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Cliquez et modifiez le titre,Cliquez pour modifier les styles du texte du masque,Deuxime niveau,Troisime niveau,Quatrime niveau,Cinquime niveau,Laboratory Training for Field Epidemiologists,Sensitivity and specificityPredictive values positive and negative,Interpretation of results,Sep 2007,Outline,Intrinsic characteristics of a test,Sensitivity,Specificity,Performance of a test in a population,Predictive value of a positive test,Predictive value of a negative test,Sensitivity,Sensitivity = True positives / Affected personsEstimate the 95% confidence interval,The sensitivity of a test in the ability of the test to identify correctly affected individuals,Proportion of persons testing positive among affected individuals,Estimating the sensitivity of a test,Identify affected individuals with a gold standard,Obtain a wide panel of samples that are representative of the population of affected individuals,Recent and old cases,Severe and mild cases,Various ages and sexes,Test the affected individuals,Estimate the proportion of affected individuals that are positive with the test,Estimating the sensitivity of a rapid test for leishmaniasis,Identify persons with leishmaniasis with a gold standard,Parasitologically proven infection,Obtain a wide panel of samples that are representative of the population of individuals with leishmaniasis,Recent and old cases,Severe and asymptomatic cases,Various ages and sexes,Test the persons with leishmaniasis,Estimate the proportion of persons with leishmaniasis that are positive with the rapid test,Sensitivity of a rapid test for leishmaniasis,Sensitivity = 148 / (150) = 98%95% confidence interval: 94%-99%,What factors influence the sensitivity of a test?,Characteristics of the affected persons?,YES: Antigenic characteristics of the pathogen in the area(e.g., if the test was not prepared with antigens reflecting the population of pathogens in the area, it will not pick up infected persons in the area),Characteristics of the non-affected persons?,NO: The sensitivity is estimated on a population of affected persons,Prevalence of the disease?,NO: The sensitivity is estimated on a population of affected persons,Sensitivity is an INTRINSIC characteristic of the test,Specificity,Specificity = True negatives / Non-affected personsEstimate the 95% confidence interval,The specificity of a test in the ability of the test to identify correctly non-affected individuals,Proportion of person testing negative among non affected individuals,Estimating the specificity of a test,Identify non affected individuals,Negative with a gold standard,Unlikely to be infected,Obtain a wide panel of samples that are representative of the population of non-affected individuals,Diverse unaffected population: Difficult to find. Ideally, those individuals that would need to be tested but not infected,Test the non-affected individuals,Estimate the proportion of non-affected individuals that are negative with the test,Estimating the specificity of a rapid test for leishmaniasis,Identify persons without leishmaniasis,Persons without sign and symptoms of the infection,Persons at low risk of infection, negative with gold standard,Obtain a wide panel of samples that are representative of the population of individuals without leishmaniasis,Persons from neighbouring villages having similar characteristics but with no transmission and no infections,Test the persons without leishmaniasis,Estimate the proportion of persons without leishmaniasis that are negative with the rapid test,Specificity of a rapid test for leishmaniasis,Specificity = 188 / 200 = 94% 95% confidence interval: 90%-96%,What factors influence the specificity of a test?,Characteristics of the affected persons?,NO: The specificity is estimated on a population of non affected,Characteristics of the non-affected persons?,YES: The diversity of antibodies to various other antigens in the population may affect cross reactivity (e.g., If malaria is endemic, polyclonal hyper gammaglobulinemia may increase the proportion of false positives),Prevalence of the disease?,NO: The specificity is estimated on a population of non affected,Specificity is an INTRINSIC characteristic of the test,Identifying the cut-off to use with a test on the basis of panel analysis: Ideal case,Cut-off,0,5,10,15,20,25,1,2,3,4,5,6,7,8,9,10,11,12,13,14,Possible values of the test,Number of tests,Sick,Well,Identifying the cut-off to use with a test on the basis of panel analysis: Real case,Cut-off,0,5,10,15,20,25,1,2,3,4,5,6,7,8,9,10,11,12,13,14,Possible values of the test,Number of tests,Sick,Well,True negatives,False negatives,True positives,False positives,To whom sensitivity and specificity matters most?,Look at denominators!,Panels of affected individuals,Panels of negative individuals,To laboratory specialists!,Outline,Intrinsic characteristics of a test,Sensitivity,Specificity,Performance of a test in a population,Predictive value of a positive test,Predictive value of a negative test,How is the test doing in a real population?,The test is now used in a real population,This population is made of,Affected individuals,Non-affected individuals,The proportion of affected individuals is the prevalence,Predictive value of a positive test,Predictive value of a positive test = True positives / Persons testing positiveEstimate the 95% confidence interval,The predictive value of a positive test is the probability that an individual testing positive is truly affected,Proportion of affected persons among those testing positive,Predictive value of a positive test,PVP = A / (A+B),This is only valid for the sample of specimens tested,What factors influence the predictive value positive of a test?,Sensitivity?,YES: To some extend.,Specificity?,YES: The more the test is specific, the more it will be negative for non affected persons. Thus, when the test is positive, it is probably truly positive (All non affected were correctly identified as testing negative).,Prevalence of the disease?,YES: Low prevalence: The test will pick up more false positives,YES: High prevalence: The test will pick up more true positives,Predictive value positive of a test according to prevalence and specificity,Specificity,Predictive value of a negative test,Predictive value of a negative test = True negatives / Persons testing negativeEstimate the 95% confidence interval,The predictive value of a negative test is the probability that an individual testing negative is truly non-affected,Proportion of non-affected persons among those testing negative,Predictive value of a negative test,PVN = D / (C+D),This is only valid for the sample of specimens tested,What factors influence the predictive value negative of a test?,Sensitivity?,YES: The more the test is sensitive, the more it captures affected persons. Thus, when the test is negative, it is probably truly negative (all affected were captured among the positive).,Specificity?,YES: But to a lesser extend.,Prevalence of the disease?,YES: Low prevalence: The test will pick up more true negatives,YES: High prevalence: The test will pick up more false negatives,Predictive value negative of a test according to prevalence and sensitivity,Sensitivity,Relation between predictive values and (1) sensitivity and (2) specificity,Positive (PPV) and negative (NPV) predictive values of a test according to the prevalence (95% sensitivity and specificity),PVP,PVN,0,50,75,100,25,To whom predictive values matters most?,Look at denominators!,Persons testing positive,Persons testing negative,To clinicians and epidemiologists!,Summary,Sensitivity and specificity matter to laboratory specialists,Studied on panels of positives and negatives,Look into the intrinsic characteristics of the test:,Capacity to pick affected,Capacity to pick non affected,Predictive values matter to clinicians and epidemiologists,Studied on homogeneous populations,Look into the performance of the test in real life:,What to make of a positive test,What to make of a negative test,Developed by:,The Department of Epidemic and Pandemic Alert and Response of the World Health Organization with the assistance of:,European Program for Field Epidemiology Training Canadian Field Epidemiology ProgrammeThailand Ministry of HealthInstitut Pasteur,Interpretation of results,
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