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NASCET Calculator: Carotid Artery Stenosis Percentage

NASCET carotid stenosis calculator. Enter stenosis and distal ICA diameters to get NASCET %, severity category, ECST equivalent, and CEA treatment thresholds.

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Patient Status:
NASCET trial thresholds apply: ≥70% = CEA strongly indicated; 50–69% = moderate benefit; <50% = surgery not indicated.

Narrowest residual lumen at maximum stenosis (inner-wall to inner-wall)

Disease-free ICA lumen distal to stenosis where walls are parallel
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How to Use This Calculator

  1. 1

    Select patient status

    Choose Symptomatic (TIA or stroke within the past 6 months ipsilateral to the stenosis) or Asymptomatic. This determines which clinical trial data applies to the treatment recommendation.

  2. 2

    Enter stenosis diameter (d_s)

    Measure the narrowest residual lumen at the point of maximum stenosis on your angiogram or CTA/MRA. Measure inner-wall to inner-wall in millimeters. This is the most critical measurement — small errors here have the largest impact on the result.

  3. 3

    Enter distal ICA diameter (d_d)

    Measure the ICA lumen at a point distal to the stenosis where the vessel walls are straight and parallel, beyond any post-stenotic dilatation. This serves as the reference normal diameter for the NASCET formula.

  4. 4

    Optionally add ECST (bulb diameter)

    Check the ECST option and enter d_b — the estimated diameter the carotid bulb would have if disease-free. This allows a direct ECST comparison rather than just the approximate conversion formula.

  5. 5

    Calculate and review the result

    The calculator returns NASCET %, severity tier, ECST equivalent, and evidence-based treatment context. A near-occlusion warning appears for stenosis ≥95%.

What Each Value Means

NASCET % Stenosis (percent)
The standard method for quantifying internal carotid artery narrowing. Uses the distal ICA (post-bulbar, disease-free) as the reference diameter. NASCET = (1 − d_s/d_d) × 100. Adopted internationally as the preferred method following the 1991 NASCET trial.
ECST % Stenosis (percent)
An alternative measurement method using the estimated original carotid bulb diameter as reference. Because the bulb is wider than the distal ICA, ECST percentages are consistently higher than NASCET for the same lesion. Approximate conversion: ECST ≈ 0.6 × NASCET + 40.
d_s (Stenosis Diameter) (millimeters)
The narrowest inner-wall to inner-wall diameter at the point of maximum stenosis on the angiogram. The numerator of the NASCET ratio. Small measurement errors here have the largest effect on the calculated percentage.
d_d (Distal ICA Diameter) (millimeters)
The ICA lumen diameter measured at a point distal to the stenosis where the artery is straight, parallel-walled, and free of disease. Serves as the reference 'normal' diameter in the NASCET formula.

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Frequently Asked Questions

What measurements do I need for the NASCET calculation?
You need two diameter measurements from the angiogram or CTA/MRA: (1) d_s — the narrowest residual lumen diameter at the point of maximum stenosis, measured inner-wall to inner-wall; and (2) d_d — the ICA lumen diameter at a disease-free point distal to the stenosis where the vessel walls are parallel and normal. Both are in mm. The ECST method additionally requires d_b, the estimated normal bulb diameter.
What is the NASCET formula for carotid stenosis?
NASCET % stenosis = (1 − d_s / d_d) × 100. For example, if the stenosis diameter is 2.0 mm and the normal distal ICA is 6.0 mm: NASCET = (1 − 2.0/6.0) × 100 = 66.7% — Moderate stenosis. The formula uses the narrower distal ICA as reference, which is why NASCET percentages are systematically lower than ECST percentages for the same lesion.
What is the treatment threshold for NASCET stenosis?
The landmark NASCET trial (1991) showed: symptomatic patients with ≥70% stenosis — CEA reduces 2-year ipsilateral stroke risk from 26% to 9% (NNT ≈ 6). Symptomatic 50–69%: moderate benefit (absolute risk reduction ~6.5% over 5 years). Below 50%: surgery offers no benefit over medical therapy. For asymptomatic stenosis, ACAS trial data (1995) suggests considering CEA for >60% in selected low-surgical-risk patients.
Why does NASCET give a lower percentage than ECST?
NASCET uses the distal ICA (a narrower, disease-free segment) as the reference diameter. ECST uses the estimated original carotid bulb diameter (wider). Because a wider denominator makes the missing tissue fraction appear larger, ECST percentages are systematically higher for the same lesion. A NASCET 70% corresponds to approximately ECST 82%. Conversion: ECST ≈ 0.6 × NASCET + 40.
What is near-occlusion and why does it matter?
Near-occlusion occurs when the ICA is so narrowed that distal ICA flow is severely reduced, causing the vessel to collapse. In this state, the residual lumen may actually appear small not because stenosis is worse but because the artery itself has narrowed. NASCET % calculation becomes unreliable when stenosis is >95–99% — the formula may overestimate or underestimate true severity. Near-occlusion should be identified clinically and on hemodynamic imaging rather than by calculated percentage alone.
Can I use NASCET for asymptomatic carotid stenosis?
The NASCET formula applies to any patient. However, treatment thresholds for asymptomatic patients derive from the ACAS trial, not NASCET. For asymptomatic stenosis ≥60% in low-surgical-risk patients, CEA provides a modest absolute risk reduction (~5.1% vs 11% over 5 years). Medical therapy has improved substantially since ACAS (1995), and the net benefit of CEA for asymptomatic patients is now debated.

References & Sources

  1. [1]
    Grant et al. — Carotid Artery Stenosis: Gray-Scale and Doppler US Diagnosis (opens in new tab)

    Supports: Evidence used for carotid duplex ultrasound velocity criteria reference, including related formulas, definitions, or reference values.

    Verified

  2. [2]
    NASCET Investigators — N Engl J Med 1991 (opens in new tab)

    Supports: Evidence used for carotid duplex ultrasound velocity criteria reference, including related formulas, definitions, or reference values.

    Verified

  3. [3]
    PMC — Quantifying Carotid Stenosis: History, Limitations and Potential (opens in new tab)

    Supports: Evidence used for nascet measurement technique: correct carotid stenosis measurement, including related formulas, definitions, or reference values.

    Verified

  4. [4]
    ACAS Trial — JAMA 1995 (Asymptomatic) (opens in new tab)

    Supports: Evidence used for how to calculate carotid stenosis using the nascet method, including related formulas, definitions, or reference values.

    Verified