Category Archives: FEV1/FVC Ratio

FEV6

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FEV6 (the volume of air forcefully exhaled after 6 seconds) has been proposed as a replacement or surrogate for FVC in spirometry. Given that using FEV6 would simplify and speed up the spirometry test this is a seductive notion.

The use of an expired volume with a fixed expiratory time as a replacement for FVC was proposed at least 25 years ago, although at that time FEV7 was proposed as being slightly more accurate than FEV6. The first reference values for FEV6 however, were not available until the results from NHANESIII study were made available in 1999 and most studies of FEV6 post-date that.

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Is it time to scuttle the FEF25%-75%?

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When we went through our hardware and software upgrade last August, one of the changes we made was to stop reporting the FEF25%-75% (AKA MMEF, MMFR, MMF). The pulmonary physicians had long since stopped using this value when assessing spirometry results and we had kept it on our reports as long as we did only for inter-laboratory compatibility. Along with other changes we made at that time we decided it was time to drop the FEF25%-75% off our reports.

FEF25%-75% has been used to assess “small airways disease” but more than one of our pulmonary physicians has said that they don’t believe there is such a thing. I’m not a clinician but I’ve always felt that tests and results need to be clinically useful in order to be performed or reported and more than one study has shown little correlation between anatomical findings and FEF25%-75%.

Regardless of whether or not small airways disease is an actual entity my first objection to the FEF25%-75% has to do with the concept that it measures flow in small airways when for most patients it lies within their FEV1. For this reason it has never been clear to me what the FEF25%-75% is measuring that the FEV1 isn’t. More importantly, I have significant concerns about the limitations involved in measuring the FEF25%-75% in the first place.

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What’s normal about the FEV1/FVC ratio?

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The FEV1/FVC ratio is used to estimate the presence and degree of airway obstruction. For well over thirty years my lab has used an FEV1/FVC ratio of 95% of predicted as the cutoff for normalcy. This value (carved onto a stone tablet by the way) had been brought to the lab by a founding physician who had come to the department from the NIH in the 1970’s. Since the software and hardware upgrade this summer our PFT Lab has switched to the NHANES III spirometry reference equations but we have so far resisted changing our 95% cutoff to the lower limit of normal (LLN). This is due in part to inertia but also in part to a mistrust in the concept of LLN. We have been steadily re-evaluating all of our testing criteria and have turned again to the FEV1/FVC ratio with the question as to whether our 95% cutoff is over-zealous or whether the LLN is too lax.

Strictly speaking LLN is a statistical concept. In the NHANES III study (and most others) it is computed as the mean predicted value minus 1.645 times the standard estimate of error. Unlike the reference equations for FVC and FEV1 which use both height and age as factors, the NHANES III reference equations for the FEV1/FVC ratio are derived solely from age. It is not clear to me this is completely correct and I have discussed some of the discrepancies between the NHANES III predicted FEV1/FVC ratio and height in a prior posting but it does make analyzing the LLN for the ratio easy. For adult, Caucasian males the reference equations are:

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FEV1/FVC ratio and height

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The PFT Lab I work with has recently gone through a major software and hardware upgrade. As part of this process we made the decision to switch our spirometry predicted equations to NHANESIII. The lab has been using the Morris predicteds for at least the last 25 years and this switch has led us to re-visit some of the issues involved in interpreting spirometry results.

More than one person that I’ve known and respected has said that spirometry is all about FEV1 and I think this is a true statement. There is a lot of other information you can get from a Forced Vital Capacity but it always comes back to FEV1.

Stepping aside from the mechanical and patient issues involved in obtaining an FEV1, once you have an acceptable FEV1 measurement how do you assess it? There is always the percent predicted and the lower limit of normal (LLN) but a reduced or normal FEV1 by itself cannot differentiate between an obstructive, restrictive or normal pattern. This is where the FEV1/FVC ratio comes in and an interesting question is where the predicted values for this ratio come from.

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