What lab tests should be done for a 5-year-old male patient with suspected iron deficiency anemia and thrombocytosis?

Laboratory Testing for Suspected Iron Deficiency Anemia with Thrombocytosis in a 5-Year-Old

Order a complete blood count (CBC) with hemoglobin, hematocrit, mean corpuscular volume (MCV), red cell distribution width (RDW), and platelet count, along with serum ferritin as the single most powerful confirmatory test for iron deficiency. 1

Initial Screening Tests

Essential First-Line Tests

• Hemoglobin and hematocrit are the most commonly used screening tests due to their low cost and ease of performance, though they only detect late-stage iron deficiency 1
• For a 5-year-old male, anemia is defined as hemoglobin below the 5th percentile for age and sex-specific reference ranges 1
• Mean corpuscular volume (MCV) should be measured, as microcytosis is characteristic of iron deficiency, though it may be absent in combined deficiencies 1
• Red cell distribution width (RDW) helps identify combined deficiencies (such as concurrent folate deficiency) and confirms the dual diagnosis when elevated 1, 2

Confirmatory Iron Studies

• Serum ferritin is the most powerful test for iron deficiency and should be obtained in all cases 1
• A ferritin level <30 μg/L indicates iron deficiency in children without inflammation, with levels <12 μg/L being diagnostic 1, 2
• Important caveat: Ferritin may be falsely elevated above 12-15 μg/L in patients with concurrent inflammation, malignancy, or hepatic disease, though levels >100 μg/L essentially rule out iron deficiency 1

Additional Iron Parameters When Diagnosis Remains Unclear

• Transferrin saturation <20-30% supports the diagnosis of iron deficiency when ferritin results are equivocal 1, 2
• Consider measuring serum iron and total iron-binding capacity if ferritin is borderline and clinical suspicion remains high 1

Addressing the Thrombocytosis

Platelet Count Assessment

• The CBC will provide the platelet count, which is essential since thrombocytosis (>400 × 10⁹/L) occurs in approximately 22% of patients with iron deficiency anemia 3, 4
• Document the degree of thrombocytosis, as it typically resolves within 3 months of iron replacement therapy in most patients 4
• Critical distinction: Iron deficiency-related thrombocytosis is a secondary (reactive) process, not essential thrombocythemia 3

Differentiating Secondary from Primary Thrombocytosis

• In a 5-year-old with documented iron deficiency anemia, the thrombocytosis is almost certainly secondary (reactive) to the iron deficiency 3, 4
• No molecular testing (JAK2, CALR, MPL mutations) is needed in this clinical context, as these are only indicated when essential thrombocythemia is suspected 3
• The presence of iron deficiency anemia itself strongly predicts secondary thrombocytosis rather than a primary myeloproliferative disorder 3

Special Considerations for Pediatric Patients

Thalassemia Screening

• If the child has an elevated red blood cell count with persistently low MCV despite iron repletion, consider beta-thalassemia trait 2
• Hemoglobin electrophoresis or HPLC should be obtained if thalassemia is suspected (HbA2 >3.5% confirms beta-thalassemia trait) 2
• Key pitfall: Do not assume microcytosis is solely from iron deficiency in certain ethnic populations where hemoglobinopathies are common 1

Interpretation in Children

• The sensitivity of anemia screening for iron deficiency is only 25% in children aged 1-5 years, with specificity of 92% 1
• This means anemia alone misses many iron-deficient children, making ferritin measurement essential for confirmation 1

Follow-Up Testing Strategy

• Reassess hemoglobin, ferritin, and platelet count after 8-12 weeks of iron therapy to confirm response 2
• Expect the platelet count to decrease in approximately 72% of patients, with thrombocytosis resolving in most cases 4
• If MCV remains low despite ferritin normalization, proceed with hemoglobin electrophoresis to evaluate for thalassemia trait 2