Salivary levels of phosphorus and urea as indices of their plasma levels in nephropathic patients.

Abstract

Phosphorus and urea are measurable in saliva. Measurements of saliva phosphorus (S-Pho) and saliva urea (S-Urea) could be useful because of low invasivity. Data are limited to saliva tests methodology and to correlations between plasma and saliva compositions. S-Pho and S-Urea were investigated focusing on blind duplicates, differences between collection sites, differences between collection times, freezing-thawing effects, and plasma-saliva correlations. Tests were performed using fresh saliva collected by synthetic swap early morning after overnight fast (standard). Methodology was investigated in fifteen healthy volunteers. Plasma-saliva correlations were investigated in thirty nephropathic outpatients. S-Pho and S-Urea in all measurements ranged above detection limits (0.3mmol/L). In healthy volunteers, S-Pho and S-Urea were similar in duplicates (results for S-Pho and S-Urea: % difference between samples≤4.85%; R between samples≥.976, P<.001), in samples from different mouth sites (≤4.24%; R≥.887, P<.001), and in samples of different days (≤5.61%; R≥.606, P<.01) but, compared to standard, were substantially lower in after-breakfast samples (-28.0% and -21.3%; R≥.786, P<.001) and slightly lower in frozen-thawed samples (-12.4% and -5.92%; R≥.742, P<.001). In nephropathic patients, S-Pho was higher than but correlated with plasma phosphorus (saliva/plasma ratio 4.80; R=.686, P<.001), whereas S-Urea and plasma urea were similar and correlated with each other (saliva/plasma ratio 0.96; R=.944, P<.001). Post-dialysis changes in S-Pho and S-Urea paralleled post-dialysis changes in plasma phosphorus and urea. S-Pho and S-Urea reflect plasma phosphorus and plasma urea. Early morning fasting fresh samples are advisable because collection time and freezing-thawing affect saliva tests.