Data required:
(1) sensitivity
(2) specificity
(3) total number of patients studied
(4) number of people with condition (or number of people without the condition)
total population =
= (TP) + (FP) + (TN) + (FN)
people with condition =
= (TP) + (FN)
people without condition =
= (total population) – (people with condition) =
= (TN) + (FP)
sensitivity =
= (TP) / ((TP) + (FN))
specificity =
= (TN) / ((TN) + (FP))
where:
• TP = true positives
• FP = false positives
• TN = true negatives
• FN = false negatives
If the above equations are rearranged:
TP =
= (sensitivity) * (people with condition)
FN =
= (TP) * (1 – (sensitivity)) / (sensitivity) =
= (people with condition) – (TP)
TN =
= (specificity) * ((total population) – (people with condition))
FP =
= (TN) * (1 – (specificity)) / (specificity) =
= ((total population) – (people with condition)) – (TN)
