Thread by @emily_fri

Because I'm interested and I want to know everyone else's thoughts, this is going to be a mega thread on the new PFT interpretation guidelines coming from ERS/ATS.

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First off, this isn't technically published yet. It says it's an "early view," but that it's been accepted to the ERJ so I assume it will be out soon.

Secondly, I'm not a PFT expert. I had to think really hard about some of this stuff to understand what it was saying. I'm just going to try to report the differences I noted and let smarter people than me comment further.

Pre-apologies for any errors or misunderstandings.

And for a reference point, I've grown up using the 2005 guidelines for all of my PFT reading needs.

The main points I'm going to highlight here are

🔹Reference equations
🔹Limits of normal
🔹Bronchodilator responsiveness
🔹Decline over time
🔹Severity grading
🔹Nonspecific pattern

Buckle up, PFT nerds. It's dense.

🔹REFERENCE EQUATIONS

We are switching to Global Lung Function Initiative reference equations for spiro, DLCO, AND lung volumes.

2005 recommended NHANES III for spiro, which covered ages 8-80.

GLI goes from ages 3-95.

2005 recommended ATS/ERS or ECCS for lung volumes.

I have heard over and over in fellowship how our data for lung volumes was much weaker than our spiro data due to lower # of patients, so this should help that.

Over 7k patients (but non-European descent data TBD??).

And 2005 didn’t even recommend a standard reference equation for DLCO, because of inter-lab variability.

There is also a lot of discussion in the 2005 paper about “race correction” for lung volumes. In this update it says broadly, “The historical approach of fixed adjustment factors for race is not appropriate and is unequivocally discouraged.”

But for the updated spiro it does appear that there are 4 different population based equations, and one “other” equation that is an average of all of them. So I’m not exactly sure how that works in regards to my previous tweet.

🔹LIMITS OF NORMAL

I have always known LLN for determining normal vs abnormal (though many places are still using set predicted cut offs).

These guidelines mention that z-score “can be used to identify individuals with unusually low or high results.”

The z-score represents standard deviations—it is based on the 5th and 95th percentile limits (-1.645 and +1.645, respectively). It describes “the chance the observed result falls within the distribution of values in healthy individuals.”

Importantly, they say “the widely used cut-offs of 80% of predicted for FEV1 and the 0.7 cut-off for the FEV1/FVC ratio are strongly discouraged."

These ignore age-related variability and lead to “systematic misinterpretation…particularly for women, children and older adults.”

🔹BRONCHODILATOR RESPONSIVENESS

200mL AND 12% was standard based on 2005 paper (shown below).

New guidelines suggest “a change of >10% of the **predicted value** indicates a positive response.”

So we are getting rid of the mL criteria and dropping to 10%.

However we are also redefining how we calculate the % change. Previously it was {post BD value – pre) / pre.

Now the denominator for FEV1 or FVC is their *predicted value*. This will avoid small absolute mL changes being deemed as significant (10%) for people who have very low baseline values.

🔹DECLINE OVER TIME

A new approach: the FEV1Q (for adults, that is).

Rather than a set 15% or 10% (shown below) decline from a year prior, the FEV1Q comes at it from the opposite side--

the FEV1Q “expresses the FEV1 in relation to a ‘bottom line’ required for survival.”

Apparently, it should remain pretty stable over time.

🔹GRADING SEVERITY

We currently use FEV1 with values of 70%, 60, 50, and 35 for 5 different severity categories.

The new paper recommends using z-scores with a three level system—

mild (-1.65 to -2.5)
moderate (-2.5 to -4)
and severe (< -4)

This graphic shows the differences in the systems for 8 different patients.

And apparently even better than z-score is our new friend the FEV1Q—it better predicts survival, AECOPD, and adverse health outcomes so maybe we’ll be using it to grade severity someday?

🔹NONSPECIFIC PATTERN

This is a thing I very much enjoy hearing about.

It's defined as a normal ratio and normal TLC, but a reduced FEV1 and/or FVC.

It may be an early indicator of restriction, or of obstruction. After 3 years of follow up, 1/3 of patients with the pattern went on to have overt obstruction or restriction.

I learned that it is often seen in obesity, neuromuscular disease, and asthma.

In people with current or former tobacco use and with no TLC/lung volumes available, this has been labelled “PRISm” (preserved ratio-impaired spirometry).

There are some other small points, but instead I’ll just leave you with the algorithms for interpretation of spiro, lung volumes, and DLCO.

And the 2005 algorithm, for comparison.

So what do you all think? Like the changes? Don't like them? Already adopting some? Still haven't adopted some of the 2005s where you are? Did I misunderstand anything?

Also, if you've stuck with this thread for this long, go get yourself a cookie because you deserve it.🍪