Air Trapping and RV/TLC Ratio Calculator
Air Trapping and RV/TLC Ratio Calculator
RV/TLC against the ratio of the report’s own predicted volumes, because there is no single RV/TLC threshold that is right at every age — and because a raised ratio can come from a high RV or a low TLC.
RV/TLC and air trapping
Measured ratio against predicted ratioRV 3.10 L (predicted 2.10), TLC 6.40 L (predicted 6.30)
Formula
- why the predicted ratio and not a threshold
- RV and RV/TLC rise with age in health, so no single figure is right across the age range, and the 2022 ERS/ATS standard sets the limit at the 95th centile of a reference set rather than at a number. Dividing the report’s predicted RV by its predicted TLC gives a comparison that is already age-, sex- and height-specific and needs no threshold at all
- predicted is not the limit of normal
- the predicted ratio is the middle of the reference distribution; the limit of normal is its 95th centile, which sits above it. A ratio modestly above predicted may still be within normal limits, and only the 95th centile on your own report settles it
- two routes to a high ratio
- RV/TLC rises either because RV rises, which is gas trapping, or because TLC falls, which is restriction. The secondary figures above separate them, and they mean entirely different things
- gas trapping against hyperinflation
- the 2022 standard’s table distinguishes them on FRC: “hyperinflation if FRC/TLC and RV/TLC elevated; gas trapping if only RV/TLC elevated”. FRC is not an input here, so this page names the possibilities rather than choosing between them
- effort and technique
- the same standard notes an increased RV/TLC “may also be seen with muscle weakness or suboptimal effort”. An expiration that stops early leaves gas behind and looks exactly like air trapping, and plethysmography and gas dilution disagree in obstruction — so RV and TLC must come from the same technique
Worked example
RV 3.10 L (predicted 2.10), TLC 6.40 L (predicted 6.30)
RV/TLC = 100 × 3.10 ÷ 6.40 = 48.4%
Predicted RV/TLC = 100 × 2.10 ÷ 6.30 = 33.3%
So the measured ratio is 145.3% of the predicted ratio
RV is 147.6% of predicted and TLC is 101.6% — the rise is almost entirely in the numerator, which is the gas-trapping pattern rather than restriction
The same ratio from the other direction. Take RV 2.00 L (predicted 2.10) with TLC 4.13 L (predicted 6.30): RV/TLC is 48.4%, indistinguishable from the case above, but RV is 95% of predicted and TLC is only 66%. The ratio rose because the denominator collapsed. Same number, opposite physiology — which is why this page prints both components
And a third point, with neither raised. RV 2.00 L with TLC 6.30 L gives 31.7% against a predicted 33.3%, below predicted, with RV at 95% and TLC at 100% of predicted
Enter RV 7.00 L against TLC 6.40 L and the page refuses: residual volume is part of total lung capacity and cannot exceed it
None of these results is a threshold crossing. The 2022 ERS/ATS standard sets the limit at the 95th centile of a reference set, which rises with age and which your report prints — being above predicted is not the same thing
One RV/TLC of 48.4%, three different lungs
| RV | TLC | RV/TLC | RV percent predicted | TLC percent predicted | What moved the ratio |
|---|---|---|---|---|---|
| 3.10 L | 6.40 L | 48.4% | 148% | 102% | RV rose — the gas-trapping pattern |
| 2.00 L | 4.13 L | 48.4% | 95% | 66% | TLC fell — a restrictive process imitating air trapping |
| 2.60 L | 5.37 L | 48.4% | 124% | 85% | Both moved — a mixed pattern |
What the 2022 standard says each pattern indicates
| Pattern | Published wording |
|---|---|
| RV or RV/TLC above the 95th centile | “may indicate hyperinflation or air trapping” |
| FRC/TLC and RV/TLC both elevated | hyperinflation |
| RV/TLC elevated alone | gas trapping, for example in COPD |
| RV/TLC elevated with no obstruction | “may also be seen with muscle weakness or suboptimal effort” |
Why there is no single RV/TLC threshold, and the two ways the ratio rises
Residual volume is the gas left in the chest after a complete expiration, and RV/TLC is the share of total lung capacity that cannot be exhaled. A rise in it is the functional signature of air left behind — small airways closing before the lung has finished emptying — and it appears early in obstructive disease, often before the FEV1/FVC ratio has moved much.
The ratio has two denominators’ worth of ambiguity built into it, and this is the trap. RV/TLC rises if RV rises, which is gas trapping. It also rises if TLC falls, which is restriction. A ratio of 48% can be a hyperinflated emphysematous chest or a small fibrotic one with an unremarkable residual volume, and the ratio alone cannot tell them apart. That is why this page prints RV and TLC as percentages of their own predicted values beside the ratio: the components carry the information the quotient throws away.
There is also no fixed threshold worth printing. RV and RV/TLC both rise with age in healthy people as elastic recoil falls, so a figure that is abnormal at 25 is unremarkable at 75. The 2022 ERS/ATS standard accordingly sets the limit at the 95th centile of a reference set — “an increase in RV or RV/TLC above the 95th percentile may indicate hyperinflation or air trapping” — rather than at a number, and distinguishes gas trapping from hyperinflation on whether FRC/TLC is raised as well as RV/TLC. This page therefore compares the measured ratio with the ratio of the patient’s own predicted volumes, which is already age-, sex- and height-specific, and says plainly that above predicted is not the same as above the 95th centile.
Two technical points decide whether any of this is interpretable. RV and TLC must come from the same method: plethysmography and gas dilution disagree in obstruction, because dilution techniques cannot reach regions that empty slowly and so understate RV in exactly the patients being assessed for air trapping. And effort matters — the same standard notes that an increased RV/TLC may be seen with muscle weakness or suboptimal effort, so an expiration that stops early produces the pattern of air trapping without any. Predicted values, LLNs and z-scores are reference-set dependent — GLI 2012, GLI Global, NHANES III and ECSC disagree for the same person — so use the set your own report names. A lung function pattern is interpreted with the clinical picture and the report’s own reference set, never on its own.
Frequently asked questions
What RV/TLC counts as air trapping?
There is no single number. RV and RV/TLC rise with age in health, so the 2022 ERS/ATS standard sets the limit at the 95th centile of a reference set rather than at a fixed figure. Compare the measured ratio with the 95th centile printed on your own report; the figures in circulation, such as 35% or 40%, are age-specific and do not transfer.
Can RV/TLC be raised without air trapping?
Yes, in two ways. A fall in TLC raises the ratio even when RV is normal, so a restrictive process can imitate air trapping; and the 2022 standard notes that an increased RV/TLC may also be seen with muscle weakness or suboptimal effort, because an expiration that stops early leaves gas behind.
What is the difference between gas trapping and hyperinflation?
The 2022 ERS/ATS standard separates them on the functional residual capacity: hyperinflation where FRC/TLC and RV/TLC are both elevated, gas trapping where only RV/TLC is. Gas trapping is air left behind at the end of expiration; hyperinflation is the whole operating volume of the lung shifted upward.
Does it matter whether RV came from plethysmography or gas dilution?
Very much, and the two must not be mixed in one ratio. Gas dilution techniques cannot sample regions that empty slowly, so they underestimate RV in obstruction — precisely the patients in whom air trapping is being looked for. Use RV and TLC from the same technique.
Why does this page ask for predicted RV and predicted TLC?
Because their ratio gives an age-, sex- and height-specific comparison without any threshold being invented. The calculator engine here has no table interpolation, so it cannot generate a GLI predicted value or a 95th centile; taking both predicted volumes from your report means the comparison carries your laboratory’s own reference set.
Related calculators
References
- Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J. 2022;60(1):2101499.
- Hall GL, Filipow N, Ruppel G, et al. Official ERS technical standard: Global Lung Function Initiative reference values for static lung volumes in individuals of European ancestry. Eur Respir J. 2021;57(3):2000289.
- Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J. 2017;49(1):1600016.
- BMJ Best Practice. Chronic obstructive pulmonary disease: diagnostic criteria. London: BMJ Publishing Group, 2025.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/