A proposed method for approximate estimates of the ion-activity products of calcium oxalate and calcium phosphate in spot-urine samples or in urine samples collected during less well defined periods of time

Abstract

The purpose of this study is to derive approximate estimates of ion-activity products of CaOx (AP_CaOx) and CaP (AP_CaP) useful for spot urine or other less well defined short-term urine collections. In accordance with previously applied and described principles for estimating ion-activity products, the intention was to derive simplified estimates of AP(CaOx)_CONC and AP(CaP)_CONC by using urine concentrations of the most important determinants of AP_CaOx and AP_CaP. A comparison thus was made between estimates derived in that manner and AP(CaOx) index and AP(CaP) index obtained from calculations based on analysis of 24 h urine samples. The best fit between estimates of AP(CaOx) index in 24 h urine and AP(CaOx) index_CONC was obtained with the following formula (r = 0.99; p = 0.0000): ${\text{AP(CaOx)index}}_{\text{CONC}} = \frac{{2.09 \times c{Ca}^{0.84} \times c{Ox} }}{{c{Cit}^{0.22} \times c{Mg}^{0.12}}}.$ The corresponding formula for AP (CaP) index_CONC was derived from comparison with the corresponding calculations of AP(CaP) index in 24 h urine (r = 0.91; p = 0.0000): ${\text{AP(CaP)index}}_{\text{CONC}} = \frac{{0.432 \times c{Ca}^{1.07} \times c{P}^{0.70} \times ({pH} - 4.5)^{6.8}}}{{c{Cit}^{0.20} }}.$ The proposed simplified formulas enable calculation of approximate estimates of AP_CaOx and AP_CaP in spot-urine samples or any kind of urine collection for which duration of the collection period is less well known and by using the concentrations (c) of the variables in mmol/L in the two formulas.