Table 2.

Sources of Variance in Sweat Chloride Measurements in a Mixed Model^*

Nested Level	Potential Sources of Variation	Sweat Chloride SD Estimate (95% CI)†	Sweat Chloride Variance Estimate (95% CI)	Percentage of Variance
CFTR genotype	CFTR genotype	18.1 (15.1–21.7)	328.2 (229.1–470.3)	56.1
Family	Common environment and intragenic CFTR modifiers	8.9 (7.8–10.0)	78.7 (61.3–101.0)	13.5
Individual	Genetic modifiers and unique environment	6.3 (4.7–8.3)	39.5 (22.5–69.3)	6.8
Date of testing	Biological variability over time	9.0 (7.9–10.2)	80.5 (62.5–103.6)	13.8
Mixed model residual	Testing variability and residual/random factors	7.6 (7.1–8.2)	58.0 (50.6–66.6)	9.9

Definition of abbreviations: CFTR = cystic fibrosis transmembrane conductance regulator; CI = confidence interval.

^*For this mixed model a total of 2,160 age-adjusted sweat chloride measurements obtained from 1,489 subjects were nested into 832 families, which were nested into 111 CFTR genotype groups.

^†The mean sweat chloride (intercept) estimated by the model was 95.6 mmol/L (95% CI, 91.8–99.5).