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Difficulty: EasyOrdering Laboratory Tests

A 28-year-old woman comes to the clinic reporting a 3-month history of generalized fatigue and decreased exercise tolerance. She notes feeling winded after climbing a single flight of stairs. Her menstrual cycles are regular but last 7 days with heavy bleeding, requiring her to change a high-absorbency pad every 2 hours during the first three days. She has followed a strict vegan diet for the past two years. Her vital signs are temperature 36.8°C (98.2°F), blood pressure 110/70 mm Hg, pulse 88/min, and respirations 14/min. Physical examination shows pale conjunctivae but no scleral icterus, organomegaly, or lymphadenopathy. A complete blood count reveals a hemoglobin level of 10.1 g/dL and a mean corpuscular volume (MCV) of 72 fL. Which of the following is the most appropriate next step in laboratory evaluation?

  1. Serum iron studies including ferritinAnswer
  2. B
    Hemoglobin electrophoresis
  3. C
    Bone marrow aspiration and biopsy
  4. D
    Direct antiglobulin (Coombs) test
  5. E
    Serum methylmalonic acid and homocysteine levels

Answer

Serum iron studies including ferritin
The correct next step is to order serum iron studies, including ferritin. This patient presents with a classic presentation of microcytic anemia (MCV < 80 fL). The differential diagnosis for microcytic anemia includes iron deficiency, thalassemia trait, anemia of chronic disease, and lead poisoning. Given her compelling clinical risk factors—chronic blood loss from menorrhagia and poor dietary iron intake due to a vegan diet—iron deficiency is the most likely diagnosis. Serum ferritin is the most accurate non-invasive test for assessing total body iron stores and is the gold standard initial screening laboratory test to confirm this diagnosis.

Step-by-Step Solution

1
Analyze the patient's presentation and complete blood count (CBC) results.
The patient has symptomatic anemia with a low hemoglobin (10.1 g/dL) and a low MCV (72 fL), indicating a microcytic anemia.
Identifying the morphological classification of the anemia (microcytic, normocytic, or macrocytic) directs the subsequent laboratory evaluation pathway.
2
Identify clinical risk factors from the patient's history.
The patient has heavy menstrual bleeding (blood loss) and follows a vegan diet (decreased dietary heme iron intake).
Correlating clinical risk factors with the laboratory abnormality establishes the most probable etiology, which is iron deficiency.
3
Select the appropriate confirmatory laboratory test.
Serum iron studies, specifically serum ferritin, are ordered to definitively diagnose iron deficiency anemia.
Ferritin is the most sensitive and specific initial non-invasive laboratory test for evaluating total body iron stores.

Key Concept

The initial laboratory evaluation of microcytic anemia (MCV < 80 fL), especially in patients with risk factors for chronic blood loss or poor dietary intake, requires serum iron studies (including ferritin) before pursuing more complex or invasive testing.
Estimated Time:50s
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Ordering Laboratory Tests

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