In patients with possible Bartonella henselae infection and elevated IgG titer, what is the best way to confirm the diagnosis: tissue biopsy with Warthin-Starry staining, tissue sent for Bartonella henselae PCR, or tissue sent for culture?

First of all, it is important to only test people with a compatible clinical syndrome. If the syndrome is not that of babesia, then any positive tests are likely to be false positives. This is a basic testing principle. A very low pretest probability is likely to lead to false positive testing.

The ...

Agree with the above commenter in that it requires a clinical context that makes sense. IgG would only be positive after several (~6) weeks, so it would be most helpful in the context of a chronic Bartonella infection. There is also cross-reactivity between B. henselae and B. quintana so understandi...