Laboratory Workup for a 17-Year-Old Female with Mild Microcytic Anemia
Immediate First-Line Testing
Order a complete iron panel (serum ferritin, transferrin saturation, serum iron, and total iron-binding capacity) together with C-reactive protein, and begin oral ferrous sulfate 60 mg daily while awaiting results. 1
The patient's laboratory values—hemoglobin 11.8 g/dL (below the WHO threshold of 12 g/dL for non-pregnant women), MCH 26.9 pg (reduced), MCHC 33.5 g/dL (low-normal), and RDW 14.3% (elevated)—strongly suggest iron-deficiency anemia. 1, 2 The combination of low MCH with elevated RDW is highly specific for iron deficiency, as MCH is the most sensitive early marker of iron-restricted erythropoiesis and falls before frank microcytosis develops. 1, 2
Specific Iron Studies to Order
Serum ferritin < 30 µg/L confirms depleted iron stores when inflammation is absent and is the single most powerful test for iron deficiency. 1, 2
Transferrin saturation < 15–16% supports iron deficiency and remains reliable even when ferritin may be falsely elevated by inflammation. 1, 2
C-reactive protein (CRP) must be measured concurrently because ferritin is an acute-phase reactant; inflammation can mask true iron deficiency by elevating ferritin into the normal range. 1, 2
Total iron-binding capacity (TIBC) > 45 µmol/L provides additional confirmation of iron deficiency. 1
Additional Helpful Tests
- Reticulocyte count should be obtained to assess bone marrow response; a low or inappropriately normal reticulocyte count in the setting of anemia confirms inadequate marrow compensation, consistent with iron deficiency. 1, 3
Empiric Treatment While Awaiting Results
Prescribe oral ferrous sulfate 60 mg elemental iron once daily, taken on an empty stomach between meals to maximize absorption. 1 This dosage is specifically recommended for adolescent females with mild anemia by CDC guidelines. 1
Provide dietary counseling to include iron-rich foods such as red meat, poultry, fish, fortified cereals, beans, and lentils, along with vitamin C-rich foods (citrus, tomatoes) to enhance absorption. 1
Four-Week Therapeutic Response Assessment
Re-measure hemoglobin and hematocrit at 4 weeks; an increase of ≥1 g/dL (or hematocrit rise ≥3%) confirms iron-deficiency anemia even if baseline iron studies were equivocal. 1 This therapeutic response validates the diagnosis and justifies continuation of therapy for an additional three months (total four-month course) to fully replenish iron stores. 1
Evaluation if No Response After 4 Weeks
If hemoglobin does not rise despite documented adherence to iron supplementation, order hemoglobin electrophoresis to exclude thalassemia trait. 1 Thalassemia minor typically shows RDW ≤ 14% (versus > 14% in iron deficiency), normal or borderline iron studies, and an MCV disproportionately low relative to the degree of anemia. 1, 2, 4, 5
The patient's RDW of 14.3% (elevated) strongly favors iron deficiency over thalassemia trait, which usually presents with RDW ≤ 14% because red cells are more uniform. 1, 2, 4 However, approximately 50% of thalassemia cases can exhibit an elevated RDW, so hemoglobin electrophoresis remains necessary if iron therapy fails. 1, 5
Assessment for Underlying Cause
In premenopausal adolescent females, menstrual blood loss is the most common cause of iron deficiency, but a careful menstrual history is essential to determine whether losses are adequate to explain the deficiency. 1 Assess for heavy menstrual bleeding, which may necessitate gynecologic referral. 1
Evaluate dietary iron intake, particularly vegetarian or vegan eating patterns, as these diets increase the risk of iron deficiency in adolescents. 1, 2
If menstrual losses seem inadequate to explain the deficiency, consider gastrointestinal sources by taking a careful history of NSAID use, gastrointestinal symptoms, and dietary factors. 1
Consider celiac disease screening with tissue transglutaminase (tTG) antibody testing, as malabsorption is a significant cause of iron deficiency with a prevalence of approximately 5% in iron-deficiency anemia. 1
Critical Pitfalls to Avoid
Do not rely on ferritin alone without CRP; inflammation can falsely elevate ferritin and mask true iron deficiency. 1, 2
Do not assume a normal MCV excludes iron deficiency; MCH typically falls before MCV in early deficiency, and the patient's MCH of 26.9 pg is already reduced. 1, 2
Do not limit evaluation to a CBC; WHO criteria for anemia require full iron studies for proper diagnosis. 1
Do not use hemoglobin electrophoresis as a first-line test; it should be reserved for cases with normal or borderline iron parameters after a therapeutic trial. 1, 2
Normal AST Interpretation
The normal AST level (11 IU/L) effectively rules out liver disease as a contributing cause, since hepatic disorders typically raise AST 2–6-fold above the upper limit of normal. 1