point iron deficiency becomes apparent. Studies show that 49.3% of patients develop overt iron deficiency after B12 treatment compared to only 9.3% before treatment 2.

The normal MCV (90.6 fL) despite B12 deficiency occurs because concurrent iron deficiency causes microcytosis that counterbalances the macrocytosis from B12 deficiency 3. This is why mixed anemia is diagnostically challenging—the MCV appears normal and peripheral smear may show morphologically normal cells 3.

Treatment Recommendations

Vitamin B12 Supplementation (Priority #1)

Initiate oral vitamin B12 1,000 µg daily immediately 4.

• Oral crystalline cobalamin is effective for most B12 deficiency cases unless malabsorption from bariatric surgery or pernicious anemia is present 4
• Check for intrinsic factor antibodies if response is inadequate
• If pernicious anemia is confirmed, switch to intramuscular/subcutaneous administration
• Increase dietary intake of B12-rich foods: turkey, pork, eggs, liver, corned beef 4

Iron Supplementation (Concurrent)

Begin oral iron supplementation at standard therapeutic doses 1.

• Most oral iron preparations are equally effective when appropriately taken 5
• Co-ingest with vitamin C to enhance non-heme iron absorption 5
• Avoid tea and coffee around meal times (impair absorption) 5
• Increase dietary heme iron from meat and seafood 5
• Expect gastrointestinal side effects (constipation, nausea) 5

Parenteral iron is NOT indicated unless pathological gut malabsorption is documented 5.

Monitoring Strategy

Recheck complete blood count, ferritin, and B12 levels in 6-8 weeks:

• Hemoglobin should increase by ≥1 g/dL with adequate response 1
• Ferritin will likely decrease initially as B12 repletion allows iron utilization for erythropoiesis 2
• If ferritin drops below 15 µg/L, intensify iron supplementation
• Reticulocyte count should increase within 1-2 weeks, indicating bone marrow response 6

Critical Pitfalls to Avoid

1. Do not treat only one deficiency—both must be addressed simultaneously. Treating B12 alone will unmask severe iron deficiency 2

2. Do not assume normal MCV excludes B12 deficiency—iron deficiency masks macrocytosis in mixed anemia 3

3. Do not delay B12 treatment waiting for further testing—neurologic damage from B12 deficiency may become irreversible 4, 7

4. Do not use ferritin alone to assess iron status—TIBC and transferrin saturation provide additional critical information 1

5. Screen for underlying causes: Consider celiac disease, atrophic gastritis, dietary insufficiency (vegetarian/vegan diet), or chronic blood loss 1, 4

The elevated BUN/creatinine ratio (26.67) suggests possible dehydration or early prerenal state—ensure adequate hydration during treatment as hematopoiesis increases metabolic demands.