Measurement of inhibitors of calcium phosphate precipitation in plasma ultrafiltrate.

Two methods are described to measure quantitatively the inhibitory activity of calcium phosphate precipitation in undiluted plasma ultrafiltrates. The first, called the "seed test," is based on the determination of the amount of apatite needed to induce the precipitation of calcium phosphate from the ultrafiltrate that previously had been brought to a constant [Ca X Pi] supersaturation. The second, called the "supersaturation test," is based on the determination of the minimum [Ca X Pi] product necessary to induce the precipitation of calcium phosphate, care being taken to eliminate the influence of chelators. With both techniques pyrophosphate was found to be the most effective inhibitor, magnesium and citrate being less active. At the physiological concentrations present in plasma ultrafiltrate, magnesium represented the greatest part of the activity in the seed test, while pyrophosphate was the most important in the supersaturation test. However, the three known inhibitors represented only part of the total inhibitory activity. In a pilot study, patients with renal failure showed an increase in inhibitory activity both in the known and unknown fractions.