Generally, the following screening tests are recommended when diabetes mellitus is suspected: a complete blood count (CBC), a serum biochemistry profile, and a urinalysis.
Why so many tests, can’t diabetes be diagnosed by an elevated blood sugar value alone?
While confirmation of elevated fasting blood and urine glucose (sugar) values is absolutely essential for the diagnosis of diabetes mellitus, these screening tests may give us additional information regarding the severity of the diabetes, about any conditions that may be contributing to the diabetes, as well as detecting complications related to the diabetic state.
Because diabetes is usually diagnosed in middle aged to older cats, your pet may have other unrelated conditions that need to be managed along with the diabetic condition. These screening tests will hopefully alert us to any such conditions.
So what might these screening tests indicate if my cat has diabetes mellitus?
The complete blood count (CBC) involves evaluation of the red blood cell, the white blood cell, and the platelet components of a blood sample.
Often with uncomplicated diabetes mellitus, these components are within the normal range. However, changes may occasionally be seen in the red or white cell values.
Although most diabetic animals drink large quantities of water, they are losing a lot of body water because they produce such dilute urine. Therefore your pet may actually be dehydrated. Dehydration can be indicated on the CBC by increases in the packed cell volume (PCV – the proportion of the blood volume that is actually occupied by red blood cells) as well as increases in the total red blood cell count.
In some severe diabetic states, lysis (rupture) of red blood cells within the blood stream may occur because of the loss of electrolytes. A reduction in the PCV and red blood cell numbers will be seen on the CBC if this is occurring.
Infections, particularly urinary tract infections, are common in diabetic patients. The presence of an infection may be indicated on the CBC by an increased number of white blood cells.
The serum biochemistry profile requires a separate blood sample from which the serum (the liquid portion of blood) is separated from the cellular portion. Serum contains many substances including glucose, enzymes, lipids (fats) proteins and metabolic waste products.
Determination of an elevated serum glucose concentration is vital to the diagnosis of diabetes mellitus. However, cats present us with a unique challenge because their glucose concentrations can become markedly increased simply due to stress. For example the stress of a veterinary office visit or the mild restraint associated with obtaining a blood sample may significantly increase your cat’s serum glucose value. Blood glucose will also be mildly elevated for several hours following a meal.
Therefore, confirmation of diabetes may require more than one blood sample collected over a period of several days.
Alternatively, diabetes may be diagnosed by an elevated serum fructosamine test. This test is described below.
Occasionally we may see changes in serum electrolytes. Electrolytes are the salt and metallic components of serum. They are involved in many of the body’s daily functions, for example nerve conduction and maintenance of proper hydration.
Because of the large volume of dilute urine that diabetic cats produce, excessive amounts electrolytes may be lost in the urine. Such losses may result in rare but serious complications. For example, severe deficits in the electrolyte phosphorus may result in the rupture of red blood cells within the blood stream.
The liver related enzymes ALT (alanine aminotransferase) and AST (aspartate transaminase) may be increased mildly in diabetic cats.
These increases may reflect mild liver cell damage that is related to decreased blood flow as a consequence of dehydration.
However, as many diabetic cats are overweight, larger increases in liver enzymes may indicate underlying liver disease such as hepatic lipidosis (fatty liver, which results from excess mobilization of body fat). Because mild changes in liver enzymes can also be noted in older cats with thyroid disease, it may be important to rule out the presence of concurrent hyperthyroidism (a condition whereby the thyroid makes too much thyroid hormone) by measuring serum thyroxine (T4) concentrations.
A urinalysis is necessary for the diagnosis of diabetes mellitus. Urine from healthy cats typically does not contain any glucose (sugar).
Occasionally a small amount of glucose may be found in the urine of a highly stressed cat because its blood glucose values were temporarily increased.
Generally, the finding of glucose in the urine (called glucosuria) as well as persistently increased blood glucose levels (called hyperglycemia) in a cat with appropriate clinical signs is diagnostic for diabetes mellitus.
Other important features of the urinalysis that need to be evaluated in diabetic animals include evidence of urinary tract infection. The presence of glucose in the urine makes conditions ideal for bacterial growth; therefore urinary tract infections are common. The urine is evaluated for the presence of red blood cells, white blood cells and bacteria. A culture of the urine may be indicated to determine what types of bacteria are present and the most appropriate antibiotics that can be used to treat the infection.
The presence or absence of ketones in the urine should be evaluated in diabetic animals. Ketones are by-products of fat metabolism. Increased utilization of fat occurs in diabetic animals because their insulin deficiency results in poor utilization of carbohydrates as an energy source. Depending upon your cat’s clinical signs, the presence of ketones in the urine may indicate a more severe or long-standing case of diabetes.
Now that my cat has been started on insulin therapy what monitoring tests can we use?
When insulin therapy is first started, we will need to monitor your cat’s response to therapy by periodic blood glucose determinations. Ideally this involves serial blood or serum glucose determinations in the form of a glucose curve. However because of the nature of most cats, the mere acts of hospitalization and serial blood sampling, no matter how gentle, will result in stress-related increases in blood glucose that will confound the results of a glucose curve.
One of the most important things you can do for your cat is to simply monitor his appetite, water consumption, energy level, and urine output.
Any changes may signify the need for additional testing and/or adjustments in the insulin dosage (it is very important that you do not make adjustments in dosage without first consulting your veterinarian).
Additional home monitoring can involve the evaluation of urine for the presence of glucose. This may involve a bit of ingenuity in obtaining a urine sample; the use of non-absorbing plastic pellets or clean aquarium gravel in the litter box for a short time period may help. A urine test strip that contains an indicator pad for glucose is then simply dipped into the urine sample and a reading is taken after a specified time interval.
New urine glucose detection products are becoming available. These products are essentially strips of material impregnated with glucose detection pads that are added directly into the litter box. When the cat urinates on this material, they change color if the urine contains glucose.
At the beginning of insulin therapy, more frequent (daily) monitoring of urine glucose is indicated. The presence of large amounts of glucose for two or three days in a row, or the complete absence of glucose may indicate the need for adjustments in insulin dosage.
Once your cat’s optimal insulin dosage has been determined and his diabetes is well regulated, monitoring may involve weekly ‘spot checks’ of urine for the presence of glucose.
Serum fructosamine concentrations are probably the most reliable and easiest way of evaluating your cat’s response to insulin therapy. Fructosamine forms through the binding of glucose to proteins in your cat’s blood stream. The higher the blood glucose, the greater the amount of fructosamine formed. Similarly, the lower the blood glucose, the smaller the amount of fructosamine formed.
Fructosamine is measured from a single blood sample. No special preparation such as fasting, and no timing of blood samples is required. Serum fructosamine provides us with a retrospective view of the average blood glucose concentration that your cat has achieved over the past 2 to 3 weeks. Therefore it is extremely helpful in long-term monitoring of diabetic patients.
However, recent changes in blood glucose concentrations will not be detected with the serum fructosamine test. Therefore if your cat is showing any behavioral changes that might signal changes in his immediate insulin requirements, then a direct blood glucose test will be more appropriate.