What laboratory tests, including vitamin D (Vit D) levels, should be checked in a patient with iron deficiency anemia, particularly those with a history of gastrointestinal disorders or chronic diseases?

Laboratory Testing for Iron Deficiency

In patients with iron deficiency, the minimum workup should include complete blood count with red cell indices (MCV, RDW), reticulocyte count, serum ferritin, transferrin saturation, and C-reactive protein (CRP). 1

Essential Initial Tests

Minimum Required Tests:

• Complete blood count (CBC) with red cell indices including mean corpuscular volume (MCV) and red cell distribution width (RDW) 1
• Reticulocyte count to assess bone marrow response 1
• Serum ferritin as the primary marker of iron stores 1, 2
• Transferrin saturation (TfS) to evaluate iron availability for erythropoiesis 1, 2
• C-reactive protein (CRP) to assess inflammation, which affects ferritin interpretation 1

Interpreting Ferritin in the Context of Inflammation

Critical pitfall: Ferritin is an acute phase reactant and can be falsely elevated in inflammatory states. 1

• Without inflammation: Ferritin <30 μg/L indicates iron deficiency 1
• With inflammation present: Ferritin up to 100 μg/L may still reflect iron deficiency 1
• Ferritin 30-100 μg/L with inflammation: Suggests mixed iron deficiency and anemia of chronic disease 1
• Ferritin >100 μg/L with transferrin saturation <20%: Indicates anemia of chronic disease 1

Extended Workup Tests

When initial tests are inconclusive or specific conditions are suspected:

• Vitamin B12 and folic acid levels to identify concurrent nutritional deficiencies, especially in patients with gastrointestinal disorders, extensive small bowel resection, or macrocytosis 1, 2, 3
• Haptoglobin and lactate dehydrogenase if hemolysis is suspected (elevated reticulocyte count) 1
• Serum creatinine and urea to evaluate for chronic kidney disease as a cause of anemia 1, 2
• Thyroid function tests in normocytic anemia to exclude hypothyroidism 1, 2
• Hemoglobin electrophoresis in microcytic anemia to rule out thalassemia or hemoglobinopathies 2

Vitamin D Testing in Iron Deficiency

Vitamin D levels should be checked in patients with iron deficiency, particularly those with chronic gastrointestinal disorders. 1, 4

Rationale for vitamin D testing:

• Vitamin D deficiency is significantly associated with iron deficiency and iron deficiency anemia 5
• Iron deficiency may impair vitamin D absorption in the small intestine 6
• Vitamin D deficiency is highly prevalent in patients with inflammatory bowel disease (53% in Crohn's disease, 44% in ulcerative colitis) 1
• Vitamin D has immunomodulatory properties relevant to chronic inflammatory conditions 1
• Vitamin D deficiency is associated with increased disease activity in IBD 1

Vitamin D interpretation:

• Deficiency: 25-hydroxyvitamin D <20 ng/mL (50 nmol/L) 4, 7, 8
• Insufficiency: 25-hydroxyvitamin D 20-30 ng/mL (50-75 nmol/L) 7
• Target level: ≥30 ng/mL 4, 8

Monitoring Frequency

For patients with chronic gastrointestinal disorders:

• In remission or mild disease: Check CBC, ferritin, and CRP every 6-12 months 1
• Active disease: Check every 3 months 1
• Vitamin D status: Routine annual monitoring recommended 1
• Vitamin B12 and folate: More frequent than annual assessment in patients with extensive small bowel resection, extensive ileal Crohn's disease, or ileal-anal pouch 1

Special Populations Requiring Additional Testing

Patients with chronic kidney disease:

• Monitor hemoglobin every 3 months if GFR <30 mL/min/1.73m² 2, 3
• Include serum bicarbonate, calcium, phosphorus, and intact PTH 2

Patients with inflammatory bowel disease:

• Differential white blood cell count if bone marrow dysfunction suspected 1
• Stool guaiac test for occult blood to identify gastrointestinal bleeding 1

Critical Pitfalls to Avoid

• Never rely on ferritin alone in inflammatory states – always measure transferrin saturation and CRP concurrently 1
• Don't overlook mixed deficiencies – concurrent B12/folate and iron deficiency can result in normocytic indices despite significant abnormalities 2, 3
• Don't assume normal MCV excludes iron deficiency – early iron deficiency presents with normal MCV but elevated RDW 2
• In men and postmenopausal women with iron deficiency, always investigate for gastrointestinal malignancy with upper endoscopy and colonoscopy 2