Diagnosis of Cat Scratch Disease (CSD)
The Bartonella species are difficult to culture, and culture is not routinely recommended. Serology is the best initial test and can be performed by indirect fluorescent assay or enzyme-linked immunosorbent assay. Although more sensitive than culture, serologic tests lack specificity because many asymptomatic persons have positive serology because of previous (often asymptomatic) exposure.17 The percentage of the general population that has a positive serologic test varies widely, but appears to be higher in cat owners.17 Immunoglobulin G titers less than 1:64 suggest the patient does not have current Bartonella infection. Titers between 1:64 and 1:256 represent possible infection; repeat testing should be performed in these patients in 10 to 14 days. Titers greater than 1:256 strongly suggest active or recent infection. A positive immunoglobulin M test suggests acute disease, but production of immunoglobulin M is brief. Immunoglobulin G has significant cross-reactivity between B. henselae and B. quintana. Polymerase chain reaction can detect different Bartonella species; specificity is very high, but the sensitivity is lower than with serology.

Consequently, when a child or adult presents with unilateral lymphadenopathy,3  the physician should consider the differential diagnoses provided in Table 1. A history of cat exposure should be sought and appropriate tests ordered, including serology for CSD. A history of cat exposure, lymphadenopathy, and elevated antibodies to B. henselae detected by enzyme-linked immunosorbent assay or indirect fluorescent assay confirms the diagnosis.

Lymph node biopsy is not indicated for most patients; however, it is appropriate in patients whose lymph nodes fail to involute and in whom diagnosis is uncertain. Lymph node specimens in patients with CSD show lymphoid hyperplasia and stellate granulomas. B. henselae is a small, curved, aerobic gram-negative bacillus that stains with silver. In bacillary angiomatosis, lobular proliferation of small blood vessels occurs with the presence of bacilli in adjacent connective tissue and blood vessels. In a series of 786 lymph node specimens from patients in whom CSD was suspected, only 245 (31.2 percent) had evidence of CSD. Thirteen of the 245 patients had concurrent mycobacteriosis or neoplasm. It is prudent that physicians follow up with patients who have unilateral lymphadenopathy, even those with confirmed CSD.