Testing for Low Blood Sugar
What might be causing low blood sugar in my pet?
There are several conditions that may cause hypoglycemia (low blood sugar, also known as low blood glucose), some of which are more serious than others. Severe liver disease, hypoadrenocorticism (Addison's disease), severe bacterial infection (sepsis), inadequate nutrition in kittens and toy breed puppies, and some types of tumors may all cause this abnormality. One of the more common tumors to cause hypoglycemia is a specific tumor of the pancreas, called an islet cell tumor or insulinoma, which produces excessive levels of insulin, thus lowering the blood sugar level. Other tumors that may result in hypoglycemia include tumors of the liver and smooth muscle.
The clinical signs of low blood sugar may include episodes of weakness, collapse, and/or seizure activity.
Occasionally a low blood sugar concentration will be noted as an incidental finding when blood testing is done for other reasons. In this instance, we need to confirm that this laboratory finding was real. It is possible that the finding is a sampling artifact, due to consumption of glucose by red blood cells within the sample after the sample was taken. To rule this out, another blood sample will be collected into a special tube that prevents blood glucose consumption and the blood sugar will be re-measured. If this repeated sample is within the reference or normal range and your pet does not show any of the clinical signs that are typically associated with hypoglycemia, then we can be assured that the initial findings were erroneous.
How do we determine the cause of low blood sugar in my pet?
A thorough history and physical examination may give us 'clues' as to the cause of low blood sugar in your pet.
For example, the presence of physical examination findings such as jaundiced (yellow) gums will suggest the possibility of underlying liver disease, whereas the presence of a fever might indicate an underlying infection.
Screening tests, including a CBC (complete blood count), a serum biochemistry profile, and a urinalysis, are also recommended.
What might these screening tests indicate?
The CBC (complete blood count) provides us with an evaluation of the red blood cells, the white blood cells and the platelet components of a blood sample. A hematology (blood) analyzer will determine the total numbers of these cells, and evaluation of a blood smear will document the physical characteristics of these cells
Anemia is indicated by decreases in the red blood cell numbers, hemoglobin, and packed cell volume (PCV) of a sample. Anemia may be a consequence of any chronic disease, including Addison's disease and liver disease.
Changes in the shape of the red blood cells may provide us information regarding the underlying cause of hypoglycemia; with liver disease increased numbers of target cells are noted, whereas with severe infection increased numbers of red blood cell fragments called schistocytes may be found. The presence of small (microcytic) red blood cells is sometimes associated with underlying blood vessel abnormalities of the liver called portosystemic shunts.
Hypoglycemia related to a severe, underlying infection might be indicated by increases in the total white blood cell numbers as well as by increased numbers of immature white blood cells in circulation.
The presence of underlying hypoadrenocorticism (Addison's disease) may be suggested by alterations in the proportions of specific types of white blood cells.
The serum biochemistry profile evaluates many organ systems, including the liver. In addition to organ related enzymes, the biochemistry profile provides information about blood sugar, proteins, cholesterol, and electrolytes.
Increases in the liver related enzymes alanine aminotransferase (ALT), alkaline phosphatase (AP), and gamma glutamyltransferase (GGT) are suggestive of underlying liver disease. Decreases in glucose, cholesterol, albumin (a blood protein), and urea (a product of protein metabolism which is made by the liver and removed from circulation by the kidneys) are supportive of severe, chronic liver disease.
Changes in the electrolyte composition may be suggestive of underlying Addison's disease. Electrolytes are salts and minerals found within the blood. Addison's disease causes altered electrolyte (sodium and potassium) ratios as well as decreases in blood sugar concentrations.
A urinalysis provides us with an evaluation of the chemical and physical characteristics of a urine sample. The microscopic evaluation of urine sediment (the solid components of the urine, which are 'sedimented' out of the sample by centrifuge) is probably one of the most helpful aspects of the urinalysis with respect to evaluation of hypoglycemia. The presence of specific crystals called ammonium biurate crystals will increase our suspicion of vascular (blood vessel) abnormalities such as a portosystemic shunt within the liver. The presence of large numbers of casts (cells shed from the kidneys in concentrated, tube-like shapes), and white blood cells may support an underlying systemic bacterial infection that is also affecting the kidneys.
If your dog is suffering from an underlying pancreatic tumor (insulinoma) or other tumor, these screening tests may be essentially normal with the exception of the presence of hypoglycemia.
What additional tests might be indicated for the evaluation of hypoglycemia?
"Any additional testing depends entirely on the combined results of the history, physical examination, and screening tests."
Any additional testing depends entirely on the combined results of the history, physical examination, and screening tests.
If either underlying liver disease or a portosystemic shunt is suspected, then additional tests will include determination of serum bile acids.
Imaging studies (radiographs, ultra-sound) of the liver and abdominal cavity may be undertaken for suspected liver disease, pancreatic tumors and other tumors. If a tumor is detected, it should be biopsies, either by fine needle aspiration, ultrasound guided biopsy or a surgical intra-operative biopsy.
Suspected Addison's disease will need to be confirmed by an ACTH stimulation test.
Systemic bacterial infections (sepsis) are confirmed by culturing a blood sample so that the type of bacteria can be identified and appropriate antibiotic therapy can be started.
In addition to finding a mass in association with the pancreas, simultaneous measurement of blood insulin and blood glucose concentrations may be used to confirm the presence of a pancreatic islet cell tumor.