Metabolic Equivalents and Functional Capacity Interpreter
Metabolic Equivalents and Functional Capacity Interpreter
The four-MET threshold is embedded in every perioperative guideline and its evidence base is thinner than its ubiquity suggests. Enter the reported capacity and how it was established — the second question turns out to matter more than the first.
Functional capacity in METs
Capacity and route → what the evidence supportsWalks one or two blocks on the level and no further; established from the patient’s own report of specific activities
The unit, the threshold, and the arithmetic that does not agree
2022 ESC guideline: VO₂max (mL/kg/min) = 0.43 × DASI + 9.6, and METs = VO₂max ÷ 3.5
- where the four-MET threshold came from
- it is a guideline convention rather than a derived cut-off. The ACC/AHA and ESC perioperative algorithms adopted “at least 4 METs” as the marker of adequate capacity, drawing on an activity classification in which four METs is roughly a flight of stairs. Reilly’s 1999 study of 600 patients is the empirical work usually cited beneath it — and that study measured blocks walked and flights climbed, chose “cannot walk four blocks and climb two flights” for its higher sensitivity over a three-block alternative, and never mentions METs at all. The number four in “four METs” and the number four in “four blocks” are not the same number
- what the comparative evidence actually showed
- the METS study compared four methods in 1,401 patients at 25 hospitals, with 28 deaths or infarctions (2 per cent). Subjective assessment: sensitivity 19.2% (14.2–25), specificity 94.7% (93.2–95.9) against inability to reach four METs on testing, and no association with the primary outcome. A structured activity questionnaire: the only method associated with the outcome, adjusted OR 0.96 per point (0.83–0.99, p=0.03). Peak oxygen consumption on exercise testing: per the ESC guideline, did not predict 30-day mortality, infarction or cardiac arrest. The authors’ conclusion was that subjectively assessed functional capacity “should not be used for preoperative risk evaluation”
- the two thresholds that are both called four METs
- the ESC guideline reports that a Duke Activity Status Index score below 34 was associated with increased odds of 30-day death or infarction, and separately gives the conversion VO₂max = 0.43 × DASI + 9.6. Put 34 through that equation and it comes out at 24.22 mL/kg/min, which is 6.9 METs — not four, and not the five METs that the study’s own substudy measured at the same score. The substudy reported a measured peak oxygen consumption of about 17 to 18 mL/kg/min at a DASI of 34, about five METs, and stated that the recommended conversion gave seven. The equation overestimates measured fitness by about two METs at the guideline’s own cut-off. Both figures are printed because both are published, and the equation’s own four-MET point corresponds to a questionnaire score near 10, far below either published cut
- why this page does not reproduce the questionnaire
- the Duke Activity Status Index is copyrighted by one of its authors and is catalogued by a licensing clearinghouse whose terms state that such instruments “are copyrighted unless placed in the public domain” and require a study-specific licence. We could not establish that it is free to reproduce, so its twelve weighted items are not reproduced here and have not been paraphrased into something that looks like them. The conversion equation above is quoted from the ESC guideline, which is a citation; the instrument is named so that a reader can go and get it properly
- self-reported and measured capacity are different quantities
- and the difference runs in one direction. Patients and clinicians both overestimate, the questionnaire-to-MET equation overestimates, and the only insensitive step in the chain is the one that happens in almost every clinic. A recorded “more than 4 METs” from a conversation is a different piece of evidence from a measured 14 mL/kg/min, and this page labels which of the two you have
Worked example
Walks one or two blocks on the level and no further; established from the patient's own report of specific activities
The reported activity sits in the 1 to 4 MET bracket of the guideline classification, so the capacity is below the four-MET threshold
It was established by report rather than by measurement, so the finding is an estimate
Reilly's cohort is the nearest empirical anchor: inability to walk four blocks and climb two flights carried a serious complication rate of 20.4 per cent against 10.4, an age-adjusted odds ratio of 1.94 (1.19 to 3.17) — in 600 outpatients before 612 procedures at one American centre in the 1990s
That study never mentions METs. The threshold this page is named after is a guideline convention laid over it
Change the route to a measurement on exercise testing and the result becomes Below the threshold on testing — the same capacity, much stronger evidence for it
Change the activity to climbing a flight of stairs, still by patient report, and the result becomes At or above the threshold, subjectively. That is the single commonest entry in a preoperative record, and it is the one with a sensitivity of 19.2 per cent
The asymmetry is the content of this page: a reassuring subjective assessment is the weakest evidence in the whole set, and it is also the most frequently recorded
The activity brackets the perioperative guidelines use
| Activity | Approximate METs | Oxygen uptake (mL/kg/min) |
|---|---|---|
| Eating, dressing, using the lavatory | about 1 | about 3.5 |
| Walking indoors around the house | about 2 | about 7 |
| Walking one or two blocks on the level; light housework | about 3 | about 10.5 |
| Climbing one flight of stairs; walking up a hill | about 4 | about 14 |
| Walking on the level at about 6 km/h; heavy housework | about 6 | about 21 |
| Running a short distance; singles tennis, football, squash | more than 10 | more than 35 |
What the four methods achieved in the METS study
| Method | Against inability to reach 4 METs on testing | Association with 30-day death or infarction |
|---|---|---|
| Clinician’s subjective assessment | Sensitivity 19.2% (14.2–25), specificity 94.7% (93.2–95.9) | None |
| Structured activity questionnaire | Not reported as sens/spec | The only method associated with it: adjusted OR 0.96 per point (0.83–0.99, p=0.03) |
| NT-proBNP | Not reported here | Reported as a comparator in the study |
| Peak oxygen consumption on exercise testing | This was the reference standard | Per the 2022 ESC guideline, did not predict 30-day mortality, infarction or cardiac arrest |
A number everyone uses and almost nobody has measured
A metabolic equivalent is a defined unit: 3.5 mL of oxygen per kilogram per minute, the approximate resting uptake of an average adult. Four METs is therefore 14 mL/kg/min, and nothing about that arithmetic is controversial. What is controversial — or ought to be — is the clinical claim built on top of it: that a patient who can achieve four METs has acceptable perioperative risk and one who cannot does not. That threshold sits in the ACC/AHA and ESC perioperative algorithms, it is asked about in every preoperative clinic in the form “can you climb a flight of stairs?”, and its evidence base is thin.
The 2022 ESC guideline is unusually direct about it. Metabolic equivalents below four “have long been considered to indicate poor functional capacity”, it says, and then: “studies using METs have been based on subjective interviews and not shown proven value”. The empirical study most often cited beneath the threshold is Reilly’s — 600 outpatients before 612 major non-cardiac procedures, in whom inability to walk four blocks and climb two flights carried a 20.4 per cent serious complication rate against 10.4 per cent, an age-adjusted odds ratio of 1.94. That is a real finding. It is also a finding about blocks and flights: the paper never mentions METs, and draws no link between its threshold and four of them.
Then the METS study measured what the clinical assessment is actually worth. In 1,401 patients at 25 hospitals, anaesthetists’ subjective grading of functional capacity in METs identified inability to reach four METs on exercise testing with a sensitivity of 19.2 per cent and a specificity of 94.7 per cent, and showed no association with 30-day death or myocardial infarction. A test that specific and that insensitive behaves in a particular way: it almost never flags a patient who is fit, and it misses four out of five of the patients it exists to find. The authors’ conclusion was that subjectively assessed functional capacity “should not be used for preoperative risk evaluation”. The only method in the study associated with the outcome was a structured activity questionnaire, and per the ESC guideline the exercise test itself did not predict the outcome either.
One more disagreement is worth printing, because it is in the guideline itself. The ESC reports a questionnaire cut-off of 34 and gives the published equation converting that questionnaire’s score to oxygen uptake: 0.43 times the score plus 9.6. Run 34 through it and the answer is 24.2 mL/kg/min, which is 6.9 METs. The study’s own substudy measured about 17 to 18 mL/kg/min at the same score — about five METs — and said the recommended conversion gave seven. The equation overestimates measured fitness by roughly two METs at the cut-off the guideline quotes, which means the arithmetic and the measurement disagree about what the threshold even is. Self-reported capacity and measured capacity are different quantities, and every step of the usual estimate runs in the optimistic direction. A score’s output is a cohort frequency, not this patient’s probability: a stratum in which 9 per cent had an event describes that stratum, not which 9 per cent. This page reports what a stratum predicted in a named study. It recommends no action.
Frequently asked questions
What is a MET and what is 4 METs?
One metabolic equivalent is 3.5 mL of oxygen per kilogram per minute, the approximate resting uptake of an average adult. Four METs is 14 mL/kg/min, conventionally anchored to climbing one flight of stairs or walking up a hill without stopping. Ten METs is 35 mL/kg/min.
Where does the 4-MET perioperative threshold come from?
It is a guideline convention rather than a derived cut-off. The ACC/AHA and ESC perioperative algorithms adopted “at least 4 METs” as the marker of adequate capacity. The empirical study usually cited underneath it, Reilly’s 1999 cohort of 600 patients, measured blocks walked and flights of stairs climbed and never mentions METs at all.
How reliable is a clinician’s estimate of functional capacity?
In the METS study it had a sensitivity of 19.2 per cent, 95 per cent confidence interval 14.2 to 25, for identifying a patient who could not reach four METs on exercise testing, with a specificity of 94.7 per cent, and it showed no association with 30-day death or myocardial infarction. The authors concluded that subjectively assessed functional capacity should not be used for preoperative risk evaluation.
Is a questionnaire better than asking the patient?
In the METS study a structured activity questionnaire was the only one of the four methods compared that was associated with the primary outcome — adjusted odds ratio 0.96 per point, 0.83 to 0.99, p = 0.03 — and the 2022 ESC guideline records that it estimated cardiac risk more precisely than subjective assessment. The caution is the conversion arithmetic rather than the instrument.
Why do two sources give different METs for the same questionnaire score?
Because one is computing and the other measured. The published equation VO₂max = 0.43 × score + 9.6 gives 24.2 mL/kg/min, about 6.9 METs, at a score of 34, while the METS substudy measured about 17 to 18 mL/kg/min at that score — about five METs — and reported that the recommended conversion gave seven. The equation overestimates measured fitness by roughly two METs at that point, and both figures are in print.
Related calculators
References
- Wijeysundera DN, Pearse RM, Shulman MA, et al. Assessment of functional capacity before major non-cardiac surgery: an international, prospective cohort study. Lancet. 2018;391(10140):2631–2640.
- Reilly DF, McNeely MJ, Doerner D, et al. Self-reported exercise tolerance and the risk of serious perioperative complications. Arch Intern Med. 1999;159(18):2185–2192.
- Halvorsen S, Mehilli J, Cassese S, et al. 2022 ESC Guidelines on cardiovascular assessment and management of patients undergoing non-cardiac surgery. Eur Heart J. 2022;43(39):3826–3924.
- Mapi Research Trust. Questionnaire licensing and the ePROVIDE catalogue entry for the Duke Activity Status Index, whose copyright is held by one of the instrument’s authors.
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