Accuracy is the degree to what a measuring tool actually measures what it claims to measure. For example, a bodyweight scale is an accurate tool to measure a person’s weight. In clinical practice, it’s unfortunately not that simple. The degree of accuracy depends on sensitivity and specificity of a measuring tool or physical examination test.
To sum it up again: A negative outcome in a 100% sensitive test can rule out the disease (SnNOut) and a positive outcome in a 100% specific test can rule in the disease (SpPIn).
With the two mnemonics SnNOut and SpPIn it´s relatively easy to put these two concepts into practice.
We have to be careful with the application of the SpPIn and SnNOut rule, if sensitivity or specificity are lower than 100%. Their ability to rule in/out always depends on both sensitivity and specificity. For further reading, we recommend you to read the following article:
Pewsner et al. (2004) – Ruling a diagnosis in or out with “SpPIn” and “SnNOut”: a note of caution.