A child with a pustular rash at a cat scratch site and tender axillary lymphadenopathy: which test best confirms cat-scratch fever?

A child with a pustular rash at a cat scratch site and tender axillary lymphadenopathy: which test best confirms cat-scratch fever?

• A. Complete blood count and C-reactive protein
• B. Immunofluorescent assay for serum antibodies
• C. Needle aspiration of the affected lymph nodes
• D. Prescribe a 5-day course of azithromycin

Answer: (B)

Cat-scratch fever is diagnosed clinically with a history of a cat scratch or bite and regional lymphadenopathy, but confirming infection with Bartonella henselae relies on serology. The immunofluorescent assay for serum antibodies against Bartonella is the best test to confirm the diagnosis because it directly detects the body's immune response to the organism, and Bartonella is difficult to culture reliably. This noninvasive test can indicate recent or past exposure, guiding confirmation when the clinical picture fits. Inflammatory markers like CBC and CRP are nonspecific, needle aspiration of the lymph node is invasive and not routinely needed for confirmation, and prescribing antibiotics is a treatment, not a diagnostic test.

Cat-scratch fever is diagnosed clinically with a history of a cat scratch or bite and regional lymphadenopathy, but confirming infection with Bartonella henselae relies on serology. The immunofluorescent assay for serum antibodies against Bartonella is the best test to confirm the diagnosis because it directly detects the body's immune response to the organism, and Bartonella is difficult to culture reliably. This noninvasive test can indicate recent or past exposure, guiding confirmation when the clinical picture fits. Inflammatory markers like CBC and CRP are nonspecific, needle aspiration of the lymph node is invasive and not routinely needed for confirmation, and prescribing antibiotics is a treatment, not a diagnostic test.