Table 3.

Sweat chloride measurements of variation, index of individuality and reference change value.

		Study 1	Study 2
			Unstratified	Young	Old
Overall Sample	Mean (mmol/L)	103.4	101.8	102.0	101.1
	SDa (mmol/L)	10.9	10.1	10.1	10.4
Method b	CVcanalytical(%)	3.9	4.1	4.1	4.1
Total individual subject variationc	SD (95% CI d) (mmol/L)	6.1 (5.6–6.6)	7.4 (7.0–7.8)	7.8 (7.3–8.3)	5.9 (5.3–6.7)
	CVtotale (%)	5.9	7.3	7.6	5.9
Within-subject variationd	N	292	687	525	162
	SD (95% CI) (mmol/L)	4.5 (3.8–5.3)	6.1 (5.6–6.6)	6.5 (5.9–7.2)	4.3 (3.2–5.2)
	CVwithinf (%)	4.4	6.0	6.4	4.2
Between-subject variation	N	28	100	78	22
	SD (95% CI) (mmol/L)	9.2 (6.6–11.7)	7.1 (6.0–8.3)	6.6 (5.4–8.0)	8.5 (6.2–11.7)
	CVbetween(%)	8.9	7.0	6.5	8.4
Reference change value (%)g		13.7	17.0	17.7	13.7
Index of individualityh		0.7	1.0	1.2	0.7

^aSD is standard deviation.

^bMethod CV_analytical is the laboratory method imprecision and does not include bias.

^cCV is coefficient of variation.

^dCI is Confidence interval.

^eCV_total was defined as: (${\text{CV}}_{\text{analtyical}}^2$ + ${\text{CV}}_{\text{within}}^2$)½.

^fCV_within was calculated as (${\text{CV}}_{\text{total}}^2$ − ${\text{CV}}_{\text{analytical}}^2$)½

^gRCV was calculated using the formula: 2^½ * Z * (${\text{CV}}_{\text{analytical}}^2$ + ${\text{CV}}_{\text{within}}^2$)1^/2, where a Z score of 1.65 was chosen to reflect a one-tailed 95% probability (p < 0.05) [9].

^hII was calculated as the ratio of total within-subject variation to between-subject variation [${\text{CV}}_{\text{analytical}}^2$ + ${\text{CV}}_{\text{within}}^2$]^½/CV_between [9].