Reference Manual - IARC Screening Group

APPENDIX BSCREENING TEST QUALITIES AND THEIRINTERPRETATION 1COMMONLY MEASURED TEST QUALITIES• Sensitivity: Proportion of women testing positive among those whoare diseased.• Specificity: Proportion of women testing negative among those whoare nondiseased.• Positive predictive value (PPV): Proportion of women havingdisease among those with a positive test result 2 .• Negative predictive value (NPV): Proportion of women having nodisease among those with a negative test result (Last 1983).Sensitivity and specificity are qualities that generally measure theintrinsic qualities of diagnostic tests. By definition, if accurately andvalidly calculated, these measures should not differ substantially acrossresearch studies. Because of this, they are good measures for comparingthe relative value of different tests with regard to identifying true diseaseor non-disease.Predictive values, on the other hand, are measures of the clinical utility ofthe test when applied to a specific population in a particular environment.Predictive values incorporate information on both the intrinsic qualitiesof the test and the prevalence of disease (i.e., probability of disease priorto testing) in the population being tested (Hulley and Cummings 1988).Figure B-1 shows how data are organized to measure the four testqualities described above.Figure B-1. Organization of Data to Measure Test QualitiesReference Test+ !Clinical + a b a+bTest ! c d c+dSensitivity = a/a+cSpecificity = d/b+da+cPPV = a/a+bNPV = d/c+db+d1 Adapted from: Gaffikin L et al (eds). 1998. Alternatives for Cervical Cancer Screening and Treatment in Low-ResourceSettings. Workshop Proceedings. JHPIEGO: Baltimore, Maryland.2 PPV and NPV are affected by the test sensitivity and specificity respectively and the prevalence of disease ornondisease in the population.Cervical Cancer Prevention Guidelines for Low-Resource Settings B-1