Urinary turbidity as a marker of crystallization: is spectrophotometric assessment useful?

We propose a simple and inexpensive in vitro crystallization assay of measuring turbidity by spectrophotometry in synthetic urine. We validated our method by investigating the effect of potassium (K) citrate on the crystallization of calcium oxalate monohydrate (CaOx), calcium phosphate, and magnesium ammonium phosphate using synthetic urine.

The crystallization of CaOx was studied using turbidimetric measurements of solution produced by mixing calcium chloride and sodium oxalate at 37 °C, pH 5.7. The turbidity of the crystal suspension was measured immediately with double-beam spectrophotometer as the absorbance of light at 660 nm wavelength. The rates of crystal formation and aggregation were obtained by measuring optical density (OD) over 30 min. The obtained results were compared to CaOx crystal concentration with and without citrate assessed by optical microscopy.

The sensitivity of spectrophotometry in measuring turbidity was confirmed by the linear correlation between the crystal concentration and OD readings at 660 nm seen on the standard curve. Under similar experimental conditions, the results were comparable to the ones obtained by optical microscopy. The OD readings over 30 min revealed an instant decrease in the number of crystals, with maximum aggregation noted at 18 min. Addition of K-citrate at 1.25 mg/ml led to initial less crystal formation (OD = 0.236 nm vs. OD = 0.527 nm), with a maximum aggregation reached at 18 min. Overall, citrate addition decreased nucleation with a small change in the aggregation (OD = 0.316 vs. OD = 0.359).

Spectrophotometric measurement of urinary turbidity is feasible and sensitive in assessing the potential clinical usefulness of different medications in inhibiting crystallization in urine.