THE FLAWS OF LAB TEST REFERENCE RANGES FOR CATS (by Siobhan Tobin - Senior Cat Group Owner)

Some lab tests provide a simple yes or no answer and are easy to interpret. Was the culture positive for bacteria? Was the test positive for viral antibodies that indicate an infection? Was there protein in the urine? But for many more tests, the meaning of the results depends on their context.

A lab report shows the results followed by a reference range, example: Creatinine 2.1 (0.6-2.4). Interpretation of results involves comparing the cat¡¦s individual result to the reference range. The reference range is often mislabeled and referred to as the "normal range" but it is merely a descriptive term which should not be empirically relied upon to determine illness. In fact, based on the laws of probability, 1 out of 20 (or about 5%) of test results will fall outside the reference range. This might be as simple as something the cat ate that day, or a lab error. Vomiting and diarrhea will often skew results. Therefore, a diagnosis should not be made based on one lab test, unless the cat is showing symptoms in support.

In human medicine, the first step in determining the reference range is to define the population to which the reference range will apply. For example, healthy non-pregnant females between the age of 20 and 30, might be one group with little variance. Over time, a large number of healthy females in this age range are tested. The results are averaged and a range is determined (usually plus or minus 2 points).

Unfortunately, in animals, labs are just not this specific. Instead, they determine a reference range for all cats from 1 to 30 (if cats lived this long). This would be considered particularly flawed in humans since the reference range for 20-30 year old non-pregnant females would be different from the reference range of 60-70 year old females. For example, a 1 year old cat may have a phosphorus level of 6.8 and still be considered totally healthy. That is because this cat is still growing healthy bones. However, a 6.8 phosphorus level in a senior cat indicates a serious health issue. Likewise, while a healthy young cat may have a T4 level of 3.5 and still be considered healthy, a senior cat with a level above about 2.5, especially if that cat is displaying symptoms, should undergo further evaluation for hyperthyroidism. Yet, the reference range for T4 often goes as high as 4.0 and even 5.2 for some labs. Those numbers would be very unhealthy for a senior cat.

Labs also do not differentiate their reference ranges based on the sex of the cat. This is also flawed. For example, creatinine is a natural byproduct of muscle breakdown excreted by the kidneys. Male cats have more muscle mass than females, therefore, they will generally have slightly higher creatinine levels than female cats.

Sometimes, the lab also includes other species in its reference ranges. For example, labs constantly use a reference range for urine specific gravity (part of urinalysis) that ranges from 0.15 - 0.60 and is the same for both dogs and cats. Cats produce very concentrated urine, whereas dog urine is not as concentrated. A cat with a specific gravity of less than 0.35 is a cat not concentrating his/her urine and is sick. A dog with a specific gravity of 1.015 would still be considered healthy, however.

A test result that is on the high or low end of the reference range is often not far removed from a test that is outside the reference range. For example, a cat with a BUN of 35 may still be considered inside a range of 14-36. However, a cat with a BUN of 37 is outside the range. BUN fluctuates depending on what the cat eats. If that same cat was tested again an hour after the first test, his/her result may be 37 or higher.

Yes, I do realize that your cat's name age and gender is listed on lab test results, but that information has NOT been taken into account in presenting the reference range.

Vets were taught about these issues in school, but it seems that after many years of getting back reports from labs grouping young animals with old animals, dogs with cats, males with females, they forget their training. They glance down the piece of paper and look for values marked in boldface that say "HIGH" or "LOW" and forget to interpret the rest of the values. This is very important to remember and it's one of the main reasons we so commonly ask members to post ALL lab results. Just because a vet says "everything was normal" doesn't mean that it is. Likewise, we cannot get too excited if we see one number outside the reference range. It may be a fluke especially if the cat is feeling and acting normally. It's always a good idea to retest in a few weeks though.

We also need to consider that lab tests are taken at one particular time in the cat's life. They are just a snapshot of how the cat is doing on that particular day at that particular time. If they were repeated later on, they may show different results. They can however be very useful in diagnosing a particular ailment that is already suspected from symptoms the cat is exhibiting.

One important factor often missed in medicine (both human and veterinary) is that each individual has his/her own "normal" levels. For example, I'm a human now suffering from hypothyroidism and it has been difficult to provide me with enough supplements to bring me up to my "normal" level. Unfortunately, because I never had my thyroid levels tested when I was healthy, I don't know what my individual normal levels are. If we could be a little more proactive about preventive medicine and test our cats' levels when they are healthy (once every 2-3 years until age 8, every year from 8-12 and twice a year thereafter), we could establish each cat's "normal" levels. If something starts to creep up or down, we will know immediately that it is "abnormal" for that particular cat.

Since we deal with senior cats on this group, here are a few important levels to keep an eye on that a lab test may not necessarily flag as "abnormal":

1. Phosphorus (blood test): Even though the reference range often goes as high as 8.0, this is not healthy for a senior cat. This level should be maintained as close to 4.0 as possible (1.30 in European/Canadian values). Up to levels of about 5.5 (1.75 in European/Canadian values) , it¡¦s not critical to begin phosphorus binders, however, an owner should begin considering lowering phosphorus content of food and/or adding phosphorus binders to some meals and should monitor levels closely for further evaluation. Higher values than 6.0 US (1.93 in European/Canadian values) require phosphorus binders added to each meal to bring the level back down.

2. Potassium (blood test): Even though the reference range often dips into the 3s, senior cats tend to urinate more than younger cats as their kidneys are older and have less functionality. Potassium is often lost in urine and deficiencies can deteriorate the kidneys. Therefore, a senior cat's potassium level should be maintained in the middle of the reference range, which is usually somewhere between 4.5 and 4.6 on most lab ranges (this is the same in European/Canadian values). Lower values than this midpoint require supplementation.

3. T4 (blood test): Even though the reference range often goes to 4.0 or 5.2, a senior cat¡¦s level becomes suspicious for hyperthyroidism above 2.5. Anything above this is considered the "grey area". It doesn't necessarily mean the cat is hyperthyroid, but it should be rechecked in a couple of weeks.

4. Urine Specific Gravity (urinalysis): Although most labs show a reference range of 1.015-1.060, this is not appropriate for cats. Any cat with a specific gravity lower than 1.035 (some sources suggest 1.040) is not concentrating his/her urine. In a senior cat, this generally means that the kidneys are becoming insufficient. This is often the first sign of kidney insufficiency before blood tests reflect the problem. While no medical treatment is necessary at this point, it¡¦s important to begin ensuring the cat has easy access to fresh water at all times and to begin monitoring blood values for further progression more frequently.

Please remember to monitor other tests for values creeping up to the high end or down to the low end of the reference range. You can chart this in Excel or any other spreadsheet program (examples and blanks available in the Files area). By doing so, you will help (not hinder) your vet and make his/her job so much easier.