Abstract
I read with interest the article by Jensen et al. (JVDI 27(5):576–580) in which the authors examined the predictive value of Canine distemper virus (CDV) neutralizing antibody titer on protection against experimental infection. The authors found that all 4 dogs with antibody titers ≥16 were protected and all 5 vaccinated dogs with titers <16 were also protected. They concluded that the positive predictive value of antibody titers was 100%, while the negative predictive value was 0%. Attribution of a negative predictive value of 0% was based on the fact that all 5 vaccinated dogs with titers <16 (“suggesting susceptibility,” according to the authors) were still protected. There is an alternate explanation for their findings. First, it would be useful to know if the 16-titer cutoff value was based on the actual dilution of the serum used in the test or the final dilution after virus is added to the test. If the latter is applicable, it would mean that their cutoff value was actually 32. Second, although “protective” cutoff titer values of 16 and 32 have been widely used in the literature, there appears to be no published experimental data to support those values. In fact, unpublished observations from this writer suggest that neutralizing antibody titers as low as 4 may be protective. The authors generated data that could be used to support another cutoff value. Accordingly, I would like to suggest that the authors reanalyze the available titer data using cutoff values of 4, 8, and 16, with the titer being defined as the reciprocal of the final dilution of serum after virus is added to the test. Such analysis could provide experimental evidence to support the possibility that all titers ≥4 may be protective against CDV.