Identifying Optimal Sample Types and Decision Thresholds for the Urinary Albumin–Creatinine Ratio

A major aim of the study by Saydah and coworkers (1) described in this issue of Clinical Chemistry was to evaluate the prevalence of albuminuria in adults participating in the National Health and Nutrition Examination Survey, 2009–2010 as an estimate of the prevalence of increased urinary albumin excretion in the US population. As their data show, increased urinary albumin is a common disorder, with >3% of a cross-sectional sample of the US population showing an increase with repeat sampling. In addition, their evaluation of >5000 paired random and first morning urine samples has provided a large data set that bears on how sample type influences test results and how values for the albumin–creatinine ratio should be interpreted. Although clinical testing of albumin excretion in the urine has been applied widely for many years, there are still questions about how best to perform such testing and interpret the results (2). Additional data that characterize more fully the sources of pre- and postanalytical variation and better approaches for standardizing measurements are needed to help resolve these remaining issues. Several previous studies reported that urine samples obtained in the first morning void yield lower values than random samples collected throughout the day (3–5), and the first morning void sample has been observed to correlate more closely with results of a 24-h collection than results for a random urine sample (5). It is logical that random urine samples collected later in the day would have higher values than first-void samples, because urinary excretion of albumin usually is higher during the …