Comparison of bovine hematology reference intervals from 1957 to 2006

Background: The most commonly used bovine hematology reference intervals were published in 1965. We found the results from healthy cattle in 2001 differed from those in many ways. Discovery of the original laboratory book used to calculate the 1965 values gave us the opportunity to evaluate whether hematology values of healthy cattle have changed over time. Objective: The purpose of this study was to establish hematology reference intervals for Holstein cows, compare selected hematologic results with similar population data from 1957, and compare these reference intervals with those of other North American veterinary schools and published values. Methods: Reference intervals were developed in 2001 using clinically healthy, bovine leukemia virus-negative, mid-lactation Holstein cows. Selected parts of the hemograms and neutrophil:lymphocyte (N:L) ratio were compared with those from healthy, age-matched Holstein cows evaluated in 1957. Bovine reference intervals were solicited from clinical pathology laboratories in North American veterinary colleges and analyzed for population characteristics and method of analysis. Results: Between 1957 and 2001, mean neutrophil counts increased significantly, whereas lymphocyte, monocyte, and eosinophil counts and hemoglobin concentration decreased significantly. Mean N:L ratio increased significantly to 1.17. Most surveyed laboratories were using the 1965 reference intervals. Two other institutions that had developed reference intervals after 2000 had results similar to ours. Conclusions: Continued use of older bovine hematology reference intervals could lead to misinterpretation of within-reference neutrophil counts as neutrophilia and under-recognition of neutropenia, eosinophilia, monocytosis, or lymphocytosis. Use of N:L>1 as evidence of inflammation should be discontinued or used with great caution.