Help Getting FEV1 Percentage From Liters - British Lung Foun...

British Lung Foundation

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Help Getting FEV1 Percentage From Liters


Hello, this is probably pretty simple for some but quite confusing for me. I have a Microlife peak flow meter and it does give the FEV1 but it does it in Liters and not percentages, which is the standard I can understand most. For example it says PEF is 447 Liters and then says my FEV1 is 2.77 Liters and I don't know how to convert that to a percentage. Any advice would be appreciated.

10 Replies

page 12 of the following with the tables at the end:-

ericgtr in reply to PMRPete

Thanks, I guess I still don't see how that's showing my actual percentage. I'm guessing I'll have to convert and calculate based off of this but not seeing how exactly.

Seems overly complex, I would probably just be better off with a peak flow meter that shows the actual percentage, any recommendations?

Are you using a peak flow meter or a spirometer?

O2Trees in reply to mrsmummy

Good question MrsM - surely a spirometer would show the percentage? And a spirometer is what you need to measure lung function assuming you have copd Eric, not a peak flow meter which is used to test asthma.

ericgtr in reply to O2Trees

First of all, all great replies and I appreciate everyone taking the time to respond. Here is the peak flow meter I am using it does give the FEV1 but it only gives in it liters and not a percentage. Some others have posted on how to do the math so I'll have to look into that.

I am 52 years oId, 175 cm tall and do have COPD, I was diagnosed as mild to moderate with a FEV1 (officially taken) of 72% and I mostly use this peak flow meter to keep track and make sure it's not slipping. At that time (almost a year ago) I was blowing around 2.72 and have been consistently blowing around 2.77 to 2.80 since, likely statistically in the ballpark.

jackdup in reply to ericgtr

I have the same Microlife as you do and they work pretty well. If you blew 2.72 when you were tested a year ago and that was 72% of predicted than your predicted at that time would have been 3.78. Did they give you a copy of your results as that would tell you what your predicted should be. Based on your height and age the link to the chart posted by PMRPete shows predicted FEV1 should be 3.50. It seems there is a several different predicted values depending on where you look. The chart shows my predicted FEV1 should be 3.25. I just had a spirometry and they used a predicted of 3.51.

If you want to use the same predicted as was used a year ago that should give you a pretty close percentage. The predicted drops a little bit each year as our lung function declines with age but it will be pretty close so using 3.78 as predicted and 2.77 as your FEV1 the percentage of predicted would be 73% and at 2.80 would be 74%.

I'm not sure how many times a day you check yours but mine can very significantly throughout the day and if I take 10 readings I can be 1.88 one time and 2.30 four hours later and then 2.10 a couple hours after that. I don't know if that is usual as I don't think most people have the ability to check theirs throughout the day and if they do, do they check it numerous times throughout the day?

ericgtr in reply to jackdup

Great information here, it seems spot on, thanks for that. I actually only check on it every couple of months or so just to make sure it's not slipping against that initial baseline when I was diagnosed.

ericgtr in reply to mrsmummy

It’s a Microlife peak flow meter.

mrsmummy in reply to ericgtr

A peak flow meter is usually used to measure asthma and a spirometer to measure COPD. If your peak flow meter only gives your actual FEV1 reading then you don't have enough information to calculate your percentage unfortunately.

Hi you need to divide the bigger number (predicted) into your actual FEV1 2.77 to work out the percentage.

For example if your fev1 is 2.15 litres and the predicted for your group is 2.77 then you divide the 2.77 into 2.15 and the result would be 83.66%. I hope you can work it out from this. x

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