Evaluation of measures of urinary albumin excretion.

Albuminuria has recently drawn much attention as a valuable risk marker for cardiovascular and renal disease progression. Albuminuria can be measured and expressed in several ways: 1) in a spot morning urine sample as urinary albumin concentration (mg/liter) or albumin:creatinine ratio (mg/mmol) and 2) in a 24-hour urine collection as urinary albumin excretion (mg/24 hours). It has not yet been clarified which measure for albuminuria is preferable in clinical practice. One of the points on which a choice should be made is which measure shows the least within-person coefficient of variation. From the perspective of their work in the Prevention of Renal and Vascular Endstage Disease Intervention Trial, 1997-2001, the authors discuss several methodological issues that are important when interpreting studies on this topic. It is argued that fresh urine should be used, since freezing at -20 degrees C results in considerable extra variability in the albumin concentration. Furthermore, it is important to use specifically collected urine samples and not portions of a 24-hour urine sample as a surrogate for a spot morning urine sample. Albuminuria follows a circadian rhythm. Consequently, values for the within-person coefficient of variation will therefore be different when they are measured in a portion of a 24-hour urine collection in comparison with a spot morning urine sample.