Acceptable False Positive Rate Research Articles

Computer-aided detection in CT colonography: initial clinical experience using a prototype system

Computer-aided detection (CAD) algorithms help to detect colonic polyps at CT colonography (CTC). The purpose of this study was to evaluate the accuracy of CAD versus an expert reader in CTC. One hundred forty individuals (67 men, 73 women; mean age, 59 years) underwent screening 64-MDCT colonography after full cathartic bowel cleansing without fecal tagging. One expert reader interpreted supine and prone scans using a 3D workstation with integrated CAD used as "second reader." The system's sensitivity for the detection of polyps, the number of false-positive findings, and its running time were evaluated. Polyps were classified as small (< or =5 mm), medium (6-9 mm), and large (> or =10 mm). A total of 118 polyps (small, 85; medium, 19; large, 14) were found in 56 patients. CAD detected 72 polyps (61%) with an average of 2.2 false-positives. Sensitivity was 51% (43/85) for small, 90% (17/19) for medium, and 86% (12/14) for large polyps. For all polyps, per-patient sensitivity was 89% (50/56) for the radiologist and 73% (41/56) for CAD. For large and medium polyps, per-patient sensitivity was 100% for the radiologist, and 96% for CAD. In conclusion, CAD shows high sensitivity in the detection of clinically significant polyps with acceptable false-positive rates.

Screening in early pregnancy for pre‐eclampsia using down syndrome quadruple test markers

To estimate the screening performance of early second-trimester prenatal serum markers for Down syndrome, in screening for the development of pre-eclampsia, and analyse the uncertainty over its screening performance. A nested case-control study was carried out on 96 women with pre-eclampsia and 5 controls for each case from among the women attending three hospitals in London for their prenatal care. Record linkage between computerized obstetric and screening databases identified women with pre-eclampsia and unaffected control women. The stored frozen serum samples collected from these pregnancies between 15 and 22 weeks' gestation were retrieved and assayed for alpha-fetoprotein (AFP), unconjugated estriol (uE(3)), total human chorionic gonadotrophin (hCG), free beta-hCG and inhibin-A. Pre-eclampsia was identified from the computerized obstetric records and confirmed by examination of the medical notes. In the pregnancies that went on to develop pre-eclampsia, early second trimester inhibin-A and hCG values were significantly raised and uE(3) values were significantly lowered, while AFP values were not significantly altered. Using the Quadruple test markers (AFP, uE(3), hCG (total or free beta) and inhibin-A), an estimated 34% of pregnancies that developed pre-eclampsia were detected at a 5% false-positive rate. If all the women who had pre-eclampsia in a previous pregnancy (assuming a pre-eclampsia prevalence of 4%) are considered as screen positive and the serum test is applied to the remaining women, then around 42% of pre-eclamptic pregnancies would be detected at a 6.5% false-positive rate. Pre-eclampsia screening performance using the Quadruple test markers was materially better than that using the Triple test markers. Adding screening for pre-eclampsia to an existing Down syndrome screening programme using the Quadruple test markers is simple and worthwhile. It will detect over 40% of pregnancies with pre-eclampsia at an acceptable false-positive rate (about 6%) and with minimal additional costs.

Computer-Assisted Reader Software Versus Expert Reviewers for Polyp Detection on CT Colonography

The purpose of our study was to assess the sensitivity of computer-assisted reader (CAR) software for polyp detection compared with the performance of expert reviewers. A library of colonoscopically validated CT colonography cases were collated and separated into training and test sets according to the time of accrual. Training data sets were annotated in consensus by three expert radiologists who were aware of the colonoscopy report. A subset of 45 training cases containing 100 polyps underwent batch analysis using ColonCAR version 1.2 software to determine the optimum polyp enhancement filter settings for polyp detection. Twenty-five consecutive positive test data sets were subsequently interpreted individually by each expert, who was unaware of the endoscopy report, and before generation of the annotated reference via an unblinded consensus interpretation. ColonCAR version 1.2 software was applied to the test cases, at optimized polyp enhancement filter settings, to determine diagnostic performance. False-positive findings were classified according to importance. The 25 test cases contained 32 nondiminutive polyps ranging from 6 to 35 mm in diameter. The ColonCAR version 1.2 software identified 26 (81%) of 32 polyps compared with an average sensitivity of 70% for the expert reviewers. Eleven (92%) of 12 polyps > or = 10 mm were detected by ColonCAR version 1.2. All polyps missed by experts 1 (n = 4) and 2 (n = 3) and 12 (86%) of 14 polyps missed by expert 3 were detected by ColonCAR version 1.2. The median number of false-positive highlights per case was 13, of which 91% were easily dismissed. ColonCAR version 1.2 is sensitive for polyp detection, with a clinically acceptable false-positive rate. ColonCAR version 1.2 has a synergistic effect to the reviewer alone, and its standalone performance may exceed even that of experts.

Evaluation of an Improved Branch-Site Likelihood Method for Detecting Positive Selection at the Molecular Level

Detecting positive Darwinian selection at the DNA sequence level has been a subject of considerable interest. However, positive selection is difficult to detect because it often operates episodically on a few amino acid sites, and the signal may be masked by negative selection. Several methods have been developed to test positive selection that acts on given branches (branch methods) or on a subset of sites (site methods). Recently, Yang, Z., and R. Nielsen (2002. Codon-substitution models for detecting molecular adaptation at individual sites along specific lineages. Mol. Biol. Evol. 19:908-917) developed likelihood ratio tests (LRTs) based on branch-site models to detect positive selection that affects a small number of sites along prespecified lineages. However, computer simulations suggested that the tests were sensitive to the model assumptions and were unable to distinguish between relaxation of selective constraint and positive selection (Zhang, J. 2004. Frequent false detection of positive selection by the likelihood method with branch-site models. Mol. Biol. Evol. 21:1332-1339). Here, we describe a modified branch-site model and use it to construct two LRTs, called branch-site tests 1 and 2. We applied the new tests to reanalyze several real data sets and used computer simulation to examine the performance of the two tests by examining their false-positive rate, power, and robustness. We found that test 1 was unable to distinguish relaxed constraint from positive selection affecting the lineages of interest, while test 2 had acceptable false-positive rates and appeared robust against violations of model assumptions. As test 2 is a direct test of positive selection on the lineages of interest, it is referred to as the branch-site test of positive selection and is recommended for use in real data analysis. The test appeared conservative overall, but exhibited better power in detecting positive selection than the branch-based test. Bayes empirical Bayes identification of amino acid sites under positive selection along the foreground branches was found to be reliable, but lacked power.

Prospective Evaluation of the Concordance between Radiation Therapists and Radiation Oncologists on Daily Verification Films

A study was implemented to determine if there was concordance between the radiation therapists and radiation oncologists in determining acceptable verification films. The protocol at the time this study was implemented stated that verification films were taken and evaluated by the radiation oncologist within three treatment days. It was hypothesized that a positive outcome could alter the protocol. Information on the acceptability of verification films taken from January 1 st through March 31 st, 2000 was documented. Radiation therapists recorded their assessment of the film, that was then compared to the radiation oncologists' assessment. There was agreement between the radiation therapist and the radiation oncologist in 822 of the 856 verification films taken, resulting in a 96.1% concordance value. Based on the high agreement, low false negative and acceptable false positive rates, it is reasonable that radiation therapists formally adopt the process of approving verification films by adding it to their scope of practice.

Screening for Chinese Children with Dyslexia in Hong Kong: The use of the Teachers' Behaviour Checklist

Primary school teachers rated the frequency of occurrence of 65 reading‐related behavioural characteristics in a sample of 251 Grade 1 to Grade 6 Chinese school children in Hong Kong. These behavioural characteristics were in the areas of general performance, reading, dictation, writing, mathematics, language, memory, concentration, sequential ability, motor co‐ordination, spatial orientation, and social/emotional adjustment. Of these 12 areas, 10 yielded scale scores that could distinguish children with dyslexia from those without dyslexia, identified on the basis of their performance in five domains of literacy and cognitive skills. Using a summary score derived from the 10 relevant scales, an optimal cut‐off score was suggested to arrive at a balance between high sensitivity and an acceptable rate of false positives in screening for children with dyslexia. The need for cross‐replication in screening children with dyslexia using the behaviour checklist with different samples of school students is emphasised.

Combined serum and nuchal translucency screening in the first trimester achieves 85% to 90% detection rate for Down and Edward syndromes

Question How accurate is combined screening, based on maternal age, using serum and nuchal translucency measurements in first trimester screening for Down and Edward syndromes? Study design Cohort study. Main results Combined screening identified 85.2% (95% CI 73.8 to 93.0%) of babies with Down syndrome with a false positive rate of 9.4% (95% CI 8.8 to 10.1%). In women over 35 years, 89.8% cases were identified and the false positive rate increased to 15.2%. Combined screening identified 90.9% (95% CI 58.7 to 99.8%) of babies with Edward syndrome (trisomy 18) with a false positive rate of 2% (95% CI 1.7 to 2.3). All cases of trisomy 18 were identified among women over 35 years ( n=8). Combined screening was significantly better than use of serum markers alone ( p=0.002) but no more accurate than use of nuchal translucency alone ( p=0.28). Authors’ conclusions Combined screening for Edward and Down syndromes during the first trimester achieved detection rates of 90% and 85%, respectively, with acceptable false positive rates.

Computerized detection of colonic polyps at CT colonography on the basis of volumetric features: pilot study.

To develop a computer-aided diagnosis (CAD) scheme for automated detection of colonic polyps on the basis of volumetric features and to assess its accuracy on the basis of colonoscopy, the standard. Computed tomographic (CT) colonography was performed in patients with use of standard bowel cleansing, air insufflation, and helical scanning in supine and prone positions. The colon was extracted from volumetric data sets generated from transverse CT sections. Volumetric features characterizing polyps were computed at each point in the extracted colon. Polyps were detected by means of hysteresis thresholding and fuzzy clustering followed by a rule-based test on the basis of feature values. Locations of the detected polyps were compared with those detected at conventional colonoscopy. Forty-one cases were analyzed: nine cases with polyps and 32 without polyps. Each case with polyps had one polyp of clinically important size (six were 5-9 mm; three, 10 mm). Thus, there were 82 volumetric data sets, 18 included polyps. Eighty-nine percent (16 of 18) of the polyps were detected. Each of the two false-negative findings was detected in the other position; thus, 100% of polyp cases were detected, with 2.5 false-positive findings per patient. The false-positive findings were similar to those due to common perceptual errors. Most of the false-positive findings were easily distinguishable from true polyps by experienced radiologists. The CAD scheme has the potential to depict polyps with high sensitivity and an acceptable false-positive rate.

The feasibility of hospital-based universal newborn hearing screening in the United Kingdom.

Current hearing screening programmes in the United Kingdom are performing unacceptably poorly. Davies et al. (1997) suggested that universal newborn hearing screening (UNHS) would be more effective and cheaper to run. However, there is concern that hospital-based UNHS would not be feasible because of early postnatal discharge, and thus babies not staying in hospital long enough to be screened. Two studies were designed to determine the viability of hospital-based UNHS in a district general hospital in the United Kingdom. Study 1 retrospectively determined the discharge age and time of discharge of all 3021 well babies born at St Helier hospital, Carshalton, and the number of babies born at home in the area, from 19 October 1997-18 October 1998. Most well babies were found to pass through hospital at a convenient time for predischarge hearing screening, and the optimal protocol was screening from 9 am-2 pm, 7 days a week. The predicted maximal screening coverage was 92.68%. Study 2 tested the calculated optimal protocol over 1 week. It was found that UNHS with otoacoustic emissions on the maternity ward from 9 am-2 pm, 7 days a week, achieved a coverage of 89.06%, with an acceptable false positive rate of 6.2%. It is likely that a similar protocol with slight modifications could be implemented successfully in other hospitals in the United Kingdom.

California Verbal Learning Test Indicators of Suboptimal Performance in a Sample of Head-Injury Litigants

Cutoff scores suggested by Millis, Putnam, Adams, and Ricker (1995) for detecting suboptimal performance on indices from the California Verbal Learning Test (CVLT) were evaluated using data from 193 compensation-seeking participants. All participants claimed to have suffered a blow to the head in an accident causing subsequent deterioration in cognitive function. The participants were divided into those with negligible or possible mild brain injuries and those with clear evidence of moderate to severe brain injuries. In addition to the CVLT, all participants were administered the Computerized Assessment of Response Bias (CARB), a two-alternative forced choice test of recognition memory that is used to detect feigned cognitive impairment. For all CVLT indices, the distributions of outcome (valid vs. suboptimal performance) was unrelated to age and brain injury severity, and only weakly associated with education. However, a significantly higher proportion of males than females obtained scores in the suboptimal performance range. The CVLT indices were not fully redundant with each other with respect to binary participant classifications; substantial disagreement between pairwise classifications was found among those participants who obtained at least one score in the suboptimal performance range. CVLT index classifications were also found to be non-redundant with classifications based on CARB scores. The CVLT may thus add useful data over and above that obtained from symptom validity testing. However, the data suggest that the use of the strategy where any one or more below-cutoff CVLT scores are considered a positive indicator of suboptimal performance may be associated with a higher than acceptable false-positive error rate.

Middle cerebral artery Doppler velocimetric deceleration angle as a predictor of fetal anemia in Rh-alloimmunized fetuses without hydrops

Objective: The purpose of this study was to evaluate the screening performance of a new middle cerebral artery Doppler velocimetric index for the prediction of fetal anemia. Study Design: Doppler velocimetry of the middle cerebral artery was performed before cordocentesis in 24 Rh-alloimmunized fetuses without hydrops on 52 occasions. The angle between the line describing the average slope during the diastolic phase of the cardiac cycle and the vertical, the middle cerebral artery standardized deceleration angle, was measured. The deceleration angle values were expressed in multiples of the median for gestational age. The screening performances of deceleration angle for the prediction of anemia (difference between expected mean hemoglobin level and measured value ≥2 g/dL) and severe anemia (hemoglobin deficit ≥5 g/dL) were determined. Results: The mean (±SD) gestational age at cordocentesis was 28.6 ± 5.7 weeks’ gestation. The risk of fetal anemia increased with decreasing deceleration angle values. The sensitivity and false-positive rate for the detection of anemia in cases with no previous transfusions (one measurement per patient) were 72.0% and 13.3%, respectively; among those with one previous transfusion the values were 90.0% and 0.0%, respectively. For severe anemia the corresponding values were 100% and 0%, respectively, among those with no previous transfusions and 100.0% and 16.7%, respectively, among those with one previous transfusion. There was no risk of severe anemia when the angle was >0.9 multiples of the median. The risk of anemia was significantly reduced with an angle greater than the median for gestational age (deceleration angle >1.0 multiples of the median; relative risk, 0.09; 95% confidence interval, 0.02-0.37). The risk was significantly increased with an angle less than the median for gestational age (deceleration angle <1.0 multiples of the median; relative risk, 30.0; 95% confidence interval, 5.9-158.4). Conclusion: The risk of fetal hydrops is remote in the absence of severe anemia. With a new Doppler velocimetric index in the middle cerebral artery the risk of severe anemia was found to be low when the deceleration angle was >0.9 multiples of the median. Anemia can also be predicted with this index. The high sensitivities and acceptable false-positive rates support the potential clinical applicability of the method to reduce the reliance on cordocentesis in Rh alloimmunization. Our findings appear to validate the utility of the deceleration angle for the prediction of fetal anemia. (Am J Obstet Gynecol 2000;183:746-51.)

Economic evaluation of prenatal screening for Down syndrome in the U.S.A.

Maternal serum screening for Down syndrome involves biochemical tests such as alpha-fetoprotein (alpha FP), human chorionic gonadotrophin (hCG) and unconjugated oestriol (uE3), either alone or in combination, that have variable detection and false-positive rates. Choosing a screening protocol requires a trade-off between a desired detection rate and an acceptable false-positive rate. Selecting a screening protocol that maximizes the net benefit to society provides one approach. We have developed a general formula for calculating the per case net social benefit of a screening test and have applied it to United States data. The maximum net benefit associated with each of the various screening options currently available is estimated and the model is further applied to determine the conditions under which the addition of a new marker to an existing protocol can be justified. For each test, or combination of tests, optimal net benefits occur at different detection and false-positive rates. Net benefits are strongly and positively dependent on maternal age; high net benefits are associated with older patients and low, or even negative, net benefits with younger patients. Also, net benefits are affected by the term risk cut-off rate. For triple testing, the 1:351 Down syndrome term risk cut-off appears to provide a higher net benefit than that obtained with 1:250 or 1:300. The optimization of societal net benefit provides a powerful approach to evaluating screening strategies, but the policies used must also consider individuals' freedom in decision making at each step of the prenatal diagnosis pathway.

Age-adjusted ultrasound risk assessment for fetal Down's syndrome during the second trimester: description of the method and analysis of 142 cases.

To describe and test a method of individual risk assessment for fetal Down's syndrome based on maternal age and second-trimester ultrasound findings. A case-control study of 142 fetuses with Down's syndrome was compared with 930 control fetuses with normal karyotype. All patients underwent second-trimester ultrasound at a single institution with a standardized ultrasound protocol without knowledge of fetal karyotype. Age-adjusted ultrasound risk assessment (AAURA) for Down's syndrome was performed by multiplying the a priori risk, based on maternal age, with likelihood ratios resulting from the presence or absence of specific ultrasound findings for each patient. Individual ultrasound findings were assigned likelihood ratios (LR) as follows: structural abnormality (LR 25), nuchal thickening (LR 18.6), echogenic bowel (LR 5.5), shortened humerus (LR 2.5), shortened femur (LR 2.2), echogenic intracardiac focus (LR 2), and renal pyelectasis (LR 1.6). A normal ultrasound was assigned a LR of 0.4. One or more ultrasound markers were identified in 68.3% (97) of fetuses with Down's syndrome compared to 12.5% of fetuses with normal karyotype. Among fetuses with positive ultrasound, 31% of those with Down's syndrome and 80% of those with normal karyotype showed a single non-structural finding. Using AAURA and a threshold of 1: 200, 74% (105 of 142) of fetuses with Down's syndrome were identified, including 61.5% (24 of 39) from women aged less than 35 years, 67.2% (45 of 67) from women aged 35-39 years inclusively, and 100% (36 of 36) from women aged 40 years or older. AAURA of 930 fetuses with normal karyotype showed an overall false-positive rate of 14.7%, including 4% (21 of 519) from women aged less than 35 years, 12.5% (42 of 337) from women aged 35-39 years inclusively, and 100% from women aged 40 years or older. AAURA permits improved individual counselling regarding the risk of fetal Down's syndrome following a second-trimester sonogram. For low-risk women under age 35 years, ultrasound assessment can identify over half of the affected fetuses with Down's syndrome with an acceptable false-positive rate (4%). For women aged 35-39 years, a normal ultrasound can substantially reduce the risk of unnecessary amniocentesis (12.5% from 100%) but will also miss approximately one-third of affected fetuses. Biochemical screening of maternal serum is also suggested for this group. Based on their high a priori risk, women aged 40 years or more should consider genetic amniocentesis regardless of a normal ultrasound.

Advances in leukocyte differential and peripheral blood stem cell enumeration.

Automated leukocyte differential counting has resulted in a marked improvement of the precision of leukocyte subclass enumeration. With advancement of this technology, algorithms have been developed to identify samples that require manual microscopic review. Complex flagging algorithms permit acceptable false-positive rates with effective use of microscopic review to identify significant morphologic abnormalities. Current literature indicates that the leukocyte differential is a poor case-finding index in both outpatient and inpatient populations. Development of highly automated technology has led to test over-utilization because of ease of performance despite recognized limitations in identifying clinically significant abnormalities. Enumeration of hematopoietic stem cells in harvested peripheral blood or bone marrow is critical for determining whether transplanted cells will produce adequate engraftment. Hematopoietic precursors express the CD34 antigen, are present as rare events in unmobilized blood, and are found in increased numbers after mobilization with chemotherapy with or without G-CSF. Flow cytometric detection of CD34+ cells is more rapid and precise than cell culture techniques and results are available in time to determine if further cell harvesting procedures will be necessary. CD34+ cell concentrations correlate with granulocyte-macrophage colony-forming unit counts as well as with time to engraftment. Although intralaboratory precision in measuring these infrequent events may be good, comparisons between different laboratories using aliquots of the same sample have yielded more variable results. Problems in determining precise and reproducible CD34+ cell concentrations may be due to nonspecific antibody binding, insufficient signal intensity of true positive events, differences in monoclonal antibodies and their fluorochrome conjugates, or failure of the total absolute leukocyte count to accurately reflect the population used for CD34 analysis.

Statistical methods for assessing groundwater compliance and cleanup: a regulatory perspective : E. F. Weber, in: Groundwater quality: remediation and protection. Proc. conference, Prague, 1995, ed K. Kovar & J. Krasny, (IAHS; Publication, 225), 1995, pp 493–500

Regulatory corrective action requirements can be imposed at sites in USA where a statistically significant groundwater quality impact is document­ ed. The regulatory basis for determining an impact is a detection monitoring decision. The basis for determining an impact no longer exists is a corrective action monitoring decision. Detection monitoring typically involves periodic comparisons between background and down- gradient monitoring well data to verify compliance of a facility. Corrective action monitoring involves comparisons between downgradient monitoring well data and a cleanup standard to evaluate remedial action effectiveness. Recommended statistical procedures for making these regulatory decisions include hypothesis tests, statistical interval estimates, and trend analyses. Application of these methods within the context of regulatory groundwater monitoring programmes raises a number of issues regarding acceptable statistical power and false positive rates, and distributional assumptions.

Reliability of attachment loss measurements in a longitudinal clinical trial

In order to appropriately carry out a longitudinal assessment of periodontal attachment loss in individuals with untreated periodontitis, reliable criteria for determining "real" changes in attachment level (AL) are required. In the present study, 25 subjects were to be examined every 2 months for up to 2 years to determine changes in AL and to relate clinical and laboratory criteria to such changes. Trained examiners for the study underwent calibration trials to determine inter-examiner and intra-examiner reliability both before the study and at intervals during the study. It was found that AL measurements were in agreement within 2 mm more than 95% of the time. The calibration trials provided an estimate of the error in attachment loss measurements, since no "real" attachment loss had occurred. From estimates of measurement error, the probability of false positive changes were determined. It was found that acceptable false positive rates (less than 5%) could be achieved if 2 examiners each detected 3 mm change at a given site or if 2 examiners each detected 2 mm change at a site and verified that this change persisted at a subsequent examination. The results of the longitudinal trial were then compared to the probability estimates calculated from the calibration trials. It was found that probabilities of AL changes detected during the longitudinal trial for less stringent conditions than described above (e.g., single examiner, 2 examiners unconfirmed) were similar to to previously estimated false positive rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimating the false positive rate for alternative measures of integrity

Estimates of the false positive rate were computed for three methods of assessing integrity: 1) a paper and pencil integrity test; 2) a weighted application blank and; 3) the traditional selection interview. Based on available validation research, the integrity test yielded the most acceptable false positive rate. This type of analysis highlights the need to compare alternative assessment procedures when evaluating the utility of a selection instrument.

Validity of clinical symptoms in benign esophageal disease, assessed by questionnaire.

A questionnaire designed to test the value of questions identifying benign esophageal disease was sent to patients representing four well-defined entities: angina pectoris (n = 30), benign esophageal disease (n = 25), gastroduodenal ulcer or former gallstones (n = 27) and normal subjects (n = 38). By means of logistic regression, three key-questions were selected. A combination of an affirmative answer to "difficulty in swallowing solid food" and to "pain in the chest" or "heartburn at night" or both did not occur in the group of normal subjects, while occurring in 68% of the patients with esophageal diseases. The combination occurred in only 10% of the patients with angina pectoris and 15% of those suffering from gastroduodenal ulcer or gallstones. The predictive accuracy of the combination of the three questions is estimated for various hypothetic prevalences of esophageal disease. These questions appear to identify approximately two-thirds of individuals with esophageal disease with acceptable false-positive rates and can therefore be used in epidemiological studies.

Diagnostic criteria for computer-aided electrocardiographic 15-lead system. Evaluation using 12 leads and Frank orthogonal leads with vector display.

The criteria for the diagnosis of myocardial infarction and ischaemic heart disease by an automated 15-lead computer-aided electrocardiographic system were examined using electrocardiograms of 543 patients. Errors in the electrocardiographic diagnosis were evaluated for each lead system (Frank orthogonal 3-lead, 12-lead, and hybrid 15-lead) using clinical and catheterization data for definitive diagnosis before review of the electrocardiograms and their reports. The effects of combinations of these diagnoses and additional ventricular conduction defects were also studied. Myocardial infarction and left ventricular hypertrophy were more reliably diagnosed using 3-lead and 12-lead systems together than with either system alone. The most sensitive criteria for anterior infarction were a Q/R ratio in Z less than 0-1 and loss of the first 20 ms of anterior forces in the horizontal and sagittal planes of the vectorcardiogram. However, false positive results were frequent, particularly in association with left ventricular hypertrophy, non-specific intraventricular conduction defects, and left bundle branch system block. Our V lead criteria were more specific whether or not these associated conditions were present. No single criterion with an acceptable false positive rate could be found to be sensitive for inferior infarction in all situations. Our most sensitive criteria were those based on the limb leads, and the presence of superior forces for the first 30 ms in the frontal plane of the vectorcardiogram, but these were better in combination. Limb lead criteria were the most specific. False positive results for inferior infarction were more frequent in the presence of left ventricular hypertrophy or ventricular conduction defects other than left anterior hemiblock. ST and T wave abnormalities were more apparent in the 12 leads than in the orthogonal leads. Specificity and sensitivity of criteria were poor, and specificity was decreased and sensitivity was not significantly improved by combining 3-lead with 12-lead criteria. Because of frequent measurement errors of ST, T, and also Q waves by the computer programme, in practice we have achieved increased sensitivity in the diagnosis of ischaemia and infarction with the combination of 3-lead and 12-lead systems. It is concluded that errors of diagnosis by a computer-aided system can be reduced by using multiple leads and that both 12-lead and orthogonal 3-lead systems are necessary for optimal computer diagnosis of left ventricular hypertrophy, myocardial infarction, and ischaemia.