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Leaf plot

In medicine, we frequently need to do tests and look for particular clinical signs to help us refine or even clinch a diagnosis. It can be difficult always to predict or interpret just how much difference a positive or negative test will make to a particular diagnosis in an individual patient.

The standard way of measuring the power of medical tests is to use statistics called sensitivity and specificity, but the calculations that you need to be made to interpret them is not intuitive. Indeed, there are forever articles appearing in journals trying to explain how to do it, but they don’t help much.

The Leaf Plot is designed to make it easy to apply a test result the diagnostic odds of your patient by showing it graphically.

First, imagine that the ‘starting odds’ are represented by the diagonal black line going from the bottom-left to the top-right (the vein of the leaf), with the diagnosis being very unlikely on the left, and changing from possible to probable, and then to very likely as you move to the right.

Then, look at the red and blue lines. If the test was positive, the chances that the diagnosis was correct would be increased by the shaded red area, up to the red line, and can be read off from the left-hand red axis. If the test was negative, then the odds of that being the diagnosis fall as far as the blue line, and can be read from the right-hand blue axis.

Thus, a weak test will produce a slim willow-leaf pattern, while a powerful test will produce a broad-leaf shape. Some tests are good at ruling tests in but not excluding them, and some are the other way round – these produce asymmetric leaves.

To use this tool

Simply download the file as an Excel document, and enter the sensitivity and specificity values for the test into the boxes, and it will draw you the Leaf Plot for that particular test.