Agreement of Immunoassay and Tandem Mass Spectrometry in the Analysis of Cortisol and Free T4: Interpretation and Implications for Clinicians

Table 3

Pearson correlations of difference between methods versus MSMS alone, for Free T4 (FT4). MSMS and (MSMS-IA) correlations reflect significant differences in the variances of the measurements by IA versus MSMS over time, and for nonpregnant women (NP) measured at one time only. (Pregnant cohort N with both ranges from 59 (T1) to 26 (T3); Nonpregnant cohort N is 28.)

DIFFERENCE	FT4 by MSMS, over time (and for NP)
	FT4 by MSMS, T1	FT4 by MSMS, T2	FT4 by MSMS, T3	FT4 by MSMS, NP
FT4, T1 MSMS-IA	0.598† ∗			—
FT4, T2 MSMS-IA		0.823† ∗		—
FT4, T3 MSMS-IA			0.723† ∗	—
FT4, NP MSMS-IA	—	—	—	0.411† ∗

† Pearson correlation coefficient significant at 𝑃<.05.∗indicates significant differences in the variances of the measurements by IA versus MSMS.