Concordance intra and inter assay

A good diagnostic method exhibits great agreement intra and inter assay.

In order to evaluate these agreements it is not enough to analyze the percentage of concordances because the influence of chance in the agreement is not considered. The Cohen´s kappa index is appropriate to solve this problem.

Contingency table for the evaluation of Cohen's kappa index

		Result of the technique (technician 1)
		Positive	Negative
Result of the technique (technician 2)	Positive	a	b	a+b
Result of the technique (technician 2)	Negative	c	d	c+d
		a+c	b+d	N

Kappa index is calculated dividing the subtraction of observed coincidence - expected coincidence by the substraction of 1 - expected coincidence, being the observed coincidence (a+d)/N and the expected coincidence [(a+b)x(a+c) + (c+d)x(b+d) ]/NxN.

In general, the following criteria based on the interpretation of Landis and Koch is used (1977):

Kappa index	Agreement
< 0.00	Less than chance
0.00 – 0.20	Slight
0.21 – 0.40	Fair
0.41 – 0.60	Moderate
0.61 – 0.80	Substantial
0.81 – 1.00	Almost perfect

Inter and intra reliability of a method