Practical Matters: "Normal" and "reference range" are not the same
Reference ranges are established by testing a group of apparently healthy normal animals. But even if a value falls within the reference range, it may not be normal for that animal if the animal is ill. For example, if an animal has a high serum ionized calcium concentration, one differential diagnosis is primary hyperparathyroidism. Many animals with primary hyperparathyroidism have serum parathyroid hormone concentrations within the reference range. However, in the face of a high serum ionized calcium concentration, a serum parathyroid hormone concentration within the reference range is not normal; in fact, it should be below the reference range because hypercalcemia normally suppresses parathyroid hormone secretion.
Other examples include serum insulin concentrations within the reference range in dogs with insulinomas that are hypoglycemic and the lack of stress leukograms in dogs that obviously are ill. Also remember that age-related changes may occur in complete blood count and serum chemistry profile results, such as lower hematocrits in young animals and mild elevations of alkaline phosphatase activities and phosphorus concentrations in growing dogs. So when interpreting the results of laboratory tests, consider the patient and whether the reference range would be normal for that patient.
Anthony P. Carr, Dr. med. vet., DACVIM (small animal internal medicine)