Table 2Projected Outcomes of Screening 100,000 Asymptomatic Adults for Carotid Artery Stenosis

True Prevalence of CAS = 1%True Prevalence of CAS = 5%
Number screened100,000100,000
Number of patients with CAS in population1,0005,000
Number of positive screening tests8,86012,300
 True positives (TPs) 940 4,700
 False positives (FPs) 7,920 7,600
Total number sent to surgery (FPs/TPs)
 a) no confirmatory testa) 8,860 (7,920/940)a) 12,300 (7,600/4,700)
 b) confirmation by angiogramb) 940 (0/940)b) 4,700 (0/4,700)
 c) confirmation by MRAc) 1,685 (792/893)c) 5,225 (760/4,465)
Strokes caused by angiogram confirmation106148
Perioperative strokes or death caused by operating on patients with false positive results
 a) no confirmatory test(a) 246(a) 236
 b) confirmation by angiogram(b)0(b)0
 c) confirmation by MRA(c) 25(c) 24
Non-fatal myocardial infarction among patients undergoing CEA: Total (FPs/TPs)
 a) no confirmatory test(a) 54 (48/6)(a) 79 (49/30)
 b) confirmation by angiogram(b) 6 (0/6)(b) 30 (0/30)
 c) confirmation by MRA(c) 10 (5/5)(c) 34 (5/29)
Outcome events in TPs
(no or angiography/MRA confirmation)
 Medical Treatment111/105555/527
 CEA Treatment60/57301/286
 Difference - events prevented by CEA51/48254/241
Perioperative events in FPs
(no/angiography/MRA confirmation)
 Medical Treatment0/0/00/0/0
 CEA Treatment246/106/25236/148/24
 Difference - events caused by CEA246/106/25236/148/24
Total stroke and perioperative death events caused or prevented by CEA (TPs + FPs)
a) No confirmatory test195 events caused18 events prevented
b) Angiography confirmation55 events caused106 events prevented
c) MRA confirmation23 events prevented217 events prevented
NNS to prevent one stroke over 5 years
a) No confirmatory testa) Events caused > preventeda) 5,556
b) Angiography confirmationb) Events caused > preventedb) 944
c) MRA confirmationc) 4,348c) 461
NNS to prevent one disabling stroke over 5 years
a) No confirmatory testa) Events caused > preventeda) 11,112
b) Angiography confirmationb) Events caused > preventedb) 1,888
c) MRA confirmationc) 8,696c) 922

Abbreviations: CAS = carotid artery stenosis; FPs = false positives; TPs = true positives; MRA = magnetic resonance angiography; CEA = carotid endarterectomy; NNS = number needed to screen

Screening and Confirmatory Testing Assumptions:


Sceening test is carotid duplex ultrasound, with sensitivity for CAS 60%–99% of 0.94; specificity of 0.92;


Confirmatory test is either (a) none, (b) cerebral angiogram (sensitivity and specificity = 100%), or (c) MRA (sensitivity = 0.95 and specificity = 0.90);


True prevalence in general > 65 year old primary care population = 1%; high risk group = 5%;


Stroke complication rate with angiography = 1.2%;


All patients with positive test go to surgery;


Perioperative stroke or death rate with CEA (whether patient is TP or FP) = 3.1% (as in ACST);


Perioperative non-fatal MI rate with CEA (whether patient is TP or FP) = 0.6% (as in ACST);


“Events” are all strokes and perioperative deaths 5 years after CEA;


Probability of event is 11.8% for medical (11.8%) and 6.4% for treatment with CEA treatment (ACST);


One-half of strokes prevented are non-disabling;


No benefit is received from medical or CEA treatment for patients with false positive screening test results.

From: 5, Discussion

Cover of Screening For Asymptomatic Carotid Artery Stenosis
Screening For Asymptomatic Carotid Artery Stenosis [Internet].
Evidence Syntheses, No. 50.
Wolff T, Guirguis-Blake J, Miller T, et al.

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