A stress leukogram is a common set of leukocyte responses caused by the release of endogenous corticosteroids that result from stress-related disease and hospitalization. Possible changes include segmented neutrophilia, lymphopenia, monocytosis, and eosinopenia, but all abnormalities are not always present,1 and it is unusual for alterations to vary >2 to 3 times the reference values.2 Lymphopenia is most common, and segmented neutrophilia is usually present.3 Monocytosis and eosinopenia are possible in dogs; however, they are more variable and frequently not present in cats.
Neutrophilia is caused by decreased adherence to the vascular endothelium from receptor downregulation, which inhibits margination of cells and therefore increases the proportion of cells in the circulating pool inside blood vessels.4 Prolonged circulation time may cause neutrophils to appear hypersegmented,5 and increased release of neutrophils from bone marrow is possible.4 The segmented neutrophil count can double in dogs and triple in cats due to a larger number of cells in the marginating pool.6 Neither a left shift to band neutrophils or toxic changes are expected due to lack of inflammatory response.
Instead of entering circulation, lymphocytes become redistributed to and retained in lymphocytic tissue (eg, lymph nodes).6 It is suspected that monocytes increase in concentration due to mechanisms similar to those of neutrophils (eg, decreased margination), although this has not been definitively proven. Eosinopenia can be difficult to detect because eosinophils are rare and only a few may be present in circulation at baseline.
The stress leukogram is transient, and cell dynamics return to normal when increased stress is resolved.3 The changes observed in a stress leukogram can also occur with consistently increased cortisol concentrations in patients with hyperadrenocorticism. Stressed patients with underlying illness may have a co-occurring inflammatory leukogram as suggested by the presence of a left shift and neutrophil toxicity; these do not occur with a stress response alone.