Input Values 

Use this to calculate the positive
and negative predictive values for a test of
known sensitivity
and specificity for a range of prior probabilities of infection.
Inputs required
 Test unit sensitivity;
 Test unit specificity; and
 Prior probability of infection.
Outputs
 Positive (PPV) and negative (NPV) predictive values for given inputs; and
 Table and plot of PPV and NPV for given sensitivity and specificity values
and varying prior probability of infection.
Formulae
PPV = p x Se/(p x Se + (1  p) x (1  Sp))
NPV = (1  p) x Sp/((1  p) x Sp + p x (1  Se))
where:
 p = Prior probability of infection
 Se = Test unit sensitivity
 Sp = Test unit specificity
References
Sergeant & Perkins, 2016. Epidemiology for Field Veterinarians: an Introduction. CABI, Wallingford, UK. pp 9598.
Thrusfield, 1995, Veterinary Epidemiology, 2nd ed. Blackwell Science Ltd, Oxford, UK. pp 277279.
Resources
Excel example
R function
Validity and accuracy
