> Can anyone give advice for calculations regarding
> specificity and sensitivity of a screening test? For
> example, suppose that 1000 patients were screened. 200 were
> tested positive and the remainder tested negative. Of the
> 200, 75 were found to be true positives and 125 were found
> to be false positives. Of the 800 test negatives, a random
> 10% sample were further tested for the disease of whom 20
> were found to be positive (ie false negatives) and 60 were
> truly negative.
> My question is: how do you calculate specificity and
> sensitivity based on the fact that only 10% of the test
> negatives were followed up?
Answer: with very wide confidence intervals! To derive the true S&S for
the whole sample, you have to know the results of all of them. Without
that, everything else is an estimate. Depending on the method of selection
of the 10%, you will have to make a judgement as to how reliable your
estimate is.
For example if you assume that the false negative rate is a valid estimate
then extrapolating to the whole sample:
True positive = 75
True negative = 600
False positive = 125
False negative = 200
Sensitivity = 75/(75+200) = 27%
Specificity = 600/(125+600) = 82.75%
Sounds like a really useful screening test!
Paul.
--
Mr Paul M Flynn MRCOG
Middlesbrough, UK
+44 (0)7768 956504; Fax: +44 (0)7092 173892
|