Carotid Stenosis NASCET to ECST Converter
Carotid Stenosis NASCET to ECST Converter
The same lesion, two percentages. NASCET divides by the distal internal carotid and ECST by the estimated original diameter at the stenosis, so ECST always reads higher and the two trials’ thresholds are not comparable numbers.
NASCET and ECST percentage stenosis
Both directions, one lesion55 per cent by NASCET criteria, converting to ECST
The two definitions and the relation between them
- NASCET%
- (1 – narrowest residual lumen / normal distal internal carotid diameter) x 100, the denominator measured beyond the bulb where the walls run parallel. BMJ Best Practice records that this method “has become the accepted worldwide standard”
- ECST%
- (1 – narrowest residual lumen / estimated original diameter at the stenosis) x 100. The denominator is the bulb, which the plaque conceals, so it is estimated by extrapolating an outline between the lesion’s margins — and it is wider than the distal vessel, one review putting it at “up to twice the diameter”, which is why ECST always reads higher
- 0.6 and 40
- the slope and intercept of the published linear relation, attributed to Rothwell and colleagues’ 1994 comparison of three measurement methods on 1,001 angiograms. Three independent sources read for this page print it identically; the Rothwell paper itself could not be read, which is stated rather than glossed over
- the only crossover
- setting the two equal gives 0.6 N + 40 = N and so N = 100. The conventions agree at a total occlusion and nowhere else, which is why a test there cannot detect a wrong implementation
- the floor, and what the relation is not
- the inverse direction is refused below an ECST value of 40, because the relation maps NASCET 0 to ECST 40. And it is a regression across a population of angiograms, not a geometric identity: Vanninen and colleagues illustrate one bifurcation at NASCET 28 per cent and ECST 46 per cent, where it predicts 56.8 — a 10.8-point miss on one real lesion
Worked example
55 per cent by NASCET criteria, converting to ECST
ECST% = 0.6 x 55 + 40 = 73.0 per cent
Both directions. Back: (73 - 40) / 0.6 = 55.0 by NASCET. A conversion that does not round-trip is wrong
The anchors, which are not equal. NASCET 50 gives ECST 70; 60 gives 76; 70 gives 82, the equivalence BMJ Best Practice states in terms; 80 gives 88 — higher by 20, 16, 12 and 8 points
The crossover, solved rather than remembered. ECST% = NASCET% requires 0.6 N + 40 = N, so 0.4 N = 40 and N = 100. The two agree at a total occlusion and at no other stenosis, so an implementation checked only at 100 per cent would look correct however wrong it was
The intercept, which is the whole argument. NASCET 0 converts to ECST 40, because the bulb is wider than the distal vessel even in a normal artery. A relation saying a normal carotid is 40 per cent stenosed is a regression, not geometry
The trial thresholds, side by side. NASCET 50 is ECST 70, and ECST 80 is NASCET 66.7. Reading one trial's threshold against the other's report is the error this page prevents
And where the sources disagree. One review attributes to the same 1994 work the anchors ECST 70 to NASCET 40 and NASCET 70 to ECST 85, where the regression gives 64 and 82 — out by 6 and 3 points
The same lesion in both conventions, and the trial thresholds
| NASCET % | ECST % (0.6 x NASCET + 40) | Gap | What sits here |
|---|---|---|---|
| 0 | 40 | 40 | A normal artery, and the intercept |
| 30 | 58 | 28 | Below both |
| 50 | 70 | 20 | NASCET’s moderate threshold. ECST 70 is not ECST’s |
| 60 | 76 | 16 | |
| 66.7 | 80 | 13.3 | ECST’s own 80 per cent threshold, in NASCET terms |
| 70 | 82 | 12 | NASCET’s severe threshold. BMJ states this pair |
| 80 | 88 | 8 | |
| 90 | 94 | 4 | |
| 100 | 100 | 0 | The only crossover |
Consensus duplex velocity criteria, and the layer they add
| Stenosis category | ICA peak systolic velocity | ICA/CCA ratio | ICA end-diastolic velocity |
|---|---|---|---|
| Under 50 per cent | Under 125 cm/s | Under 2.0 | Under 40 cm/s |
| 50 to 69 per cent | 125 to 230 cm/s | 2.0 to 4.0 | 40 to 100 cm/s |
| 70 per cent to near occlusion | Over 230 cm/s | Over 4.0 | Over 100 cm/s |
| Near occlusion | Variable — high, low or undetectable | Variable | Variable |
Two conventions, two sets of numbers, one artery
NASCET and ECST both express carotid stenosis as a percentage reduction in diameter, and they disagree about the denominator. NASCET divides the residual lumen by the normal distal internal carotid artery, measured beyond the bulb where the walls run parallel. ECST divides it by the estimated original diameter at the stenosis — the bulb itself, which the plaque conceals and which has to be reconstructed by extrapolating an outline between the lesion’s margins. Because the bulb is wider, the same residual lumen is a larger proportional loss and ECST reports a substantially higher percentage for the identical lesion.
The published relation between them, attributed to Rothwell and colleagues’ 1994 comparison of measurement methods on 1,001 angiograms and printed identically by three of the sources read for this page, is ECST = 0.6 x NASCET + 40. NASCET 50 is about ECST 70; NASCET 70 is about 82, the equivalence BMJ Best Practice states explicitly. Set the two equal and the only solution is 100 per cent. Run it to zero and it says a normal artery is 40 per cent stenosed, which is the clearest proof that this is a population regression and not geometry.
That is why the two landmark trials’ thresholds cannot be compared. NASCET found benefit in symptomatic stenosis of 50 to 69 per cent and above 70 per cent by its own convention; ECST found benefit only at 80 per cent or more by its own. Those sentences contain the numbers 70 and 80 and they describe arteries of similar severity: ECST 80 is about NASCET 66.7. A report saying “70 per cent stenosis” without naming its convention could be either trial’s threshold or neither, which is why BMJ Best Practice insists that “physicians treating carotid disease must specify which methodology was used”.
Two honesties. The relation is a population average and individual lesions depart from it: Vanninen and colleagues illustrate a bifurcation at NASCET 28 per cent and ECST 46 per cent where it predicts 56.8, and one review attributes to the same 1994 work anchors of ECST 70 to NASCET 40 and NASCET 70 to ECST 85, where it gives 64 and 82. And both definitions are angiographic, while most stenoses in practice are graded from Doppler velocities. This category owns the limb and the named vessel. Risk estimation in somebody with no vascular diagnosis belongs to the pooled cohort equations, the cost of an operation to the Revised Cardiac Risk Index and the AUB-HAS2 index, and central circulatory arithmetic to systemic vascular resistance and mean arterial pressure.
Frequently asked questions
What is the conversion between NASCET and ECST carotid stenosis?
ECST per cent is about 0.6 times the NASCET per cent plus 40, and NASCET per cent is about (ECST per cent minus 40) divided by 0.6. The relation is attributed to Rothwell and colleagues’ 1994 comparison on 1,001 angiograms, and three independent sources read for this page print it identically. NASCET 50 is roughly ECST 70.
Why does ECST give a higher percentage than NASCET?
Because its denominator is bigger. NASCET divides by the normal distal internal carotid artery; ECST divides by the estimated original bulb diameter, which one review puts at up to twice the distal diameter.
At what stenosis do the two conventions give the same number?
Only at 100 per cent. Setting ECST equal to NASCET gives 0.6 N + 40 = N, so N = 40 / 0.4 = 100. That matters practically: at a total occlusion every wrong implementation looks right, so the check has to be done at 50, 60, 70 or 80 per cent.
Are the NASCET and ECST intervention thresholds comparable?
No, and that is the error this page exists to prevent. NASCET found benefit at 50 to 69 per cent and at 70 per cent or more by its own convention; ECST only at 80 per cent or more by its own. ECST 80 is about NASCET 66.7, so the two trials described arteries of broadly similar severity with numbers ten points apart.
Does this conversion work for an ultrasound report?
Not straightforwardly. NASCET and ECST are angiographic definitions, while a duplex report grades stenosis from velocities against consensus thresholds — and neither source read for this page states which angiographic convention those categories are calibrated against. Published velocity schemes also disagree with each other. Read the convention off the report, or ask.
Related calculators
References
- BMJ Best Practice. Carotid artery stenosis: diagnostic criteria. London: BMJ Publishing Group, 2025.
- Extracranial Carotid Stenoses — radiology textbook chapter, radiologykey.com.
- RadioGyan. NASCET and ECST carotid stenosis measurement. radiogyan.com, 2025.
- Vanninen R, Manninen H, Koivisto K, Tulla H, Partanen K, Puranen M. Carotid stenosis by digital subtraction angiography: reproducibility of the European Carotid Surgery Trial and the North American Symptomatic Carotid Endarterectomy Trial measurement methods and visual interpretation. AJNR Am J Neuroradiol. 1994;15(9):1635–41.
- A review of carotid stenosis measurement conventions. Neurovascular Imaging. 2015;1. doi:10.1186/s40809-015-0002-1
- WikiJournalClub. NASCET — summarising Barnett HJM, Taylor DW, Eliasziw M, et al. Benefit of carotid endarterectomy in patients with symptomatic moderate or severe stenosis. N Engl J Med. 1998;339(20):1415–25.
- WikiJournalClub. ECST — summarising European Carotid Surgery Trialists’ Collaborative Group, Lancet. 1998;351(9113):1379–87.
- AuntMinnie. SRU conferees select thresholds for ultrasound diagnosis of carotid disease. auntminnie.com, 2002.
- Radiopaedia. Ultrasound assessment of carotid arterial atherosclerotic disease. Melbourne: Radiopaedia.org, 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/