Why do ranges matter?

[Jellybabyhead]

Stupid question klaxon!

I keep seeing “we can’t analyse your results without know the lab’s ranges” or similar. Why do the ranges matter? Don’t they all test the same units, so a FT4 result of 13 from a lab with a range of 9-19 would be the same as a result of 13 from a lab with a range of 10-20?

Please tell me what I’m missing... I am a total newbie as you can probably tell. I should know more considering I had half of my thyroid removed in Feb 😳

[SeasideSusie Remembering]

Don’t they all test the same units, so a FT4 result of 13 from a lab with a range of 9-19 would be the same as a result of 13 from a lab with a range of 10-20?

Yes the units are the same, but ranges generally are based on local population, there is nothing universal about them.

Taking your examples:

FT4: 13 (9-19) = 40% through range

FT4: 13 (10-20) = 30% through range

My area's range is 7-17 so 13 would be 60% through range

Medicheck's range is 12-22 so 13 would be 10% through range

So you can see why the range is important. Your result will be worked out with the lab's own range, it can't be used with any other lab's range.

[Jellybabyhead in reply to SeasideSusie]

Thank you Susie, I had no idea the ranges were based on local population

[humanbean]

As well as reference ranges being determined by local population there are probably quite a few different companies in the business of making test machines. Not all labs will have the same make of test machines or testing protocols. These will also impact reference ranges.

Another thing to bear in mind - within the UK units of measurement will be consistent between labs, but if you compare a UK lab to a US lab the units and the reference ranges might be completely different, and this affects what optimal levels are. As an example :

Optimal level of vitamin D in the UK = 100 - 150 nmol/L

Optimal level of vitamin D in the US = 40 - 60 ng/mL

These UK and US levels are actually the same.

[Jellybabyhead in reply to humanbean]

Thanks humanbean, that’s really interesting

[Gambit62]

as well as variation due to using different types of machinery (and different test protocols) test are not always done in the same units - eg folate and B12 can be done in pg/L or in pmol/L. Not much difference for B12 but a huge difference for folate. Similar with other tests for micronutrients.

[silverfox7]

Good question to have asked though as I've sure reading the thread will help others who aren't aware of the reasons. My local hospital for instance the top of the range is 6.8 for T3 but I go to a nearby town where the top is 5.7 so quite a big difference!