It was with great interest that I read the article “Biological variability of C-reactive protein and specific canine pancreatic lipase immunoreactivity in apparently healthy dogs” by Carney et al. There are relatively few veterinary publications that address the appropriate use of reference intervals and mean critical difference (or reference change values). The publication of biological variability data for canine C-reactive protein (CRP) and pancreatic lipase immunoreactivity (cPLI) permits laboratories to calculate maximum allowable error for analytical quality specifications. Moreover, these data provide insight into the utility of population-based reference intervals (RIs) versus subject-based RIs via the calculated index of individuality.
While this information is valuable to diagnosticians and practitioners, it is important that it be disseminated correctly. The authors state that, “For any assay, an index of individuality of <0.7 supports use of a population-based reference range, whereas a value >1.7 indicates negligible utility of a population-based reference range,” and cite Fraser and Harris. In fact, Fraser and Harris state that when the index of individuality is “less than 0.6, conventional population-based reference values are of very limited value in the detection of unusual results for a particular individual. In contrast, when [the index of individuality] is more than 1.4, observed values can be compared usefully with reference values.”
The authors contribute valid data to the veterinary literature, but their presentation is confusing. The reported individuality indices of 0.26 for cPLI and 0.78 for CRP are actually inverse indices of individuality. Use of subject-based RIs is appropriate for analytes that have a high degree of inter-individual compared to intra-individual variation (i.e., a low index of individuality).