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The target plot: a new way of displaying the performance of a screening test
1Department of Environmental and Preventive Medicine, Wolfson Institute of Preventive Medicine, St Bartholomew's and the Royal London School of Medicine and Dentistry, UK.
Objectives:
To produce a graphical method to represent the performance of a screening test that illustrates the prevalence of the disorder being screened for, as well as its discriminatory potential.
Concept:
A target plot was constructed in which the risk of the disorder is represented by a series of concentric circles of constant risk (isorisks) equivalent to specified false positive rates, with the highest risk in the centre and lower risks spreading outwards towards the circumference. Dots were drawn to represent cases of the disorder; these were of a size such that their total area as a proportion of the area of the whole target plot equalled the prevalence of the disorder in the screened population. The discriminatory power of the test was seen as the clustering of dots around the centre or bull's eye of the target. The detection rate could be estimated as the proportion of dots which fell within the isorisk corresponding to a specified false positive rate.
Application:
The target plot was applied to second trimester antenatal screening for Down's syndrome using different combinations of screening markers, and also to screening for ischaemic heart disease using protein components of cholesterol (apolipoproteins A I and B and Lp(a) lipoprotein), systolic blood pressure, and smoking status.
Discussion:
The efficacy of the different methods of screening for Down's syndrome is readily apparent using the target plot, as is the poorer performance of screening for ischaemic heart disease.
Conclusions:
The target plot is a simple and quantitative way of displaying the performance of a screening test that may be useful in teaching and educational material.