If Hemoccult test is negative for screening of colonic cancer, no fuher test is done. If the hemoccult test is positive the individual will have a second stool sample tested with Hemoccult II test. If this second sample is positive for blood, the individual will be referred for more extensive evaluation. The effect of net sensitivity and net specificity of this method of screening is

SCREENING TEST USED IN SERIES: A population is subjected to one screening test followed by a second screening test is applied on those individuals only who test positive on th e1st screening test. Combined sensitivity of 2 tests A and B in series: Sensitivity (A) * Sensitivity (B) Combined specificity of 2 tests A and B in series: Specificity (A) + Specificiity (B) - SCREENING TEST USED IN PARALLEL: A population is subjected to two or more screening tests at the both (or all) screening tests. Combined sensitivity of 2 tests A and B in parallel: Sensitivity(A) + Sensitivity (B) - Combines specificity of 2 tests A and B in parallel: Specificity(A)*Specificity(B) Test in series Tests in parallel Combined sensitivity Decreases Increases Combined specificity Increases Decreases Combined PPV Increases Decreases Combined NPV Decreases Increases Ref: Simple Biostatistics by Indrayan and Indrayan 1st edition Pgno: 56