Hi Omima,
For calculation of sensitivity, specificity, PPV and NPV; firstly individually calculate all these performance indicators for the respective diagnosis test at the selected confidence interval like 95% and then based on comparison of both diagnostic results, derive the most optimal diagnostic result i.e. the one having highest value for sensitivity and NPV while low value for specificity and PPV. For a disease,
Sensitivity – (true positive)/(true positive + false negative)
Specificity – (true negative)/(true negative + false positive)
PPV – (true positive)/(true positive + false positive)
NPV – (true negative)/ (true negative + false negative)