Titers alone insufficient to diagnose cat scratch disease

The authors present two cases of cat scratch disease (CSD) in which initial acute titers were negative or equivocal for Bartonella henselae while convalescent titers were positive. The cases illustrate that a single acute negative titer is insufficient to exclude the diagnosis of CSD and that convalescent titers should be obtained in patients for whom there is a high clinical suspicion of the disease.

In the first case, a 43-year-old woman developed acute unilateral vision loss in the lower portion of her right visual field. She reported being scratched by kittens two weeks previously and had cat scratches on her lower extremities. Serological studies for B. henselae were equivocal for IgG and negative for IgM. The right fundus showed optic disc edema with peripapillary hemorrhage and exudate. The patient was started on azithromycin (500 mg daily for five days). Two weeks later, convalescent serologies were positive for B. henselae IgG and remained negative for IgM. The patient's signs and symptoms progressively improved to 20/20 in each eye with normal visual field testing.

The second case was of a 6-year-old boy who developed blurry vision after being hospitalized for surgical drainage of a psoas muscle abscess. Funduscopy of the right eye showed optic disc edema with a macular star of exudates. Initial B. henselae serological results were negative for IgG and IgM. The patient was started on azithromycin (60 mg daily for four weeks) and rifampin (175 mg twice daily for four weeks). Repeat testing two weeks later showed convalescent titers for B. henselae of IgG while they remained negative for IgM.

Over the following month, the patient's vision improved to 20/40, right eye, and 20/30, left eye. There was resolution of the optic disc swelling in the right eye and reduction in the size of the lesion overlying the left optic nerve head.

The authors note that it is their practice to obtain a convalescent titer two weeks following a negative initial result, as the initial test may have been performed before IgG titers have risen or after IgM titers return to normal. They write that a high seroprevalence in the general population limits the utility of a single IgG titer. In addition, there appears to be limited association between the serological titers and either the clinical manifestations of CSD or the duration of symptoms, so titers alone should not be used to measure the outcome or prognosis of disease. Laboratory testing with polymerase chain reaction also might be valuable in patients with negative serologic results.