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64Ana-Maria ŠimundićThe shape of a ROC curve and the area under thecurve (AUC) helps us estimate how high the discriminativepower of a test is. The closer the curve is locatedto upper-left hand corner and the larger the area underthe curve, the better the test is at discriminating betweendiseased and non-diseased. The area under thecurve can have any value between 0 and 1 and it is agood indicator of the reliability of the test. A perfectdiagnostic test has the AUC 1.0. whereas a nondiscriminatingtest has an area 0.5. Generally we can saythat the relation between AUC and diagnostic accuracyapplies as described in Table II.Table II. Relationship between the area under the ROC curveand diagnostic accuracyareadiagnostic accuracy0.9 – 1.0 excellent0.8 - 0.9 very good0.7 - 0.8 good0.6 - 0.7 sufficient0.5 - 0.6 bad< 0.5 test not usefulAUC is a global measure of diagnostic accuracy. Ittells us nothing about individual parameters, such assensitivity and specificity. Out of two tests with identicalor similar AUC, one can have significantly highersensitivity, whereas the other significantly higher specificity.Furthermore, data on AUC state nothing aboutpredicative values and about the contribution of the testto ruling-in and ruling-out a diagnosis. Global measuresare there for general assessment and for comparisonof two or more diagnostic tests. By the comparisonof areas under the two ROC curves we can estimatewhich one of two tests is more suitable for distinguishinghealth from disease or any other two conditions ofinterest. It should be pointed that this comparisonshould not be based on visual nor intuitive evaluation[4]. For this purpose we use statistic tests which evaluatethe statistical significance of estimated differencebetween two AUC, with previously defined level ofstatistical significance (P).DIAGNOSTIC ODDS RATIO (DOR)Diagnostic odds ratio is also one global measure fordiagnostic accuracy, used for general estimation ofdiscriminative power of diagnostic procedures and alsofor the comparison of diagnostic accuracies betweentwo or more diagnostic tests. DOR of a test is the ratioof the odds of positivity in subjects with disease relativeto the odds in subjects without disease [4]. It iscalculated according to the formula: DOR =(TP/FN)/(FP/TN).DOR depends significantly on the sensitivity andspecificity of a test. A test with high specificity andsensitivity with low rate of false positives and falsenegatives has high DOR. With the same sensitivity ofthe test, DOR increases with the increase of the testspecificity. For example, a test with sensitivity > 90%and specificity of 99% has a DOR greater than 500.DOR does not depend on disease prevalence; howeverlike sensitivity and specificity it depends on criteriaused to define disease and its spectrum of pathologicalconditions of the examined group (diseaseseverity, phase, stage, comorbidity etc.).DIAGNOSTIC EFFECTIVENESS (ACCURACY)Another global measure of diagnostic accuracy isso called diagnostic accuracy (effectiveness), expressedas a proportion of correctly classified subjects(TP+TN) among all subjects (TP+TN+FP+FN). Diagnosticaccuracy is affected by the disease prevalence.With the same sensitivity and specificity, diagnosticaccuracy of a particular test increases as the diseaseprevalence decreases. This data, however, should behandled with care. In fact, this does not mean that thetest is better if we apply it in a population with lowdisease prevalence. It only means that in absolutenumber the test gives more correctly classified subjects.This percentage of correctly classified subjectsshould always be weighed considering other measuresof diagnostic accuracy, especially predictive values.Only then a complete assessment of the test contributionand validity could be made.YOUDEN'S INDEXYouden's index is one of the oldest measures fordiagnostic accuracy [6]. It is also a global measure of atest performance, used for the evaluation of overalldiscriminative power of a diagnostic procedure and forcomparison of this test with other tests. Youden's indexis calculated by deducting 1 from the sum of test’ssensitivity and specificity expressed not as percentagebut as a part of a whole number: (sensitivity + specificity)– 1.For a test with poor diagnostic accuracy, Youden'sindex equals 0, and in a perfect test Youden's indexequals 1. Youden's index is not sensitive for differencesin the sensitivity and specificity of the test,
Measures of diagnostic accuracy: basic definitions 65which is its main disadvantage. Namely, a test withsensitivity 0,9 and specificity 0,4 has the sameYouden's index (0,3) as a test with sensitivity 0,6 andspecificity 0,7. It is absolutely clear that those tests arenot of comparable diagnostic accuracy. If one is toassess the discriminative power of a test solely basedon Youden's index it could be mistakenly concludedthat these two tests are equally effective.Youden’s index is not affected by the diseaseprevalence, but it is affected by the spectrum of thedisease, as are also sensitivity specificity, likelihoodratios and DOR.DESIGN OF DIAGNOSTIC ACCURACY STUDIESMeasures of diagnostic accuracy are extremely sensitiveto the design of the study. Studies suffering fromsome major methodological shortcomings can severelyover- or under-estimate the indicators of test performanceas well as they can severely limit the possibleapplicability of the results of the study. The effect ofthe design of the study on the bias and variation in theestimates of diagnostic accuracy can be quantified [7].STARD initiative published in 2003 was a very importantstep toward the improvement of the quality ofreporting of studies of diagnostic accuracy [8, 9]. Accordingto some authors, the quality of reporting ofdiagnostic accuracy studies did not significantly improveafter the publication of the STARD statement[10, 11], whereas some others hold that the overallquality of reporting has at least slightly improved [12],but there is still some room for potential improvement[13, 14].Editors of scientific journals are encouraged to includethe STARD statement into the Journal Instructionsto authors and to oblige their authors to use thechecklist when reporting their studies on diagnosticaccuracy. This way the quality of reporting could besignificantly improved, providing the best possibleevidence for health care providers, clinicians and laboratoryprofessionals; to the best for the patient care.REFERENCES1. Irwig L, Bossuyt P, Glasziou P, Gatsonis C, Lijmer J.Designing studies to ensure that estimates of test accuracyare transferable. BMJ. 2002;324(7338):669-71.2. Raslich MA. Markert RJ, Stutes SA. Selecting and interpretingdiagnostic tests. Biochemia Medica2007;17(2):139-270.3. Deeks JJ, Altman DG. Diagnostic tests 4: likelihoodratios. BMJ 2004;17;329(7458):168-9.4. Obuchowski NA, Lieber ML, Wians FH Jr. ROC curvesin clinical chemistry: uses, misuses, and possible solutions.Clin Chem. 2004;50(7):1118-25.5. Glas AS, Lijmer JG, Prins MH, Bonsel GJ, Bossuyt PM.The diagnostic odds ratio: a single indicator of test performance.J. Clin Epidemiol. 2003;56(11):1129-35.6. Youden WJ. Index for rating diagnostic tests. Cancer.1950;3:32-35.7. Rutjes AW, Reitsma JB, Di Nisio M, Smidt N, van RijnJC, Bossuyt PM. Evidence of bias and variation in diagnosticaccuracy studies. CMAJ. 2006;14;174(4):469-76.8. Bossuyt PM, Reitsma JB, Bruns DE, Gatsonis CA,Glasziou PP, Irwig LM, et al. Towards complete and accuratereporting of studies of diagnostic accuracy: theSTARD initiative. Clin Chem 2003;49:1-6.9. Bossuyt PM, Reitsma JB, Bruns DE, Gatsonis CA,Glasziou PP, Irwig LM, et al. The STARD statement forreporting studies of diagnostic accuracy: explanation andelaboration. Clin Chem 2003;49:7-18.10. Wilczynski NL. Quality of reporting of diagnostic accuracystudies: no change since STARD statement publication--before-and-afterstudy. Radiology. 2008; 248(3):817-23.11. Bossuyt PM. STARD statement: still room for improvementin the reporting of diagnostic accuracy studies. Radiology2008;248(3):713-4.12. Smidt N, Rutjes AWS, Van der Windt DAWM, OsteloRWJG, Bossuyt PM, Reitsma JB, et al. The quality of diagnosticaccuracy studies since the STARD statement:has it improved?. Neurology 2006;67:792-797.13. Bossuyt PM. Clinical evaluation of medical tests: still along road to go. Biochemia Medica 2006;16(2)89–228.14. Bossuyt PM. The quality of reporting in diagnostic testresearch: getting better, still not optimal. Clin Chem.2004;50(3):465-6.Corresponding author:e-mail: am.simundic@gmail.comReceived: 9.12.2008Accepted for publication: 13.01.2009
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