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The Use of Random Urine Samples for Quantification of Urinary Anion Excretions † 1820

Measurements of urinary anion excretions (UAE) are critical in the understanding of renal stone formation, chronic metabolic acidosis and inborn errors of metabolism such as oxalosis. 24-h urines are difficult to collect, and require several for all anion excretions. We hypothesized that by application of a newly designed, specific and sensitive ion-chromotography method that allows simultaneous determinations of urinary oxalate, citrate, sulfate and phosphate levels, & related to creatinine excretion, we could predict accurately quantitative UAE from randomly voided urines. We designed an analytic system employing HPLC (Dionex-500) equipped with analytical(AS-11)& guard (AG-11) columns as stationary phases; NaOH,as mobile phase,continually degassed with N2, and run as a linear gradient from 5-52.5 mM over 21 minutes. The eluent background conductivity was suppressed with an anion self-regenerating suppressor (ASRS-1) to a level < 1.5μS at the highest [NaOH]. We determined the intra- and interassay CVs to be 0.9-1.4 and 3.1-3.3% for controls of 25μM and 75μM, respectively. We analyzed 272 24h urines by our HPLC method and compared them to reference lab levels. Although we found an excellent correlation (citrate, r=0.875; oxalate, r=0.807; p<10-3for both) between the methods, a low bias in citrate samples and a high bias in oxalate samples in the reference lab was found because of mis-identified peaks. We next determined the relationship between the 24-hour UAE (related to creatinine excretion), and a randomly voided urine within a week of that collection in 161 samples. We found an excellent correlation for each analyte (r2 from 0.6-0.8,p<10-3). Normal ranges were determined, based on literature and our own data. Abnormal results were concordant 100% of the time in both 24-hour and random urines. UAEs were measured in an additional 323 random urines to evaluate potential differences in normal values versus 24-hour urinary excretions. We found none. Therefore, we can conclude that using an innovative HPLC-based system for simultaneous determination of four major urinary anions, we can accurately and reproducibly measure each. Further, normative ranges have been established for random UAE which do not differ than those derived from 24-hour urinary excretions. This method will enhance the ability to determine relevant UAE in disorders of stone formation and metabolic diseases.

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Langman, C., Sailer, D. & Hoppe, B. The Use of Random Urine Samples for Quantification of Urinary Anion Excretions † 1820. Pediatr Res 43, 310 (1998).

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