***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
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    ***EXAMPLES*** 
    Input: A multicenter retrospective audit of  carotid endarterectomies performed during 1981 was completed with 46 institutions contributing 3,328 cases . Overall , there was a 2.5 % risk of transient neurological dysfunction following surgery and a 6 % risk of stroke or death . The intra-institutional combined major morbidity and mortality varied from 21 % to 0 . Those institutions with greater than 700 beds had a statistically lower incidence of stroke or death than did other institutions . The incidence of stroke or death postoperatively was significantly lower for patients who were operated on for amaurosis fugax or for unspecified reasons . Those patients who were operated on for a progressing stroke had a higher incidence of stroke but this group was at greatest risk for stroke without surgery . The incidence of postoperative stroke or death was related to the type of arterial repair ; vein patch grafting was statistically better than both fabric patch grafting and primary closure . When all patients who were not monitored during surgery were compared to all patients who had  electroencephalographic ( EEG ) monitoring , there was found to be a significant statistical difference in favor of the EEG group . Endarterectomy combined with coronary artery bypass or simultaneous bilateral endarterectomies had a statistically significant higher incidence of stroke or death than did unilateral carotid endarterectomy Background and Purpose R and omized clinical trials have shown that carotid endarterectomy decreases the risk of subsequent stroke in patients with high- grade carotid stenosis and ipsilateral transient ischemic attack or minor stroke . The benefit of surgery is highly dependent on surgical risk . We previously found that patients with ipsilateral hemispheric symptoms were at greater risk of carotid endarterectomy complications compared with those who were asymptomatic or had nonipsilateral symptoms . The goals of the present study were ( 1 ) to identify preoperative clinical factors that may increase the risk of complications after  carotid endarterectomy  in patients with ipsilateral hemispheric symptoms and ( 2 ) to develop a risk index based on this patient-level data . Methods Records from 1160 carotid endarterectomies performed at 12 academic medical centers composed the primary data set . Hospital charts for the admission during which carotid endarterectomy was performed were systematic ally review ed by abstract ors using a defined protocol . The present analysis was carried out on data from the subset of patients who had carotid endarterectomy for ipsilateral hemispheric symptoms . C and i date variables were identified based on univariate Fisher 's exact tests or x2 tests . A risk index was then developed using those variables with a greater than 90 % probability of being associated with adverse outcomes . Results Of the 697 patients with ipsilateral symptoms , 8.5 % had either stroke , myocardial infa rct ion , or died during the postoperative period of hospitalization . Those over the age of 75 had a greater risk of myocardial infa rct ion ( 6.6 % versus 2.3 % , P=.024 ) but not of stroke or death ( P<.10 ) . The overall frequencies of adverse outcomes were also higher in the 5 patients with complete ipsilateral carotid occlusions ( 40 % versus 8.2 % , p<.01 ) , the 28 patients with ipsilateral intraluminal thrombus ( 17.9 % versus 8.1 % , P=.01 ) , and the 65 patients with ipsilateral carotid siphon stenosis ( 13.9 % versus 7.9 % , P=.10 ) . There were no differences in adverse outcomes among those with different degrees of ipsilateral stenosis ( 30 % to 49 % , 50 % to 69 % , and 70 % to 99 % ) . Adverse outcome rates were similar regardless of the type of symptom ( transient ischemic attack , recent ipsilateral minor stroke , remote ipsilateral minor stroke ) . There were no significant differences in adverse outcome rates based on sex , race , history of angina , recent myocardial infa rct ion , congestive heart failure , chronic obstructive pulmonary disease , hypertension , degree of stenosis of the contralateral carotid artery , or presence of ulceration in the ipsilateral artery ( Fisher 's exact tests , P>.10 ) . A count of variables with greater than 90 % probability of being associated with adverse outcomes ( age ≤75 years or angiographic evidence of ipsilateral carotid occlusion , stenosis in the region of the carotid siphon , or intraluminal thrombus ) was used to form a simple risk index . “ High-risk ” patients ( one or more risk factors ) had more than two times the risk of complications compared with “ low-risk ” patients who had no risk factors ( odds ratio , 2.18 ; 95 % confidence interval , 1.25 to 3.81 ) . Conclusions Certain preoperative clinical variables may place patients with ipsilateral symptoms at greater risk of perioperative complications after  carotid endarterectomy  . Prospect i ve validation of a simple risk index would provide an additional method for assessing preoperative risk in endarterectomy c and i date In a prospect i ve study using a multigate pulsed Doppler system , minor ( less than 30 % diameter reduction ) carotid artery lesions were diagnosed by detecting not only abnormalities in the blood flow pattern , but also local changes in artery wall distensibility . For the diagnosis of more severe lesions , additional information was obtained from disturbances in the Doppler audio spectrum . Biplane arteriography was used as a reference . The observed agreement , sensitivity , and specificity were 86.6 % , 90.3 % , and 88.6 % , respectively , for all lesions and 85.7 % , 82.1 % , and 88.6 % , respectively , when only minor lesions were considered . kappa ( a chance-corrected measure of agreement ) was 81.7 % . If only blood flow abnormalities were used to detect minor lesions , 43.5 % would be missed . Our results indicate that minor carotid artery lesions can be diagnosed noninvasively more accurately when both local blood flow irregularities and local changes in vessel wall distensibility are taken into account We prospect ively compared duplex ultrasonography with digital subtraction angiography ( DSA ) and conventional angiography in the evaluation of  carotid atherosclerosis by study ing 494 arteries . Arteries were grade d independently as normal , mildly stenotic , moderately stenotic , severely stenotic , or occluded . There was absolute agreement between duplex ultrasonography and DSA as to disease severity in 296 ( 68 % ) of 434 vessels , good correlation in an additional 119 vessels ( 28 % ) , and poor correlation in 19 vessels ( 4 % ) . When duplex studies were compared with conventional angiograms , the correlations were similar : absolute for 60 ( 71 % ) of 85 vessels , good for 17 ( 20 % ) of 85 vessels , and poor for eight ( 9 % ) of 85 vessels . With conventional angiography as the st and ard , duplex studies tended to overestimate the severity of disease in 64 % of vessels and underestimate it in 36 % , v 28 % and 72 % , respectively , for DSA . We concluded that duplex ultrasonography is as accurate as DSA in assessing  carotid bifurcation atherosclerosis .  The tendency of DSA to underestimate disease severity may lead to diagnostic errors in patients with nonobstructive plaques A representation and interpretation of the area under a receiver operating characteristic ( ROC ) curve obtained by the " rating " method , or by mathematical predictions based on patient characteristics , is presented . It is shown that in such a setting the area represents the probability that a r and omly chosen diseased subject is ( correctly ) rated or ranked with greater suspicion than a r and omly chosen non-diseased subject . Moreover , this probability of a correct ranking is the same quantity that is estimated by the already well-studied nonparametric Wilcoxon statistic . These two relationships are exploited to ( a ) provide rapid closed-form expressions for the approximate magnitude of the sampling variability , i.e. , st and ard error that one uses to accompany the area under a smoothed ROC curve , ( b ) guide in determining the size of the sample required to provide a sufficiently reliable estimate of this area , and ( c ) determine how large sample sizes should be to ensure that one can statistically detect differences in the accuracy of diagnostic techniques BACKGROUND AND PURPOSE Carotid endarterectomy has been shown to be beneficial in patients with high- grade carotid stenosis and ipsilateral transient ischemic attack or stroke . This benefit will be realized only if the operation is performed safely . We sought to determine the extent to which clinical ly significant adverse events occurring after carotid endarterectomy can be predicted from clinical data available before surgery . METHODS Eleven hundred sixty patients were r and omly selected from all patients who underwent carotid endarterectomy and were discharged during the calendar years 1988 , 1989 , and 1990 in 12 academic medical centers in 10 states . Clinical data abstract ed from hospital charts were analyzed retrospectively . A model was developed and vali date d to predict the occurrence of stroke , myocardial infa rct ion , or death during the postoperative period of hospitalization . RESULTS Eight patients ( 6.9 % ) suffered at least one adverse event . Rates for individual complications were as follows : death , 1.4 % ; nonfatal stroke , 3.4 % ; nonfatal myocardial infa rct ion , 2.1 % ; and nonfatal stroke or death , 4.8 % . Significant predictors of adverse events were age 75 years or older , symptom status ( ipsilateral symptoms versus asymptomatic or nonipsilateral symptoms ) , severe hypertension ( preoperative diastolic blood pressure of greater than 110 mm Hg ) , carotid endarterectomy performed in preparation for coronary artery bypass surgery , history of angina , evidence of internal carotid artery thrombus , and internal carotid artery stenosis near the carotid siphon . The presence of two or more of these risk factors was associated with a nearly twofold increase in risk of an adverse event ( relative risk , 1.7 ; 95 % confidence interval , 1.0 to 3.0 ) . CONCLUSIONS Clinical data can be used to stratify patients undergoing carotid endarterectomy  according to risk of postoperative in-hospital stroke , myocardial infa rct ion , or death Findings of two-dimensional time-of-flight magnetic resonance ( MR ) angiography projection angiograms were prospect ively compared with those of color Doppler sonography by using angiography as a st and ard in 23 consecutive patients ( 42 carotid bifurcations ) to evaluate their utility in determining the presence of carotid artery stenosis .  MR angiography helped detect 50 % or greater lumen diameter stenosis ( sensitivity , 0.96 ; specificity , 0.64 ) . Color Doppler sonography with 1.25 m/sec peak systolic velocity as a threshold had a sensitivity of 0.96 and a specificity of 0.71 . Statistical analysis showed a correlation between percentage of lumen diameter narrowing and the length of the zone of signal intensity loss with MR angiography ( r = .69 ; P less than .0001 ) . A stronger relationship was obtained between angiographic narrowing and peak systolic velocity derived from color Doppler sonography ( r = .80 ; P less than .0001 ) . Two-dimensional time-of-flight MR angiography displayed as projection angiograms and combined with carotid artery and combined with carotid artery sonography is a useful approach for helping detect and potentially grade the severity of stenoses of the carotid artery OBJECTIVE To determine whether carotid endarterectomy provides protection against subsequent cerebral ischemia in men with ischemic symptoms in the distribution of significant ( greater than 50 % ) ipsilateral internal carotid artery stenosis . DESIGN Prospect i ve , r and omized , multicenter trial . SETTING Sixteen university-affiliated Veterans Affairs medical centers . PATIENTS Men who presented within 120 days of onset of symptoms that were consistent with transient ischemic attacks , transient monocular blindness , or recent small completed strokes between July 1988 and February 1991 . Among 5000 patients screened , 189 individuals were r and omized with angiographic internal carotid artery stenosis greater than 50 % ipsilateral to the presenting symptoms . Forty-eight eligible patients who refused entry were followed up outside of the trial . OUTCOME MEASURES Cerebral infa rct ion or crescendo transient ischemic attacks in the vascular distribution of the original symptoms or death within 30 days of r and omization . INTERVENTION Carotid endarterectomy plus the best medical care ( n = 91 ) vs the best medical care alone ( n = 98 ) . RESULTS At a mean follow-up of 11.9 months , there was a significant reduction in stroke or crescendo transient ischemic attacks in patients who received carotid endarterectomy ( 7.7 % ) compared with nonsurgical patients ( 19.4 % ) , or an absolute risk reduction of 11.7 % ( P = .011 ) . The benefit of surgery was more profound in patients with internal carotid artery stenosis greater than 70 % ( absolute risk reduction , 17.7 % ; P = .004 ) . The benefit of surgery was apparent within 2 months after r and omization , and only one stroke was noted in the surgical group beyond the 30-day perioperative period . CONCLUSIONS 
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Real-time B-mode ultrasonographic imaging allows for direct visualization of the carotid artery and calculation of the widths of obstructed and unobstructed arteries and of the lesions themselves ; thus , investigators can determine the percentage of stenosis [ 34 , 35 ] .
The results of these studies can be translated into estimates of carotid patency .
We also calculated a new measure for diagnostic tests termed test effectiveness , which attempts to avert the forced tradeoffs between