C MANNIX1, ATY WONG1, JQ ZHANG1, G RANGAN1, C WOOLNOUGH2, A RANGAN3
1Centre for Transplant and Renal Research, Westmead Institute for Medical Research, University of Sydney, Westmead, NSW; 2Department of Chemical Pathology, Royal Prince Alfred Hospital, Camperdown, NSW; 3Nutrition and Dietetics, School of Life and Environmental Science, The University of Sydney, Sydney, NSW
Aim: The aim of this study was to assess the relative validity of the beverage frequency questionnaire (BFQ) compared to 24-hour urine volume and osmolality.
Background: Increasing interest in the relationship between fluid intake and health outcomes has highlighted the need for a tool that specifically measures fluid intake. The BFQ is a semi-quantitative questionnaire designed to measure the usual volume and frequency of fluid consumption over the past month.
Methods: Patients with chronic kidney disease (CKD) due to autosomal dominant polycystic kidney disease (ADPKD; 18-65 yrs old; eGFR ≥30ml/min1.73m2) completed the BFQ with the assistance of a dietitian, and provided two 24-hour urine collections and a blood test, from which urine volume, osmolality and serum copeptin (B.R.A.H.M.S assay) were measured. Pearson correlation coefficients, paired t test and Bland-Altman plot were used to evaluate agreement between the methods.
Results: A sample of 80 patients [54% male, mean (SD) age 43 (11) years, BMI 27 (5)] completed the BFQ and at least one 24-hour urine collection. Moderate correlations were observed between the BFQ and 24-hour urine volume (r=0.596) and osmolality (r=-0.331), but not with serum copeptin. However, serum copeptin was correlated with 24-hour urine osmolality (r=0.451). Bland-Altman plots revealed good agreement between BFQ measured fluid intake and 24-hour urine volume with no obvious bias, however, the limits of agreement were wide. The BFQ tended to over-estimate fluid intake compared to 24-hour urine volume (2708 ml vs 2455 ml, P<0.05).
Conclusions: The BFQ shows potential as a valid tool for estimating fluid intake in people with CKD due to ADPKD.