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Ever Wonder How “Normal” Ranges for Blood Tests Are Determined? And Why “Normal” Vs. “Optimal”?

Just had some blood work done a few weeks ago in what seems like a never-ending quest to figure out some of these symptoms I’ve been battling for years. While I was reading through my results, I wondered where these reference ranges came from and how they’re determined.

Turns out that each lab has a range, or set of numerical values, that is derived from tests done on 95% of a healthy, a.k.a. normal, target population. Normal ranges will differ from lab to lab depending on sample populations, testing procedures and equipment. Normally these factors are taken into account when generating a range, and while they can differ from lab to lab, most normal ranges at most labs tend to be pretty close.

Now before we touch on optimal ranges for blood work, consider this: If someone’s number on say, a simple Vitamin D test, were 33 ng/mL they would be considered normal, based on the range (30-100) used at the lab my blood was drawn at. Though within the established range, it’s at the very low end of that range. A few more digits and it is considered an unhealthy level that calls for treatment.

Being a numbers-driven society, I get our need to quantify what is good or bad. And there has to be a basis on which to send people for more testing, prescribe medication, etc. But everyone is different, and responds differently to different deficiencies or excesses; and at the end of the day 4 digits might simply mean you only feel slightly less crappy than the person whose number drops below 30 in the example of a Vitamin D deficiency.   

Take the ranges for thyroid. First of all, most doctors only test for TSH levels to begin with, and the range I saw on my paperwork was for .350-5.50. After researching this a bit, I found that way back in 2002, the National Academy of Clinical Biochemistry (NACB) issued new guidelines (at the time anyway) for the diagnosis and monitoring of thyroid disease. Basically, they said that when more sensitive screening was done (by more sensitive, they meant by excluding people with thyroid disease (not making that up)) that 95% of the healthy population tested had a TSH level between .4 and 2.5. A year later, in January 2003, the American Association of Clinical Endocrinologists backed NACB’s guidelines, saying doctors should “consider treatment for patients who test outside the boundaries of a narrower margin based on a target TSH level of .3 to 3.0.”

It’s 10 years later and nothing has changed in the TSH ranges that most labs use. There are numerous reasons for this, but it’s indicative of why “normal values” are tricky. And this is just one example, one test. Taking thyroid testing even one step further, there are numerous tests such as Total T3 and Total T4, as well as Free T3 and Free T4, along with antibody tests and a bunch more that are available. All of which have different values, depending on who you talk to, and different significance, if they’re taken at all. There is a whole cadre of practitioners and patients out there who believe that Free T3 is the most important measure, as it is a much more significant marker, biologically speaking, than all the others.

So where does that leave us? Feeling faint most likely, and not from the amount of blood you had drawn on your last visit. And that’s to say nothing of optimal ranges, which aren’t tested for. I still don’t quite understand why we don’t aim for optimal vs. normal. The standard definition of optimal range is a “reference range or limit that is based on a concentration or levels that are associated with optimal health or minimal risk of related complications and diseases.”

Granted, the optimal — as well as the normal — range is going to be dependent on sex, race, age, diet and a host of other factors. But if you have a doctor worth his/her salt, is it too much to expect some blood work and values that is based off the population you’re representative of? And if you present with symptoms that make you miserable, should you simply be told you’re healthy because your values fall within “normal” ranges?

As with all things medicine, seems like there are more questions than answers more often than not. And in the case of the United States in particular, where way too many people have some sort of undiagnosed chronic condition these days, it makes you wonder about the 95% of the population being used as a baseline.  What if they’re not as healthy as we think? What if a state of disease is becoming the new “normal”?

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