SCREENING FOR DISEASES

Screening for Diseases



Screening for Diseases

Question # 1

Test Result

Present

Absent

Total

Positive

True Positive

False Positive

All positive results

Negative

False Negative

True Negative

All negative results

Total

All with Antibody

All without antibody

Total

Sensitivity

Sensitivity - the likelihood that result of the test will be positive when administered to subjects who actually have the antibody. = true positives / all with antibody. Algebraically, sensitivity = A / (A+C). When the cutoff will be moved from A to B, it will show true positive sensitivity. In other words, test will give positive result of all subjects with antibodies. When the cutoff mark will move from A to C, test will show false negative. It means that even subjects with antibodies will not be detected in the test (Turnock, 1990).

Specificity

Specificity - the probability that the test result will be negative when administered to persons who are actually without the antibody = true negatives / all without antibody. Algebraically, specificity = D / (B+D). When the cutoff mark will be moved from A to B, it will show false positive result. In other words, it will show gave false result that the subjects have antibodies, which is not true. When the cutoff mark will be moved from A to C, it will show true negative results meaning the subjects don't have antibodies in them (Turnock, 1990).

Cutoff mark

Cutoff mark should be towards more sensitive test. Such test has a tendency to miss very few subjects with the antibodies. In other words, it will have few positive negatives. The downside of this is that it will wrongly detect subjects without antibodies. It means there would be large number of false positive (Walker, Et al, 1990).

Relationship between Sensitivity and Specificity

There is a tradeoff between sensitivity and specificity of screening tests. An ideal test should be 100% on both specificity and sensitivity but such screening test ...