In a 17-year-old with normal comprehensive metabolic panel and complete blood count except for mild anemia (hemoglobin 11.8 g/dL, hematocrit 35.2 %), what laboratory test should be ordered next?

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

Order a serum ferritin level as the first-line test, followed by a complete iron panel including transferrin saturation, serum iron, and total iron-binding capacity to evaluate for iron deficiency—the most common cause of anemia in adolescents. 1

Rationale for Iron Studies

In a 17-year-old with mild anemia (hemoglobin 11.8 g/dL, hematocrit 35.2%), iron deficiency is the primary concern and represents the most prevalent nutritional deficiency in this age group. 1, 2

Serum Ferritin as the Primary Test

• Serum ferritin is the most specific and powerful test for iron deficiency and should be ordered first. 1
• A ferritin level <15 μg/L is diagnostic of iron deficiency, while levels <30 μg/L generally indicate depleted iron stores. 1
• In adolescents without inflammation, a ferritin <30 μg/L confirms iron deficiency; however, if inflammation is present, ferritin up to 45-100 μg/L may still be consistent with iron deficiency since ferritin is an acute phase reactant. 1

Complete Iron Panel Components

Beyond ferritin, obtain: 1, 3

• Transferrin saturation (a value <30% supports iron deficiency) 1
• Serum iron and total iron-binding capacity 1
• Red blood cell indices including mean corpuscular volume (MCV) and red cell distribution width (RDW) (already available from the CBC) 1, 4

Additional Considerations Based on MCV

If Microcytic (Low MCV)

• Low MCV with normal or elevated RDW suggests iron deficiency anemia, while low MCV with normal RDW (<14%) may indicate thalassemia trait. 1
• If iron studies are normal despite microcytosis and the patient has appropriate ethnic background, hemoglobin electrophoresis should be ordered to exclude thalassemia or hemoglobinopathy. 1

If Normocytic or Macrocytic

• Normocytic anemia with normal iron studies warrants evaluation for chronic disease, hemolysis (reticulocyte count), or bone marrow disorders. 4
• Macrocytic findings would prompt vitamin B12 and folate levels, though this is less common in adolescents. 4

Critical Pitfalls to Avoid

Do not rely on hemoglobin and hematocrit alone—these parameters decrease only with severe iron depletion and can miss early iron deficiency. 5 Many iron-depleted individuals have normal CBC parameters but abnormal ferritin and iron studies. 5

Do not assume dietary insufficiency without testing—while menstrual blood loss is the most common cause of iron deficiency in adolescent females 1, other causes including malabsorption (celiac disease), chronic blood loss, or restrictive diets must be considered. 1

Age-Specific Context

For adolescent girls (12-18 years), the CDC recommends screening all nonpregnant females for anemia every 5-10 years during routine examinations, with annual screening for those with risk factors including extensive menstrual blood loss, low iron intake, or previous iron deficiency. 1

Confirmatory Testing if Iron Deficiency is Identified

If ferritin is low (<15-30 μg/L) confirming iron deficiency: 1

• Initiate oral iron therapy (60-120 mg elemental iron daily for adolescents) 1, 2
• Recheck hemoglobin in 4 weeks—an increase of ≥1 g/dL confirms iron deficiency anemia as the diagnosis 1
• If no response to iron therapy despite compliance, further evaluation with celiac screening (tissue transglutaminase antibodies) and assessment for gastrointestinal blood loss may be warranted 1