TP, true positives, FN, false negatives, FP, false positives, TN, true negatives. In the context of this article, positive test means clinical sign present, and negative test means clinical sign not present. Sensitivity is a/a+c, the probability of having the clinical sign, given that you have the disease, specificity is d/ b+d, the probability of not having the clinical sign, given that you do not have the disease. Predictive value is a/ a+b, the probability of having the disease, given that you have the clinical sign.