Newsletter Highlight :

By Prof. Joseph Lau, Chinese University of Hong Kong.

The HKEA Newsletter 2002, 6(2), p.5.

 

HOW GOOD IS IT? EVALUATION OF SCREENING PROGRAMMES

 

Sensitivity and specificity are often used to measure accuracy of screening tests. When a screening test has a range of values, different cut-off points can be used and they result in different sets of sensitivity and specificity. There is always a trade-off between these sets of sensitivity and specificity values and a choice should be made based on the application of the screening test. A highly sensitive test can be used to "rule-out" a diagnosis in face of a negative screening result and a highly specific test can be used to "rule-in" a diagnosis in the presence of a positive result. Like other statistics, sensitivities and specificities are subject to chance and can be described by confidence intervals; cautions should be given to cases where these statistics are derived from small samples.

When the sensitivity values are plotted against the (1-specificity) values, a receiver-operating characteristics curve is obtained. The Area Under the Curve (AUC) measures the performance of a screening test. The maximum area of 1.0 says that the screening test is as good as the gold standard; an area of 0.5 says that the test is totally useless. One can use special statistics to compare the areas of two different ROC curves generated by applying two screening test to same individuals, so as to determine whether the performance of one test is statistically different (better or worse) from another one.

A more updated method to assess the performance of a screening test is the likelihood ratio, defined as sensitivity/(1-specificity). It is used to generate post-test odds and probabilities which can be compared with pre-test odds and pretest probabilities (prevalence of disease). A large change between the pre-test and post-test values would suggest that screening is worthy as it would change a clinical practice. Positive and negative predictive values are defined as the probabilities that someone with a positive or negative screening result is actually a case or a non-case. They are intuitively important. However, as they are dependent on prevalence of disease; they cannot be used to assess accuracy of screening test.


Copyright © HKEA. All Rights Reserved.