Okay, this may be wrong but at the moment I’m can’t seem to find a reason why it should be. A report like this came across my desk a couple of days ago.
| Observed: | %Predicted: | Post-BD: | %Predicted: | %Change: | |
| FVC: | 4.59 | 94% | 4.87 | 100% | +6% |
| FEV1: | 3.38 | 89% | 3.58 | 94% | +6% |
| FEV1/FVC: | 73.6 | 95% | 73.5 | 95% | 0 |
Not particularly unusual and it would usually be interpreted as being within normal limits without a significant post-BD change. If you calculate the FEV1/VC ratio using the pre-BD FEV1 and the post-BD FVC however, it’s 89% of predicted and this indicates mild airway obstruction. But you’re not supposed to use the post-BD FVC this way, are you?
Well, why not?
The ATS/ERS standards for spirometry states:
“The largest FVC and the largest FEV1 (BTPS) should be recorded after examining the data from all of the usable curves, even if they do not come from the same curve.”
The ATS/ERS standards for interpretation further states:
“An obstructive ventilatory defect is a disproportionate reduction of maximal airflow from the lung in relation to the maximal volume (i.e. VC) than can be displaced from the lung.”
Nowhere does it say that the VC can’t come from a post-BD FVC. The vital capacity is a relatively fixed value based on rib cage dimensions and the distance the diaphram can travel and bronchodilators do not change this. Any increases that are seen in a post-BD FVC are almost always related to a reduction in airway resistance and gas trapping, and not to a change in lung volume. This being the case, a VC is a VC, regardless of when it is measured.
The ATS/ERS standards for interpretation seconds this point of view to some extent when it states:
“The VC, FEV1, FEV1/VC ratio and TLC are the basic parameters used to properly interpret lung function. Although FVC is often used in place of VC, it is preferable to use the largest available VC, whether obtained on inspiration (IVC), slow expiration (SVC) or forced expiration (i.e. FVC).”
So other than saying that’s not the way it’s supposed to be done, why can’t you use the post-BD FVC this way? If you accept the idea that the FEV1/IVC ratio and FEV1/SVC ratio are valid approaches to determining the presence of airway obstruction you are also accepting the idea that the FVC is often underestimated as a consequence of airway obstruction. If you accept this idea, then why isn’t the post-BD FVC as valid a measurement of VC as IVC and SVC?
I can’t think of a reason why not and the ATS/ERS standards for spirometry and interpretation don’t explicitly forbid it. They don’t explicitly approve it either, but once you accept the notion of using the FEV1/VC ratio instead of the FEV1/FVC ratio it becomes a relatively logical consequence of that train of thought.
The downside (or is it the upside?) of using the pre-BD FEV1/post-BD FVC ratio is that some individuals whose spirometry would otherwise be considered to be within normal limits would instead be considered to have mild airway obstruction. I’d point out however, that this is also what would probably happen if SVC was routinely measured and the FEV1/SVC ratio reported instead of the FEV1.
So, why isn’t the post-BD FVC used this way right now? I think the answer is mostly psychological. Pretty much from the beginning of modern spirometry in the 1950’s post-BD testing has been performed only to compare FVC to FVC and FEV1 to FEV1. Although the ATS/ERS standards mandated the use of the FEV1/VC ratio (with the VC coming from the SVC, IVC or FVC, whichever was largest) a dozen years ago, this is recent history and the idea still hasn’t trickled completely down to all PFT labs and all pulmonologists. Even with the mandate from the ATS/ERS SVC and IVC maneuvers are performed much less frequently than FVC maneuvers so most practitioners aren’t in the habit of making substitutions. This adds up to a sort of institutional blindness and I will admit to having been part of it because I’ve been in the field for over 40 years and this is the first time it ever occurred to me.
This issue really only applies when the baseline FEV1/FVC ratio is withn normal limits. I can’t find any logical reason not to insert a post-BD FVC into the FEV1/VC ratio and if it increases the number of individuals diagnosed with airway obstruction then I don’t think this is any different than it would be if we performed SVC maneuvers as part of routine spirometry.
References:
Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of lung function testing. Standardisation of spirometry. Eur Respir J 2005; 26: 319-338.
Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of lung function testing. Interpretive strategies for lung function tests. Eur Respir J 2005; 26: 948-969.
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What you have observed is pretty frequent in asthmatics. The point is that even if the preBD test is in normal range, it is not necessarily the ceiling for the patients. The predicted value is just the 50th percentile for the population and an individual’s personal best (free of obstruction) can be way beyond this. The increase in FVC and FEV1 is 200+ in this case and represents a true BD response if both pre and post trials have met the repeatability criteria.
The ATS/ERS do state that you can use the largest FEV1 and FVC from all the curves…but pre and post BD curves cannot be pooled. They are measured under different conditions. So use of PreFEV1/PostFVC would have no scientific rationale.
You are right about FEV1/SVC increasing the sensitivity to detect airways obstruction. You can look up my paper in J Asthma . 1998;35(4):361-5. Another way to do that is to allow relaxation in effort in the latter part of the curve (my paper Indian J Chest Dis Allied Sci 43 (4), 205-210).
Why people do not measure SVC or IVC? Because our softwares do not allow that. 99% people I know would go for the “best” curve because of convenience. Lost lower end softwares do not allow you to select FEV1 and FVCs.
Dr. Chhabra –
It may be assumed that this is so obvious it did not need to be said but nowhere in the ATS/ERS standards is it stated that you cannot “pool” pre- and post-BD results. And just how are post-BD conditions for FVC any more different than they are for an SVC or IVC? As best as I can see it any post-BD increases in FVC are due either to a reduction in airway resistance and gas trapping (or to just testing variability) and not to an actual change in lung capacity. Unless bronchodilators actually increase lung capacity (and what’s the mechanism for that?) then all that’s changed is our ability to more accurately measure the VC and that’s no different than changing the testing maneuver to SVC or IVC. Other than saying “that’s just not done” I’m coming up blank trying to find a reason not to use the post-BD FVC if it’s the largest VC.
I agree in general that the ATS/ERS standards of +12% and +0.20 L as the threshold for bronchodilator response probably misses the mark. I would say this is particularly true when a 0.15 L increase for an FEV1 of 0.50 L in severe COPD is not considered significant. But I’m having trouble seeing a 0.20 L increase as being significant when the FEV1 is 5.00 L (+4%).
Software is both a boon and a bane but nobody should be surprised that the software for inexpensive spirometers usually has limited features. Having said that, I’m familiar with the software for several different systems that are intended to be used in hospital-based pulmonary function labs and even there the ability to select results can be limited. I talked with the software team for one of the major equipment manufacturers several years ago and they thought it was a good thing to prevent (stupid, un-educated) technicians from being able to choose which results were to be reported since they thought the software was smarter. This is wrong on many levels but is still part of the design philosophy for many manufacturers and still a selling point for some lab managers and medical directors.
Regards, Richard
Dear Richard
When I say the pre and post BD curves are under different conditions, I am referring to the bronchomotor tone. It is reduced after salbutamol inhalation that decreases air trapping and thus increases FVC.
Any definition of what is a significant increase is arbitrary. Statistically speaking, if the increase is greater than spontaneous variability, it is a true drug effect. The upper limit of spontaneous variability also sets the acceptable limit of repeatability. If the pre BD trials meet the repeatability criteria of 150 ml, then a post BD measurement that is higher by 200 ml would represent a true drug response. Thus, an absolute increase of 200ml in FEV1 or FVC should be looked at as a bronchodilator response if spirometry is acceptable and repeatable.
Softwares do sacrifice lots of things to make things simpler for large scale use. Selecting the largest values or using the “Best” curve gives mostly similar results or differences that are not clinically significant.
Best Wishes
Dr Chhabra
Dear Richard,
Thanks for such a great site !
If you would use the post BD FVC then you should use the largest FEV 1 as well which in this case is in the Post BD curve as well and so the ratio is the same.
It will hold indeed true what you are saying if the largest FEV1 is from the pre -BD trial which sometimes it happens and then It is acceptable and correct in my mind.
Also inwhat you wrote there is a typo
If you calculate the FEV1/VC ratio using the pre-BD FEV1 and the post-BD FVC however, it’s 89% of predicted and this indicates mild airway obstruction.
You meant 69.
Stylianos –
The FEV1 is primarily a measurement of expiratory flow rates whereas FVC is a measurement of lung volume. Expiratory flow rates are affected by bronchodilators so in this case I am not advocating that the largest FEV1 also be taken from the post-BD results. My point is that FVC is different from FEV1, particularly since you can substitute SVC or IVC for FVC.
You’re right in that the actual pre-BD FEV1/post-BD FVC ratio is 69 but since I didn’t include the predicted FEV1/VC ratio you couldn’t see that it also happens to be 89% of predicted.
Regards, Richard
Thank you for a great blog!
I agree with your point of view, and I must admit that I sometimes have used the preFEV1/postFVC to argue (at least for myself) that there is some degree of obstruction in a patient.
I have limited statistical training and no PhD, but I wonder if this also could be a statistical question. If the difference is not significant – how can you be sure that the postFVC is the true FVC? Would it therefore be wrong to use results from different tests to obtain a new ratio?
Best regards,
Erik Dyb Liaaen, MD
Norway
Dr. Liaaen –
I’m not sure that’s a statistical issue since we already treat the largest FVC, SVC or IVC as if it was the “true” FVC, so if you accept that bronchodilators have no significant effect on lung volume then there is no reason not to accept the post-BD FVC as the “true” FVC. Unless you are referring to taking a VC from a different testing session (different day, different equipment) then I have no qualms about using the SVC that’s performed as part of lung volume measurements or the IVC that’s performed as part of the DLCO and substituting them in the FVC/VC ratio.
Statistics come into play when we calculate the LLN from a study population. But I’ve noticed that we treat the FEV1/VC ratio LLN threshold as if it was a binary choice, ie. ratio above LLN = no obstruction | ratio below LLN = obstruction. The LLN is really just the point where we think there is a reasonable probability that obstruction is present but in fact the probability of obstruction being present when the ratio is above the LLN is not zero and the probability that obstruction is present when the ratio is below then LLN is not 100%. So we end up quibbling about thresholds when we should be discussing probabilities instead.
Regards, Richard
The 2005 Standard states that a significant change with bronchodilator can either come from improvement in the FEV1 at 12% and 200 ml or FVC of 12% and 200ml. Indeed we often see the later. Therefor you shouldn’t do a crossover ratio.
The Standing commitee made a mistake in omitting it. I heard recently that the standard is up for review very soon.
Recent article in Resp Care mentioned same point.
Resp Care 2016;61(11):1523-1529
Dr. Rashkin –
Thank you for pointing this article out to me. Although I am a subscriber I somehow had missed the entire November issue. Interestingly they made some of the same arguments I did and the outcome was that there was an increase of 6% in the diagnosis of airway obstruction when the pre-BD FEV1/post-BD FVC ratio was used.
Ragards, Richard