Kappa = (Po - Pe)/(1 - Pe)

Where:

Po = A + D

Pe = [(A+B)*(A+C) + (C+D)*(B+D)]/N2

Po = Proportion of subjects observed to be concordant*

Pe = Proportion of subjects expected to be concordant*, based on chance alone

N =Total number of subjects

*Where both sources of data agree that subject either has presence or absence of disease