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Use of the standard urine phosphate assay for measuring the phosphate content of beverages: Validation and variation
Elizabeth Lindley1,2,4, Seán Costelloe3, Julienne Freeman3, David Keane1,2,4 and Douglas Thompson3
Departments of 1Renal Medicine, 2Medical Physics and 3Blood Sciences, Leeds Teaching Hospitals NHS Trust, 4NIHR Devices for Dignity Healthcare Technology Co-operative
INTRODUCTION: Hyperphosphataemia is strongly associated with cardiovascular morbidity and mortality in patients with chronic kidney disease. Control of serum phosphate is particularly challenging for haemodialysis patients as the phosphate generated by an adequate protein intake cannot be removed with standard treatment regimes. Binders that reduce absorption of phosphate can have unwanted side-effects and often become an intolerable pill burden.
Beverages can contain phosphate in a form that is readily absorbed and are often consumed between meals, without a binder. Manufacturers are obliged to declare use of additives, including phosphoric acid in colas, but not the amount added. The phosphate present in beer and wine is not mentioned on the label as it is not added. For patients on low phosphate diets (26-32 mmol/day), it would help to know which types or brands of beverages are higher in phosphate.
METHOD: The inorganic phosphate concentration in samples of thirty-two soft drinks, beers, ciders, and wines was measured with and without spikes of a solution containing a known amount of phosphate (as H2NaPO4). All samples were measured in duplicate using the reaction between inorganic phosphate and ammonium molybdate on a Siemens Advia 1800 autoanalyser. In the assay conditions used, the blue compound formed is quantified by absorbance at 340/658 nm. The measured increase in phosphate concentration in the spiked samples was compared with the expected increase to see if the beverages contained anything that would enhance or diminish the assay signal. A difference of less than 20% was considered acceptable. Fourteen additional beverages were tested as standard urine samples (without spikes) to further investigate the variation between brands.
RESULTS: Recovery of the phosphate spike varied from 84.8 to 90.9% for soft drinks, from 82.2 to 103.5% for beers, and ciders, and from 80.5 to 86.1% for wines, indicating that our standard urine phosphate assay can be used to accurately measure the phosphate concentration in these beverages. There was considerable variation in the phosphate concentrations measured. Soft drinks that did not contain phosphoric acid had undetectable levels (<0.03 mmol/L), whilst the colas contained 3.2 to 5.0 mmol/L. The ciders tested had concentrations of 1.8 to 2.6 mmol/L, which was lower than the beers (2.7 to 8.1 mmol/L) and the wines (3.6 to 9.9 mmol/L).
DISCUSSION: The BDA Renal Nutrition Group recommends a minimum protein intake for haemodialysis patients of 1.1 g/kg ideal weight/day, i.e. 77 g/day for a 70 kg patient. Using the figures in Daugirdas et al. (Sem Dial 2011), this will lead to a phosphate load of about 26 mmol/day. Good conventional dialysis should remove about 100 mmol phosphate/week. This leaves an anuric patient with about 12 mmol/day of excess phosphate for which 4 to 12 binder tablets (depending on the type and size) will be required. This substantial pill burden is needed simply to manage the phosphate generated from adequate protein ingestion. Any extra mmols of phosphate from beverages will add to this burden and/or lead to high serum phosphate levels.
CONCLUSION: We have shown that this assay for inorganic phosphate in urine, which is based on the phosphomolybdate method and is similar to the methods used in the vast majority of hospital laboratories, can give accurate measurements of phosphate in a range of beverages. The information the assay provides can enable patients to compare locally available beverages and avoid or limit their intake of those with high phosphate content.