Table 2. Intraclass correlation coefficients (ICC) for serum 25-hydroxyvitamin D levels between twins in a pair with 95% confidence intervals.

Variable	ICC (95% CI)	P-value
HPLC-APCI-MS	0.66 (0.54−0.76)	ref	NA
RIA	0.54 (0.39−0.66)	0.004	ref
CLIA	0.40 (0.22−0.55)	<0.001	<0.001

The p-values correspond to a test of equality of the observed correlations.