A 7-year-old child with hereditary spherocytosis presents with sudden severe anemia and reticulocytopenia. Parvovirus B19 is suspected. Serum IgM is negative. What is the most appropriate diagnostic test?

• A Repeat IgM testing after 4 weeks, since IgM is uniformly delayed in aplastic crisis
• B Electron microscopy of stool for viral particles
• C Bone marrow culture for parvovirus B19, the reference standard
• D Parvovirus B19 DNA by PCR, because viremia is intense during aplastic crisis while antibody response may be absent or delayed ✓

Explanation

In transient aplastic crisis the infection occurred some days earlier, so the brisk viremic phase drives marrow arrest and PCR for D19 DNA is highly positive, while IgM may not yet have appeared or may be blunted. Culture of D19 is impractical because it grows only in erythroid progenitors, and electron microscopy of stool is irrelevant since transmission is respiratory. Waiting 4 weeks risks missing the treatable window in a hemolytic patient.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

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