Changes in laboratory results can be physiologic (i.e. expected due to age, breed, sex, physiologic status such as pregnancy, lactation) or associated with disease. Thus, interpretation of results need to take into account expected physiologic changes in order to determine what is expected due to the animal and what is indicative or supports underlying disease.
Studies in premature foals have shown that they have lower red blood cell counts, but larger red blood cells (high MCV), because fetal RBCs are larger than adult RBCs. The high MCV causes a high hematocrit, which is discrepant with the RBC count. Due to immaturity of the fetal hypothalamic-pituitary-adrenal axis, premature foals have lower corticosteroids, lower neutrophils with a neutrophil to lymphocyte ratio of <1:1. They have slightly lower sodium and chloride than adults, higher calcium and lower glucose than adults or foals born at term. Creatinine is also higher due to the allantois being richer in creatinine (Lester 2005, Axon and Palmer 2008).