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Sensitivity and specificity Predictive values positive and negative

Interpretation of results. Sensitivity and specificity Predictive values positive and negative. 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.

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Sensitivity and specificity Predictive values positive and negative

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  1. Interpretation of results Sensitivity and specificityPredictive values positive and negative Sep 2007

  2. 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

  3. Sensitivity The sensitivity of a test in the ability of the test to identify correctly affected individuals Proportion of persons testing positive among affected individuals Sensitivity = True positives / Affected personsEstimate the 95% confidence interval

  4. 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

  5. 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

  6. Sensitivity of a rapid test for leishmaniasis Sensitivity = 148 / (150) = 98%95% confidence interval: 94%-99%

  7. 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

  8. Specificity 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 Specificity = True negatives / Non-affected personsEstimate the 95% confidence interval

  9. 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

  10. 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

  11. Specificity of a rapid test for leishmaniasis Specificity = 188 / 200 = 94% 95% confidence interval: 90%-96%

  12. 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

  13. Identifying the cut-off to use with a test on the basis of panel analysis: Ideal case 25 Cut-off 20 15 Sick Number of tests Well 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Possible values of the test

  14. Identifying the cut-off to use with a test on the basis of panel analysis: Real case False negatives False positives True negatives True positives Cut-off 25 20 15 Sick Number of tests Well 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Possible values of the test

  15. To whom sensitivity and specificity matters most? • Look at denominators! • Panels of affected individuals • Panels of negative individuals • To laboratory specialists!

  16. 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

  17. 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

  18. Predictive value of a positive test 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 = True positives / Persons testing positiveEstimate the 95% confidence interval

  19. Predictive value of a positive test PVP = A / (A+B) This is only valid for the sample of specimens tested

  20. 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

  21. Predictive value positive of a test according to prevalence and specificity Specificity

  22. Predictive value of a negative test 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 = True negatives / Persons testing negativeEstimate the 95% confidence interval

  23. Predictive value of a negative test PVN = D / (C+D)This is only valid for the sample of specimens tested

  24. 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

  25. Predictive value negative of a test according to prevalence and sensitivity Sensitivity

  26. Relation between predictive values and (1) sensitivity and (2) specificity

  27. Positive (PPV) and negative (NPV) predictive values of a test according to the prevalence (95% sensitivity and specificity) PVN PVP 0 50 75 100 25

  28. To whom predictive values matters most? • Look at denominators! • Persons testing positive • Persons testing negative • To clinicians and epidemiologists!

  29. 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

  30. Interpretation of results 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 Programme Thailand Ministry of Health Institut Pasteur

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