Laboratory reference ranges: Does anyone know why... - Thyroid UK

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Laboratory reference ranges

Redporti
Redporti

Does anyone know why some labs have different reference ranges when it comes to blood tests? As an example I had my bloods done at hospital A and the reference range for FT3 was 4.3 - 8.1. Then I had my bloods done at hospital B and the FT3 range was 3.1 - 6.8. The FT3 result was the same from both labs at 3.2. So how can one lab say it’s normal and one say that it’s below value? Who is right?

20 Replies
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helvella
helvellaAdministrator

Lots of reasons.

First, they might use technology produced by different companies.

Second, they might have adjusted the ranges on the basis of what they see in the results their lab gets.

I assume that was a typo? You posted "RT3" and "FT3" - should both have been "FT3"?

SeasideSusie
SeasideSusieAdministrator

Redporti

if I had my bloods done at hospital A and the reference range for RT3 was 4.3 - 8.1. Then I had my bloods done at hospital B and the FT3 range was 3.1 - 6.8.

Are you talking about two different tests here - rt3 (reverse T3) and FT3 (free T3), or is that a typo? I'm guessing a typo because rT3 isn't normally done by the NHS and the range is vastly different anyway.

If you are talking about just the one test - FT3 - then different labs have different machines which use different ranges, and it's said that ranges are based on local population, so here in Wales my lab's range for FT4 is 7-17 and we often also see 9-19,11-23, 12-22 and others mentioned on the forum. So there is no standard range.

Yes it was a typo. So what result is right? Both labs are in London.

SeasideSusie
SeasideSusieAdministrator in reply to Redporti

Each result is correct for the range given by the lab that did the test.

If you want to compare the results when they have different ranges then you have to work out the percentage through range the result is. Post the result/range for each hospital and we'll work out the percentage.

Edited to add:

Sorry, I've just seen that you've given the result and it's the same for both hospitals.

3.2 (4.3 - 8.1) = minus 28.94% through range

3.2 (3.1 - 6.8) = 2.7% through range

Were both tests done under the exact same conditions:

* same time of day

* last dose of thyroid meds take the same length of time before blood draw

* same dose of thyroid meds

Nothing is static but there is a big difference if the test conditions were exactly the same. Which then poses the question of whether there was any interference with the testing procedure?

Redporti I have edited this post.

Yes taking at same times of the day but obviously different days. So when it comes to deciding what treatment is needed what one do I go by?

3.2 (4.3 - 8.1) = 28.94% through range

Are you sure about that percentage? The result is under-range. I make it - 28.95% (minus 28.95%).

Redporti, are you sure about that range? I've never seen one quite so wide.

Redporti
Redporti in reply to greygoose

Yes that’s the range on the printout.

greygoose
greygoose in reply to Redporti

I think, if it were me, I'd question that range, next time you go back. It's very unusual, and mistakes can be made. :)

SeasideSusie
SeasideSusieAdministrator in reply to greygoose

Yes, GG, you are correct, I missed off the minus sign. I'll go and correct it.

SeasideSusie
SeasideSusieAdministrator in reply to greygoose

Sorry greygoose I forgot to thank you for pointing that out. It's so easy to miss something off, and my fingers do work faster than my brain, so I'm always grateful when someone lets me know if I've made an error :)

Oh, I know the problem! lol I type fast, too. I tend to miss out things like 'a' or 'I', or the middle letter of a word. :)

First one is FT3 3.1 reference range 4.3-8.1. Second is FT3 3.2 reference range 3.1-6.8. Thanks.

SeasideSusie
SeasideSusieAdministrator in reply to Redporti

Can I refer you back to SlowDragon's replies to your previous thread here:

healthunlocked.com/thyroidu...

She has given you sage advice and I absolutely agree with her that you need to get your TSH down to (in my opinion) 1 or below to see where that takes your FT4 and whether it improves your FT3. Then you decide whether or not adding T3 to your Levo is the right course of action. So, as your endo and SlowDragon have said, at this stage you need to increase your Levo.

All your nutrient levels need to be optimal for thyroid hormone to work. I've read somewhere that Ferritin needs to be 70 for good conversion of T4 to T3 to take place, but that 55-70% of it's range is optimal (higher end for men) according to web.archive.org/web/2018083... so with your range from the results in your previous post you are looking at raising your ferritin to around 89. You can help raise your level by eating liver regularly, maximum 200g per week due to it's high Vit A content, also liver pate, black pudding, and including lots of iron rich foods in your diet.

Thank you for your reply & I understand that I need to increase my Levothyroxine to get my TSH down & then my T3. What has confused me is if I use a different lab for my blood test how will I know what my levels are if they all have different ranges.

SeasideSusie
SeasideSusieAdministrator in reply to Redporti

What has confused me is if I use a different lab for my blood test how will I know what my levels are if they all have different ranges.

Use this calculator to work out the percentage through range, then compare the percentage:

chorobytarczycy.eu/kalkulator

SlowDragon
SlowDragonAdministrator

Your Ft3 should improve as your levothyroxine is increased

Redporti
Redporti in reply to SlowDragon

I understand that but my question is why are the two labs producing different ranges? Who’s range is right? If my result is the same but the ranges are different then does that mean I’ve still got a FT3 level of 3.2 whatever lab I use?

SlowDragon
SlowDragonAdministrator in reply to Redporti

Many labs use very different ranges

It’s the percentage through the individual range that’s important

Ft3 varies depending when test is done

Both results are low because you are still on too low levothyroxine. You should see some slow steady improvement as levothyroxine increases

Wetsuiter
Wetsuiter in reply to Redporti

i think it may be rare that you get the same result with different ranges. Maybe just a confusing coincidence.

if they were done 2 months apart, (presume they were) then they could both be right.

helvella
helvellaAdministrator in reply to Wetsuiter

The correct procedure if you are trying to compare two labs is to split the drawn blood - sending one sample to one lab, the other to the second.

As you point out, two months is an eternity in terms of blood tests, especially at the more subtle end of things. After all, we are pretty well aware how time of day can make a difference.

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