Positive and Negative Predictive Values for a test

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))

• p = Prior probability of infection
• Se = Test unit sensitivity
• Sp = Test unit specificity

References

Resources