Reference vs normal ranges

[HarryE]

Somewhere in all the reading I have done, I saw an article which expressly said reference ranges should not be referred to as normal ranges. Of course, now I want it, I can't find it! Does anyone recall where it might have been? Thaks

[Hidden]

I don't know which article you refer to HarryE but here's the Wiki take on it:

Regarding the target population, if not otherwise specified, a standard reference range generally denotes the one in healthy individuals, or without any known condition that directly affects the ranges being established. These are likewise established using reference groups from the healthy population, and are sometimes termed normal ranges or normal values (and sometimes "usual" ranges/values).

However, using the term normal may not be appropriate as not everyone outside the interval is abnormal, and people who have a particular condition may still fall within this interval.

[Hidden]

Is this any use to you: labtestsonline.org/understa...

[HarryE in reply to Hidden]

Thank you. The item I am thinking of related specifically to thyroid tests, and I think within the NHS too. I was hoping to reference it in a letter

[Hidden in reply to HarryE]

Sorry HarryE. Hope you find it.

[humanbean]

As far as I know, reference ranges are generally determined by taking the population ranges and knocking off the lowest 2.5% and the highest 2.5%. The 95% in the middle is deemed to be the normal reference range.

There are several problems with this that I can see.

1) The "population" being used doesn't actually mean that everyone has been tested. "Population" usually refers to everyone that the the testing organisation has access to. And who are the people who are usually tested? Sick people. So over time the reference range could drift further and further away from what is normal for people who truly are healthy.

2) Knocking off 2.5% at the top and the bottom of the range implies that no more than 2.5% of the population can ever be sick with a low level of something, and ditto for having a high level. I have seen mention of doctors over-treating for some conditions because less than 1% of the population has a particular condition. But I've never seen any worries being raised about a condition being MORE common than the 2.5% limit at either end of the range. Doctors don't seem to believe that any unhealthy condition (like hypothyroidism for instance) can be as common as that.

3) There is always an assumption that the distribution of anything being measured is a normal distribution i.e. a bell shaped curve with the largest proportion of people being in the middle of the range. TSH has a highly skewed distribution. But people are frequently being told they are "nicely mid-range" (my mother for instance) and therefore they are fine.

4) Where is the evidence that the 95% included in the reference range should be in the middle of the population range? Perhaps in a population range the 4% at the bottom of the range need treatment and 1% at the top need treatment. I've never seen that particular idea mentioned.

[HarryE in reply to humanbean]

Thanks! Still can't find the article I'm thinking of, but all very useful

[helvella Administrator Thyroid UK]

A classic example of where the word "normal" is inappropriate would be a range of heights. One developed in southern Africa (where pygmies live), the other in Finland (renowned for extreme height). A person travelling from one location to the other would very likely suddenly find themselves out of the local reference range - but they are, for themselves, just as "normal" as ever they were.

The word "normal" should, in my view, most definitely be avoided simply because of the confusion with the "normal distribution" (often called the bell-curve, or Gaussian distribution). There may be many other reasons to avoid it, but this is sufficient on its own merits.

Hypnoteq's Wiki reference is pretty good.

Rod

[Hidden]

Been hunting but can't find it yet. Found this, which is interesting in its own right:

thyroid.about.com/library/n...

[HarryE in reply to Hidden]

Bless you, thanks xx