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Les Irwig, MBBCh, PhD, FFCM; Paul Glasziou, MBBS, PhD; Andrew Wilson, BMedSci, MBBS(Hons), FRACP; Petra Macaskill, BA(Hons), MAppStat

JAMA. 1991;266(12):1678-1685.

Abstract
An individual's blood cholesterol measurement may differ from the true level because of short-term biological and technical measurement variability. Using data on the within-individual and population variance of serum cholesterol, we addressed the following clinical concerns: Given a cholesterol measurement, what is the individual's likely true level? The confidence interval for the true level is wide and asymmetrical around extreme measurements because of regression to the mean. Of particular concern is the misclassification of people with a screening measurement below 5.2 mmol/L who may be advised that their cholesterol level is "desirable" when their true level warrants further action. To what extent does blood cholesterol change in response to an intervention? In general, confidence intervals are too wide to allow decision making and patient feedback about an individual's cholesterol response to a dietary intervention, even with multiple measurements. If no change is observed in an individual's cholesterol value based on three measurements before and three after dietary intervention, the 80% confidence interval ranges from a true increase of 4% to a true decrease of 9%.

(JAMA. 1991;266:1678-1685)

Author Affiliations
From the Department of Public Health, University of Sydney (Drs Irwig and Glasziou and Ms Macaskill), and the Department of Community Medicine, Westmead Hospital, Sydney (Dr Wilson), Australia.

Footnotes
Reprint requests to Department of Public Health, University of Sydney, New South Wales, Australia 2006 (Dr Irwig).

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