Dr. Hechemy states that his experience confirms our conclusion that the indirect fluorescent antibody (IFA) and indirect hemagglutination (IHA) appear to be the most sensitive serologic tests currently used to diagnose Rocky Mountain spotted fever (RMSF). Therefore, with regard to the major message of our paper, we appear to be in agreement.
Regarding our laboratory criteria for confirming cases of RMSF, Dr. Hechemy raises an important issue that, in our experience, is frequently a source of contention between laboratory-based scientists and epidemiologists. For national surveillance, epidemiologists are often more willing to accept a degree of nonspecificity in a case definition to improve sensitivity, recognizing that a few errors will be unlikely to affect conclusions based on large data sets. In the case of RMSF surveillance, we are willing to accept an occasional erroneous diagnosis in which confirmation is obtained by a single high titer (or stable high titers) by complement fixation (CF) (≥ 1:16) or IFA (≥ 1:64).