The reason I bring this up is that I find many clients – and even some veterinarians – will look at laboratory blood test results in something of a vacuum. So, for example, a concerned owner will ask for a copy of blood test results (results that they have a perfect right to see, of course), look for “highs” and “lows,” and then try to come up with some sort of reason for the “abnormality.” First, let’s look at a lab test.
You’ll notice that by each of the values, there’s a little bar graph. It’s divided into thirds, with the middle third being the lab “normal.” Everything else is either above or below the lab normal. In case you’re wondering, this little bar graph is called the reference interval (RI).
Lab “normal ranges” are simply an average of many blood samples. The bar, as it were, is moved both above and below the average (this is a matter for statisticians). Moving the bar up and down eventually produces a group that includes 95% of the subjects studied. Another way to look at the normal range is that 95% of the horses should fall in that range. That’s great, as far as it goes.
The thing is, “Normal” is not an absolute condition: it’s relative to the overall well-being of your horse. “Normal” typically lacks any sort of a universal definition. Take your horse’s front legs. Does he toe in? Toe out? Are his legs perfectly straight? What’s normal?
Blood tests work that way, too. It’s entirely possible for your normal, healthy horse to have a blood test that’s on either side of the laboratory range. It’s not necessarily abnormal. Here are a couple of examples.
- You submit a complete blood count for your horse because you want to see how he’s doing (not necessarily a good reason, but it happens). A complete blood count (CBC) looks at things like the total number of red and white blood cells and the amount of protein in the blood. The laboratory range is 6.3 to 10.9 M/µL (if you like these sorts of units, you were probably very good in chemistry). Your horse’s level is 6.0. Is this something that you need to do something about? Is he anemic (that is, does he lack red blood cells)?
- Take the test that I posted earlier. Let’s say it’s summertime and it’s been hot. You have some blood drawn because you’re worried about him. His sodium and his chloride – important salts, to be sure – are just below the lab “normal.” Should you start giving him salt?
Overinterpretation of blood tests can go the other way, too. So, for example, say you have an older horse that you’re concerned might have Pars Pituitary Intermedia Disorder (mercifully abbreviated as PPID and more commonly known as Equine Cushing’s Disease). Let’s say you run a screening test for ACTH (which is one of the more commonly performed tests) and the lab test comes back at the high end of the laboratory’s normal range (for the lab that I use, the normal range is 9 – 35 pg/mL). Let’s say that your horse’s test reads 34 pg/mL. Does that mean that he’s close to developing PPID? Does that mean you need to start him on medication? Nope. In the absence of any other clinical signs, it most likely means that on that day, he was at the higher end of the laboratory’s normal range. If you’re still worried, you can always run another test somewhere down the road. In and of themselves, if your horse seems otherwise normal, he probably is.
Of course, blood tests are very useful. If your horse is running a fever, has a snotty nose, has a cough, and isn’t eating, a blood test showing an elevated white blood cell count and increase inflammatory proteins has meaning. If you horse has a curly hair coat, won’t shed, and is 23 years old, an elevated ACTH level is likely to mean something. But in the absence of other signs of a problem, some laboratory result that’s on the border of what a laboratory has established as “normal” is more likely than not to be normal for your horse.
BOTTOM LINE: If your horse is otherwise healthy, you probably don’t need to worry too much about some lab value that’s near the top or the bottom of a laboratory established reference range. Your horse probably doesn’t like having needles stuck in him anyway.
Cue Bobby McFerrin.