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This page gives signposting and links for practical guidance and methods that are common to the analysis of more than one nutrient.
The Association of Clinical Biochemistry, UK, provides peer-reviewed analyte monographs for many elements and biological molecules of interest.
Questionnaire: Use of a questionnaire to assess 24 h urine collection completeness is reported in the US NHANES surveys (see link below). However, standardised procedures across study fieldworkers is paramount to reduce operator variation, and rigorous and intensive protocols are recommended to uphold data quality, including supervised urine collection. See this study for more details:
24 h urine samples are usually the gold standard for the measurement of most nutrient biomarkers. However, they can be cumbersome and require intensive and strict fieldwork protocols, and can introduce bias (i.e. from missed collection) in some settings.
Where collection of 24 h urine samples is impossible, spot urine samples may be a valuable alternative. Further, since spot samples are often routinely used in health surveys, e.g. for the measurement of urinary iodine, additional assay can be easily integrated into survey or research protocols. Spot samples also remove the need for multiple visits, and therefore may be a more efficient use of resources.
Example spot collection methods:EUThyroid: instructions and guidance for taking urine samples is given on page 35 of this document prepared by the EUThyroid group. It relates to urine collection for iodine assessment, however can be adapted for use for other biomarkers. Urine samples handling advice is also included.Sample size estimates may need adjustment to provide the correct power and precision.See these selected references for example guidance on sample size:
There are several ways to assess urinary hydration using proxy biomarkers, including creatinine (see below), specific gravity or osmolality. These are described in the following reference:
Middleton DR, Watts MJ, Lark RM, Milne CJ, Polya DA 2016 Assessing urinary flow rate, creatinine, osmolality and other hydration adjustment methods for urinary biomonitoring using NHANES arsenic, iodine, lead and cadmium data. Environ Health 15:68.
Creatinine concentration can be used to assess hydration, and is an alternative method for 24 h urine collection completeness assessment. Creatinine values should be interpreted dependent on sex, protein intake, muscle mass, degree of malnutrition and ethnicity. Since standard cut-offs do not widely exist, creatinine should be used with caution.
Measurements of creatinine concentration and sodium concentration in a spot urine sample combined with details of the individual’s sex, weight, height and age allow application of the Kawasaki formula which estimates 24 h urine excretion (16). It is recommended that this method, which is less accurate than using 24 h collections, can be used for population assessments provided that the sample size of the group is adequate (5). Review the Kawasaki method.
Kawasaki T, Itoh K, Uezono K, Sasaki H 1993 A simple method for estimating 24 h urinary sodium and potassium excretion from second morning voiding urine specimen in adults. Clinical and experimental pharmacology & physiology 20:7-14.
Instructions and guidance for taking tried blood spot (DBS) samples can be found in the WHO/CDC/ETH Zürich protocol following: Procedure for DBS collectionKindly provided by Human Nutrition Laboratory, ETH Zurich, Switzerland
PABA is a non-toxic B-complex vitamin that is thought to be fully absorbed and is readily analysed. PABA is an accepted measure to assess the completeness of 24 h urine collection. It involves the concomitant administration of 80 mg PABA tablets usually with main meals. The use of PABA in national surveys has been reported in the National Diet and Nutrition Survey: assessment of dietary sodium in adults 19-64 years in England (2014) (page 15, section 2.6).
Though used as an objective measure of urine collection completeness, the use of PABA is not without issue due to potential variation in excretion rate with age, non-adherence to the dosage regimen and potential interaction with medication although this is less of a problem when PABA is measured by HPLC.
Method: SOP NBL PABA by HPLCKindly provided by the NIHR BRC Nutritional Biomarker Laboratory, University of Cambridge, Cambridge UK
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Guidance on how to maintain internal quality control is contained in each biomarker fact-sheet…