What laboratory test should be ordered for a 17-year-old female with normal comprehensive metabolic panel and complete blood count except for a hemoglobin of 11.8 g/dL and hematocrit of 35.2%?

Laboratory Workup for Mild Anemia in a 17-Year-Old Female

Order a complete iron panel including serum ferritin, transferrin saturation, serum iron, and total iron-binding capacity (TIBC), along with C-reactive protein (CRP) to evaluate for iron deficiency—the most common cause of anemia in adolescent females. 1, 2

Rationale for Iron Studies

Why This Patient Needs Further Testing

• The hemoglobin of 11.8 g/dL meets WHO criteria for mild anemia in non-pregnant women over 15 years (defined as Hb 11.0–11.9 g/dL), requiring confirmation of iron deficiency before treatment. 1, 2
• A complete blood count (CBC) alone is insufficient and often misleading—individuals can have normal red cell indices while harboring significant iron depletion that will progress to frank anemia if undetected. 3
• Iron deficiency affects 7.8 million adolescent girls and women of childbearing age in the United States, with prevalence exceeding 50% in some adolescent cohorts due to menstrual blood loss, rapid growth, and inadequate dietary iron intake. 4, 1

Essential Iron Panel Components

Serum ferritin is the single most powerful and specific test for iron deficiency:

• Ferritin <15 µg/L is diagnostic of iron deficiency. 2
• Ferritin <30 µg/L indicates depleted iron stores in the absence of inflammation. 1, 2
• Critical caveat: Ferritin is an acute-phase reactant and can be falsely elevated by inflammation, infection, or chronic disease—this is why concurrent CRP measurement is mandatory. 1

Transferrin saturation <15–16% supports iron deficiency and is less affected by acute inflammation than ferritin, helping differentiate true iron deficiency from anemia of chronic disease. 1

C-reactive protein (CRP) must be measured simultaneously because inflammation can mask iron deficiency by elevating ferritin into the "normal" range; a normal CRP is required to interpret low ferritin accurately. 1

Total iron-binding capacity (TIBC) >45 µmol/L is another indicator of iron deficiency and completes the iron panel. 1

Additional Helpful Tests Already Available

The CBC with differential provides red cell indices that offer clues but are not diagnostic alone:

• Mean corpuscular hemoglobin (MCH) is the most sensitive early marker of iron deficiency, detecting iron-restricted erythropoiesis before microcytosis develops. 1
• Mean corpuscular volume (MCV) and red cell distribution width (RDW) help distinguish iron deficiency (low MCV with elevated RDW) from thalassemia trait (low MCV with normal RDW <14%). 2
• Do not assume normal MCV excludes iron deficiency—early deficiency often presents with normocytic indices. 1

Clinical Pitfalls to Avoid

• Never rely on ferritin alone without CRP: Inflammation falsely elevates ferritin and will cause you to miss true iron deficiency. 1
• Never stop at CBC alone: Many iron-deficient patients have normal hemoglobin, hematocrit, and red cell indices until severe depletion occurs. 3
• Never use risk-based questionnaires: Studies show that screening questionnaires (including the Bright Futures Adolescent Previsit Questionnaire) fail to predict objective iron deficiency or anemia in adolescent females. 5

If Iron Studies Confirm Deficiency

Once iron deficiency is confirmed (ferritin <15–30 µg/L with low transferrin saturation):

• Initiate oral iron therapy with 60–120 mg elemental iron daily. 2
• Recheck hemoglobin after 4 weeks; an increase ≥1 g/dL confirms iron-deficiency anemia and adequate response. 4, 2
• If hemoglobin fails to rise despite documented compliance, further evaluate with celiac disease screening (tissue transglutaminase antibodies) and assess for gastrointestinal blood loss. 4, 2
• Menstrual blood loss is the most frequent cause in this population, but malabsorption and restrictive diets must also be considered. 1, 2

If Iron Studies Are Normal

When iron studies return normal despite anemia:

• Order vitamin B12 and folate levels to evaluate for megaloblastic anemia. 1
• Consider hemoglobin electrophoresis if the patient has microcytosis with normal iron studies and belongs to an at-risk ethnic group (African, Mediterranean, or Southeast Asian ancestry) to exclude thalassemia or hemoglobinopathies. 4, 2