Day-to-day variability in spot urine protein-creatinine ratio measurements

Accurate measurement of proteinuria is important in the diagnosis and management of chronic kidney disease (CKD). The reference standard test, 24-hour urinary protein excretion, is inconvenient and vulnerable to collection errors. Spot urine protein-creatinine ratio (PCR) is a convenient alternative and is in widespread use. However, day-to-day variability in PCR measurements has not been evaluated.

Prospective cohort study of day-to-day variability in spot urine PCR measurement.

Clinically stable outpatients with CKD (n = 145) attending a university hospital CKD clinic in Australia between July 2007 and April 2010.

Spot urine PCR.

Spot PCR variability was assessed and repeatability limits were determined using fractional polynomials.

Spot PCRs were measured from urine samples collected at 9:00 am on consecutive days and 24-hour urinary protein excretion was collected concurrently.

Paired results were analyzed from 145 patients: median age, 56 years; 59% men; and median 24-hour urinary protein excretion, 0.7 (range, 0.06-35.7) g/d. Day-to-day variability was substantial and increased in absolute terms, but decreased in relative terms with increasing baseline PCR. For patients with a low baseline PCR (20 mg/mmol [177 mg/g]), a change greater than ±160% (repeatability limits, 0-52 mg/mmol [0-460 mg/g]) is required to indicate a real change in proteinuria status with 95% certainty, whereas for those with a high baseline PCR (200 mg/mmol [1,768 mg/g]), a change of ±50% (decrease to <100 mg/mmol [<884 mg/g] or increase to >300 mg/mmol [>2,652 mg/g]) represents significant change.

These study results need to be replicated in other ethnic groups.

Changes in PCR observed in patients with CKD, ranging from complete resolution to doubling of PCR values, could be due to inherent biological variation and may not indicate a change in disease status. This should be borne in mind when using PCR in the diagnosis and management of CKD.