Copyright © 2004 by the Johns Hopkins Bloomberg School of Public Health
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Estimating Laboratory Precision of Urinary Albumin Excretion and Other Urinary Measures in the International Study on Macronutrients and Blood Pressure
1 Department of Preventive Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
2 Department of Epidemiology and Public Health, Faculty of Medicine, Imperial College of Science, Technology and Medicine, London, United Kingdom.
3 Akademisch Ziekenhuis St. Rafael, Leuven, Belgium.
4 Department of Health Sciences, Shiga University of Medical Science, Otsu, Japan.
Microalbuminuria is a risk factor for renal failure, stroke, and cardiovascular disease. However, estimating laboratory precision for albumin excretion is problematic because of its highly skewed distribution and the presence of values below assay detection limits. The authors used 781 quality control pairs from 24-hour urine samples collected between 1996 and 1999 in the International Study on Macronutrients and Blood Pressure (INTERMAP) to compare percentage of technical error (%TE), the usual estimate of laboratory precision, with the mean and median values of within-pair coefficients of variation (CVs) for urinary albumin concentration and other urinary variables. In INTERMAP, %TE was larger than mean CV for all variables. Exclusion of potentially mislabeled samples reduced this difference; for example, for sodium, estimates of %TE and mean and median CV were 2.37%, 0.75%, and 0.28%, respectively, for all 781 pairs and 0.84%, 0.48%, and 0.27%, respectively, with possibly mislabeled pairs excluded. For urinary albumin concentration, exclusion of one mislabeled pair changed estimates for %TE and mean CV from 29.6% and 20.8% to 20.6% and 20.6%, while median CV was unchanged at 9.4%. After exclusion of urinary albumin concentration pairs with values below the detection limit, estimates were 15.4%, 11.4%, and 6.4%, respectively. Results indicate that mean and median CV are not equivalent to %TE and that values below the detection limit can markedly affect estimates and should be excluded.
albumins; albuminuria; clinical laboratory techniques; potassium; research design; sodium; urine
Abbreviations: Abbreviations: CV, coefficient of variation; INTERMAP, International Study on Macronutrients and Blood Pressure; percent TE, percentage of technical error.
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