DLCO, KCO and Alveolar Volume Calculator
DLCO, KCO and Alveolar Volume Calculator
KCO is DLCO divided by alveolar volume — not DLCO corrected for lung volume. Treating it as a correction is the standard error, and this page shows what the two numbers actually separate.
DLCO, KCO and alveolar volume
KCO equals DLCO over VADLCO 18.0, VA 4.00 L, plethysmographic TLC 6.40 L
Formula
- KCO
- the rate constant for carbon monoxide uptake — strictly, the logarithmic rate of alveolar CO disappearance per unit breath-hold time and dry barometric pressure. The 2017 ERS/ATS standard notes that it is independent of flow, of lung volume and of the barometric pressure measured during the manoeuvre, and that VA is not a component of it
- VA
- the single-breath alveolar volume: “an estimate of lung gas volume into which carbon monoxide is distributed and then transferred across the alveolar capillary membrane”. It is derived from tracer dilution in one breath, so poorly ventilated lung is simply absent from it
- why DLCO = KCO × VA matters
- the 2017 standard calls the product “the key index that is interpreted for gas transfer”. Reading the two factors separately is what distinguishes a small lung with intact alveoli from a normal-sized lung with damaged ones — and a DLCO alone cannot make that distinction
- VA is not TLC
- the two diverge in obstruction, where tracer gas does not reach poorly ventilated regions. The expected VA/TLC in adults is about 0.85 to 0.90; below that, impaired gas mixing is contributing to a low DLCO, and the 2022 standard warns that a single-breath TLC must not be read as evidence of restriction because it systematically underestimates the real one
Worked example
DLCO 18.0, VA 4.00 L, plethysmographic TLC 6.40 L
KCO = 18.0 ÷ 4.00 = 4.50 mL/min/mmHg per litre
Round trip: 4.50 × 4.00 = 18.00, the DLCO back
VA ÷ TLC = 4.00 ÷ 6.40 = 0.625, well below the expected adult 0.85 to 0.90. The single breath reached only 62.5% of the thoracic gas, leaving 2.40 L unsampled — the gas-mixing pattern of obstruction, and a reason the DLCO itself is low
Three points, to show the relationship is not proportional. Hold DLCO at 18.0 and move VA: at VA 6.00 L, KCO is 3.00; at 4.00 L, 4.50; at 3.00 L, 6.00. Halving VA doubles KCO exactly, because KCO is a quotient — and that is precisely why it cannot be a correction for volume, since the real fall in DLCO when a lung is made smaller is much less than proportional
The two patterns the page exists to separate. A patient with extrapulmonary restriction — pleural disease, obesity, neuromuscular weakness, or simply an incomplete inspiration — has a small VA with a normal or high KCO: the alveoli are intact and the lung is merely under-expanded. A patient with parenchymal disease has a low KCO, with or without a small VA: alveolar-capillary units have been lost
Enter VA 7.00 L against TLC 6.40 L and the page refuses rather than returning a figure. VA is single-breath lung gas minus anatomical dead space and cannot exceed total lung capacity, so that pair cannot both be right
The four combinations, and what the 2022 standard attributes each to
| VA | KCO | Typically | Published wording |
|---|---|---|---|
| Low | Low or normal | Loss of alveolar-capillary units | “loss of alveolar capillary structure such as in emphysema or interstitial lung disease with loss of lung volume” |
| Low | High | A low lung volume state, not a lung disease | lung resection, or an incomplete inspiration |
| Normal | Low | A low DLCO with a normal-sized lung | “pulmonary vascular impairment, emphysema with preserved lung volume or anaemia” |
| Normal | Normal | Gas transfer not the limiting problem | Check the VA/TLC ratio before accepting this: a normal VA with poor gas mixing is still an incompletely sampled lung |
KCO against VA at a fixed DLCO of 18.0
| VA (L) | KCO | VA ÷ TLC at a TLC of 6.40 L |
|---|---|---|
| 6.00 | 3.00 | 0.938 |
| 5.00 | 3.60 | 0.781 |
| 4.00 | 4.50 | 0.625 |
| 3.00 | 6.00 | 0.469 |
| 2.50 | 7.20 | 0.391 |
What dividing by alveolar volume does and does not achieve
DLCO is the product of two things a report usually prints separately: how fast carbon monoxide leaves alveolar gas, which is KCO, and how much alveolar gas the single breath reached, which is VA. The 2017 ERS/ATS standard calls their product “the key index that is interpreted for gas transfer”, and reading the factors separately is what distinguishes a small lung with intact alveoli from a normal-sized lung that has lost them.
What dividing by VA does not do is correct DLCO for lung volume. The 2017 standard says so in as many words — KCO “should not be reported using the term DLCO/VA”, because that notation implies a normalisation that is not available. The relationship between lung volume and carbon monoxide uptake is non-linear and less than one for one: make a lung smaller and DLCO falls by considerably less than the volume does, so dividing by VA over-corrects and KCO rises as volume falls. A KCO inside the reference range at a markedly reduced VA is therefore not a normal result; it is the expected arithmetic of a small sampled volume.
That is the distinction the page is for. A small VA with a preserved or high KCO points away from the lung parenchyma — pleural disease, chest-wall restriction, obesity, neuromuscular weakness, lung resection, or an inspiration that was simply not complete. A low KCO, with or without a small VA, points at the parenchyma or the pulmonary circulation: emphysema, interstitial disease, pulmonary vascular disease. The 2022 standard adds the third pattern, a low DLCO with a normal VA, and attributes it to “pulmonary vascular impairment, emphysema with preserved lung volume or anaemia”.
One further trap. VA from a single breath is not TLC. The expected ratio in adults is about 0.85 to 0.90, and a markedly lower value means tracer gas did not reach part of the lung — common in obstruction, and a reason the DLCO is low in its own right. The 2022 standard warns that “a low TLC from a single-breath test (such as VA from the DLCO) should not be interpreted as demonstrating restriction”, because single-breath measurements systematically underestimate the real total lung capacity. 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
Is KCO the same as DLCO corrected for lung volume?
No, and the 2017 ERS/ATS standard specifically asks that it not be written as DLCO/VA for that reason. The relationship between lung volume and carbon monoxide uptake is non-linear and less than one for one, so dividing by VA over-corrects. KCO rises as lung volume falls, which means a normal-looking KCO at a low VA is not a normal result.
What does a low VA with a normal or high KCO mean?
Usually that the lung is under-expanded rather than diseased: pleural disease, chest-wall or abdominal restriction, obesity, neuromuscular weakness, previous resection, or an incomplete inspiration during the manoeuvre. The 2022 standard attributes a low VA with a high KCO to a low lung volume state.
What does a low KCO mean?
That carbon monoxide is transferring slowly out of the gas that was reached, which points at the parenchyma or the pulmonary circulation — emphysema, interstitial lung disease, pulmonary vascular disease — or at anaemia, which reduces transfer without any lung abnormality and should be corrected for before the KCO is interpreted.
Can I use the single-breath VA as the total lung capacity?
No. The 2022 ERS/ATS standard warns that a low TLC from a single-breath test such as VA should not be read as demonstrating restriction, because single-breath measurements systematically underestimate TLC. The expected VA/TLC in adults is about 0.85 to 0.90, and the gap widens in obstruction where tracer gas cannot reach poorly ventilated regions.
Why is my VA so much smaller than my plethysmographic TLC?
Because the single breath only samples lung the tracer gas reached. A ratio well below 0.85 indicates impaired gas mixing contributing to the low DLCO, which the 2022 standard treats as informative in itself — it is the signature of obstruction and of bullous disease rather than a measurement error.
Related calculators
References
- 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.
- 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.
- ndd Medical Technologies. Adjustment of DLCO Measurements (EasyOne Connect application note). Zurich: ndd, 2021.
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/