USMLE NBME Step 1 Question: Hereditary Spherocytosis
Clinical Vignette
A 22-year-old man presents to his primary care physician with a 3-month history of fatigue. He reports that his father and paternal grandmother had similar symptoms and required splenectomy in their 30s. Physical examination reveals mild scleral icterus and splenomegaly extending 4 cm below the left costal margin. Laboratory studies show:
- Hemoglobin: 9.5 g/dL
- MCV: 96 fL
- MCHC: 36 g/dL
- Reticulocyte count: 4%
- Total bilirubin: 3.0 mg/dL (indirect: 2.2 mg/dL)
- Direct antiglobulin test (DAT): Negative
- Peripheral blood smear: Numerous spherocytes
What is the most appropriate next diagnostic test?
Answer Options
A. Osmotic fragility test or eosin-5'-maleimide (EMA) binding test
B. Hemoglobin electrophoresis
C. Glucose-6-phosphate dehydrogenase (G6PD) enzyme assay
D. Bone marrow biopsy
E. Serum haptoglobin level
Correct Answer: A
Detailed Explanation
The most appropriate next step is confirmatory testing with osmotic fragility test or EMA binding test, as this patient's presentation is classic for hereditary spherocytosis (HS) and requires specific diagnostic confirmation before considering definitive management such as splenectomy. 1
Diagnostic Approach to This Case
The patient presents with the diagnostic triad of HS:
- Hemolytic anemia (low hemoglobin with elevated reticulocytes and indirect bilirubin)
- Spherocytes on peripheral smear
- Positive family history (autosomal dominant inheritance pattern in 75% of cases)
- Negative DAT (excluding autoimmune hemolytic anemia)
Why Each Answer is Correct or Incorrect
Option A (CORRECT): The ICSH guidelines specifically recommend confirmatory testing for HS when screening parameters are suggestive 1. The EMA binding test has become the preferred confirmatory test with sensitivity >92% and specificity >99% for HS 1, 2. The osmotic fragility test (particularly the incubated version) remains an acceptable alternative confirmatory test 3, 1. These tests directly assess the membrane protein defects that define HS and are essential before proceeding to splenectomy, which is the definitive treatment for moderate-to-severe cases.
Option B (Incorrect): Hemoglobin electrophoresis evaluates for hemoglobinopathies like thalassemia or sickle cell disease. While thalassemia can present with microcytic anemia and splenomegaly, this patient has a normal-to-elevated MCHC (36 g/dL), which is characteristic of HS 4, 3. Thalassemia typically shows hypochromic (low MCHC) microcytic cells. The spherocytes and family history of splenectomy make HS far more likely 5.
Option C (Incorrect): G6PD deficiency causes episodic hemolytic anemia triggered by oxidative stress (infections, certain drugs, fava beans), not chronic compensated hemolysis with persistent spherocytosis. The continuous nature of symptoms and spherocytes on smear make this diagnosis unlikely 4.
Option D (Incorrect): Bone marrow biopsy is not indicated in HS diagnosis. The reticulocytosis (4%) demonstrates appropriate bone marrow response to hemolysis 6. Bone marrow examination would only be considered if there were cytopenias in multiple cell lines suggesting marrow failure or infiltrative process 7.
Option E (Incorrect): While haptoglobin is decreased in hemolysis and can support the diagnosis, it is a nonspecific marker that doesn't distinguish between different causes of hemolysis 6. The patient already has clear evidence of hemolysis (elevated indirect bilirubin, reticulocytosis), so haptoglobin adds no diagnostic value. The critical next step is confirming the specific etiology (HS) with definitive testing.
Key Diagnostic Parameters in HS
Modern hematology analyzers provide screening parameters that strongly suggest HS 4, 3, 8:
- Elevated MCHC (>35 g/dL): Present in this patient at 36 g/dL
- ΔMCV-MSCV (difference between mean cell volume and mean sphered cell volume) >10 fL: Sensitivity 82.75%, specificity 95.9% 3
- Elevated mean reticulocyte volume (MRV) with normal immature reticulocyte fraction (IRF) 8
However, these screening parameters must be followed by confirmatory testing (EMA binding or osmotic fragility) before making treatment decisions 1, 9.
Clinical Pitfall
The most common diagnostic error is confusing HS with autoimmune hemolytic anemia (AIHA), as both present with spherocytes and hemolysis. The negative DAT in this case excludes AIHA 5, 6. However, in cases where DAT is equivocal, the reticulocyte parameters can help: HS shows elevated MRV without proportional increase in IRF, while AIHA typically shows both elevated 8.