Table 4. Linear regression analyses for the relationship between the absolute differences85 in serum values between the methods against the mean of the two values.

Variable	Slope	95% CI	P-value
CLIA vs HPLC-APCI-MS	0.19	0.09−0.29	<0.0001
RIA vs HPLC-APCI-MS	0.22	0.12−0.31	<0.0001

*Adjusted for twin-ship dependence.