THE VALUE OF MULTIPLE 24-HOUR URINE SAMPLES FOR ESTIMATING SODIUM INTAKE

OBJECTIVE: A single 24-hour urine collection is frequently used to estimate daily sodium intake. Long-term balance studies, however, observed large discrepancies between 24-hour sodium intake and sodium excretion. In a controlled setting with stable sodium intake, the use of up to 7 consecutive 24-hour urine samples has been shown to increase the accuracy of sodium intake estimates. The value of multiple 24-hour urine collections for estimation of sodium intake has not been investigated in the clinical setting with uncontrolled intake, use of natriuretic medication and comorbidities that may affect sodium excretion. DESIGN AND METHOD: In a retrospective cohort study, we included adult subjects who collected an outpatient 24-hour urine sample between 1998 and 2000, and had collected at least 6 additional 24-hour urine collections during follow-up. Mean sodium excretion was calculated for 2, 3, 4, 5, 6, and 7 consecutive 24-hour urine collections. We calculated the absolute change in 24-hour urine sodium excretion with each added urine sample. We considered a 0.4-gram change in sodium (1 gram salt) intake clinically relevant. RESULTS: We included 316 subjects with an average age of 44 years, and 46% were male. 50% of the subjects had a history of hypertension, 30% had diabetes and 9% had a history with a cardiovascular event. Diuretics were used by 22% of the subjects and 44% used ennin-angiotensin-aldosterone system inhibitors. The median follow-up period was 16.4 years. The median time to collection of 7 urine samples was 2.5 years (IQR 1.3-4.8). Mean sodium excretion was 159 mmol/day (SD 62). With addition of a second 24-hour urine sample, individual estimated sodium intake changed with 0.43 gram sodium (IQR 0.17-0.90) and changed > 0.4 gram in 52% of the subjects. When a fifth or seventh collection was added, estimated sodium intake changed > 0.4 gram in 14% and 7% of our cohort (Figure). CONCLUSIONS: Collection of multiple 24-hour urine samples substantially changes estimated individual sodium intake in the clinical setting. The added value of collecting > 5 24-hour urine samples is limited and changes sodium intake estimations only in 14% of the subjects.