Diurnal variation of canine blood leukocyte counts.

A diurnal variation was detected in the numbers of total leukocytes, neutrophils, lymphocytes and eosinophils in the blood of 19 dogs (10 beagles and nine German shepherds). Blood samples were collected every fourth hour for 24 hours and once a day for 7 days. The neutrophil count increased during the day and reached its maximum in the late afternoon. The lymphocyte count had its maximum values in the late evening and its minimum values in the early morning. The eosinophil numbers were low around noon, increased during the afternoon and reached their maximum numbers in the late evening. Leukocyte numbers had statistically significant diurnal variation, so in designing research protocols with repeated blood sampling and closely controlled factors it may be important to take blood samples at the same time every day. The mild normal leukocyte changes during the day are not likely to confuse interpretation of clinical cases where patient results are compared with wide reference ranges.