Urinary biomarkers in 24-h urine samples fairly reproducible
Various urinary biomarkers such as minerals, electrolytes, bisphenol A (BPA) and most polyphenols can be reasonably reproduced in 24-hour urine samples collected within a few days or ≤1 year, suggests a recent study.
“The majority of urinary biomarkers show fair-to-excellent reproducibility in 24-hour urine samples, which did not very substantially by sample collection interval, frequency and other variables,” researchers said.
“The time integration, versatility and feasibility of 24-hour urine samples make these biospecimens potentially useful for the assessment of various nutritional and environmental exposures in epidemiologic studies,” they added.
A total of 3,168 participants in the Nurses’ Health Study (NHS), Nurses’ Health Study II (NSHII) and Health Professional Follow-up Study provided 24-hour urine samples, from which intraclass correlation coefficients (ICCs) of biomarkers were calculated.
“Overall, within a given period, the reproducibility was robust to the number of 24-h urine samples that were collected or to the time intervals between collections, although the biomarker concentrations varied less in samples collected within 1 month compared with those collected at intervals of 3 months to >1 year,” researchers said.
ICCs for sodium were 0.32 and 0.34 in the NHS and NHSII, respectively, in 742 women with four samples each collected over the course of 1 year. In 2,439 men and women with two samples each collected over 1 week to ≥1 month, the ICCs for sodium ranged from 0.33 to 0.68 at various intervals between collections. [Am J Clin Nutr 2017;105:159-168]
Reproducibility was generally higher (ICCs >0.4) for the urinary excretion of potassium, calcium, magnesium, phosphate, sulfate and other biomarkers.
ICCs for polyphenol metabolites in 41 women with two 24-hour urine samples ranged from 0.15 (catechin) to 0.75 (enterolactone). For phthalates, ICCs were generally ≤0.26 except for monobenzyl phthalate (ICC, 0.55), while for BPA the ICC was 0.39.
It was estimated that, for the large majority of the biomarkers, two to three 24-hour urine samples could provide a stable measurement of long-term concentration, with the exception of phthalates and a few polyphenol metabolites.
“Our findings show that three 24-h samples are sufficient for the measurement of long-term exposure status in epidemiologic studies,” researchers said.
In an accompanying editorial, Lars Ove Dragsted from the University of Copenhagen in Denmark said that if a transition from subjective to objective assessment of dietary intakes will take place in the coming years, research themes might likely centre on several aspects of quality, reliability, validity and usefulness of biomarkers. [Am J Clin Nutr 2017;105:8-9]
“The number of samples collected will still be the single factor determining the level of detail observable by biomarkers, but the observation that two to three 24-h urine samples are already providing considerable coverage of the average intakes of many food-derived compounds is likely to bring about additional impetus to such a transition,” he said.