Use of Para-aminobenzoic Acid for Validating 24-hour Urine Completeness in a Sodium-Blood Pressure Population Study

Abstract

Despite the fact that high sodium intake has been linked to high blood pressure and an increased risk for cardiovascular disease (CVD), sufficient and irrevocable evidence to support extremely low dietary sodium intake has been lacking. The best method used for the estimation of daily sodium intake is indirectly by using 24-hour urinary sodium excretion, combined with para-aminobenzoic acid (PABA), which is used for validating completeness of urinary collection. However, the PABA validation method is not favored among researchers and the validity of high-performance liquid chromatography (HPLC) analysis for PABA has not been evaluated in a large-scale study. This study validated an improved PABA HPLC method and applied it in the sub-set of the Prospective Urban Rural Epidemiology (PURE) – 24-hour Urinary Excretion (24USE) Study for the examination of the sodium-blood pressure association. In addition, participants’ self-reported assessment of urinary collection completeness was compared to the PABA method. The HPLC analysis method for measuring PABA levels was downscaled and the reaction time lengthened to achieve higher through-put and reaction yield. By applying the optimized PABA method for evaluating urinary completeness, 612 of 681 participants’ samples contained 70%-110% PABA recovery and were extracted from the PURE24USE Study for further analysis. The average adjusted sodium excretion or intake was 3,673 ± 1,637 mg/day. The participants’ self-reports predict urinary collection completeness as measured by PABA with a sensitivity of 76.9% (95% CI: 74.4%-79.6%) and specificity of 31.7% (95% CI: 27.0%-36.4%). This moderate agreement suggests that PABA still has to be considered the gold standard, until further convincing evidence is available that self-report is more accurate than PABA.