Assessing FEV1 trends, re-visited

There are a couple of different ways to assess changes in FEV1 from one patient visit to another. For several decades my lab has used a change of >=10% and >=200 ml as the threshold for a significant change. Recently the ATS released standards for occupational spirometry that included an age-adjusted change in FEV1 of >=15% as the threshold for significant change. For the time being we have continued to use the 10% threshold when comparing results that are relatively close in time and are using the 15% threshold when they are separated by a much longer period. Since we haven’t actually gotten around to defining what is recent and what isn’t there is still a bit of uncertainty in how we apply this but even though there are differences in thresholds and how the numbers are calculated both approaches are essentially numerical. Recently a couple of reports crossed my desk that have caused me to wonder whether a qualitative change should also be a consideration.

Trend Table 1

In the 14 years between these two tests the FEV1 has decreased by 0.56 L or -12.6%. By the 10% threshold criteria this is a significant change but I think that 14 years is a reasonably long period of time and the age-adjusted change is only 5.1% which indicates this change is not significant.

Trend Table 2

In the year between these two tests the FEV1 has decreased by 0.22 L or 7.0%, which doesn’t meet either criteria for a significant change.

But what has changed between these tests is that in both instances the spirometry went from normal to showing mild obstruction. This is a qualitative change and I think it is likely significant.

Spirometry is a “noisy” measurement and this applies both to a patient’s ability to perform a repeatable effort and for a spirometer to accurately measure it. My lab’s 10% threshold is based on studies from several decades ago but is probably not too far off the mark. A relatively recent study that looked at the normal distribution of change indicates that a decrease greater than approximately 8% within a year is likely not normal. This still doesn’t make a 7% absolute or 5% age-adjusted change clinically significant and in fact leaves them within the normal range of variability.

However, when a patient’s best spirometry effort is reported it is this result that is used to categorize the patient as being normal or as having airway obstruction (we’ll ignore restriction for the time being since spirometry cannot diagnose it). Obviously this categorization has significant implications for the patient’s clinical management so it would seem to me that when a patient goes from “normal” to “mild obstruction” (or vice versa) this should be considered a significant clinical change.

Note: This raises an interesting (and critical) point and that is where do you draw the line between “normal” and “obstruction”? The ATS-ERS advocates the use of LLN but this has been far from universally adopted. My lab (and others) uses an FEV1/FVC ratio less than 95% of predicted and other labs I know of use the predicted FEV1/FVC ratio minus 5. A patient that is considered to be obstructed at one institution can be considered to be normal at another. My personal opinion is that airway obstruction is multi-factorial and the reliance on any single standard will inevitably end up mis-categorizing at least some patients (at the moment I’m particularly curious about patients with elevated peak flows and a normal-ish FEV1). I suspect that more labs than not follow the ATS-ERS standard but I (and many others) have reservations about this approach since it is primarily statistical in nature. My medical director and I would be interested in hearing from any lab that uses different criteria than those outlined above or from anybody that has their own opinions about this point.

The problem is that a difference of just 1% can lead to placing results in either category. This admittedly is an artifact of the need to create dividing lines between categories. We may all publicly agree that these dividing lines are really fuzzy in nature but that doesn’t stop us from taking a numerical result and firmly placing it in one category or another and this is an artifact of human psychology. This doesn’t mean that there isn’t a lot of value in deciding what is normal and what isn’t but it still seems to me that a 1% change isn’t significant.

What does seem to be significant would be when a FEV1 is “reasonably” normal and changes to “reasonably” obstructed (or vice versa) even if it doesn’t meet any of the numerical criteria for a significant change. I don’t have a strong notion of where “reasonable” is for this purpose but I think that both of these examples I’ve presented meet this criteria.

The reason I think this is worthwhile is that at present the only criteria for alerting an ordering physician about a significant change is numerically based and does not alert a physician to a change in categorization. Of course an interpretation will include the categorization of the current results but when a comparison is made with prior results, the statement “there has been no significant change in FEV1” does not and cannot indicate that there has been a change from “normal” to “mild obstruction” (or vice versa). Saying “no significant change” is not necessarily being misleading since it is a true statement that meets ATS-ERS standards, but at the same time I think it is not being as clinically informative as it could be.

If I was setting up standards for this I think that a good starting point would be the category change between “normal” and “mild obstruction” (and vice versa, of course) that was accompanied by an age-adjusted numerical change of at least 5%. When this was present I think that a comment like “although the change in FEV1 is not statistically significant, results were previously categorized as normal/obstructed” should be made.

At the moment I think that the change between normal and mild obstruction is the only one that is really important since it is the most critical one and as importantly it is also well studied and characterized. Although the changes from mild to moderate to severe obstruction are important in their own way and may deserve mention in the same way the boundaries between them are far more arbitrary and are differences in degree rather than kind.

It could easily be argued that this is not a valid approach towards assessing change and I would agree that there are good statistical reasons it isn’t. Nevertheless, I think that the fact that non-significant changes can cause spirometry results to be categorized differently and that this difference has clinical significance means the change is significant.

 

References:

Brusasco V, Crapo R, Viegi G, et al. ATS/ERS Task Force: Standardisation of Lung Function Testing. Interpretive strategies for lung function testing. Eur Respir J 2005; 26: 948-968.

Kangalee KM, Abboud RT. Interlaboratory and intralaboratory variability in pulmonary function testing. A 13-year study using a normal biologic control. Chest 1992; 101: 88-92.

Redlich CA, Tarlo SM, Hankinson JL, Townsend MC, Eschenbacher WL, Von Essen SG, Sigsgaard T, Weissman DN. Official American Thoracic Society Technical Standards: Spirometry in the occupational setting. Amer J Respir Crit Care Med 2014; 1889: 984-994.

Wang ML, Petsonk EL. Repeated measures of FEV1 over six to twelve months: what change is abnormal? J Occup Environ Med 2004; 46: 591-595.

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