Table 3. Definition of sensitivity, specificity, positive predictive value and negative predictive value.

Carriage of meningococci (confirmed by the gold standard in Oxford):
PCR outcome/carriage	Carriage present	No carriage
Positive	True Positive (TP)	False Positive (FP)
Negative	False Negative (FN)	True Negative (TN)

Sensitivity (SS): Proportion of samples positive by the gold standard technique (true carriers) that were positive by PCR. Sensitivity = TP/(TP+FN)

Specificity (SP): Proportion of samples negative by the gold standard technique (true non-carriers) that were negative by PCR. Specificity = TN/(TN+FP)

Positive Predictive Value (PPV): Probability to have a positive result with the new PCR when it is positive with the gold standard (carriage of meningococci or carriage of the tested strain). PPV = TP/(TP+FP)

Negative Predictive Value (NPV): Probability to have a negative result with the new PCR when it is negative with the gold standard (no meningococcal carriage or carriage of the tested strain). NPV = TN/(TN+FN)