Agglomeration inhibition reflected stone-forming activity during long–term potassium citrate therapy in calcium stone formers

Abstract

Objectives. The agglomeration of preformed crystals of calcium oxalate has been hypothesized to be the rate-limiting step in renal stone-forming activity (SFA). The effect of urine on the in vitro inhibition of agglomeration of seed crystals of calcium oxalate monohydrate, designated [tm], has been used to monitor SFA in calcium oxalate stone formers (CaOxSF). The objective of the present study was to determine whether [tm] could be used to help monitor the long-term effectiveness of oral potassium citrate therapy (K-Cit-Rx) in CaOxSF. Methods. Clinic and radiographic (or ultrasound) reports were evaluated for 80 patients, aged 20 to 72 years, 55 men and 25 women, who were treated with oral K-Cit for recurrent calcium oxalate urolithiasis at the Ochsner Stone Clinic between January 1992 and July 1996. Seventy-five of these patients had at least one 24-hour citrate excretion rate of less than 3.0 mm/day before or after K-Cit-Rx. SFA graded on a scale of −2 to +2 by radiographic criteria was combined with information on stone passage to evaluate clinical stone status, and 24-hour urine collections were evaluated for volume, pH, calcium, citrate, uric acid, oxalate, creatinine, and [tm] on free diet before and after 6 to 53 months of K-Cit-Rx. Historical information on procedures performed for urolithiasis before and on K-Cit-Rx was also reviewed. Results. K-Cit-Rx resulted in increased urine pH ( P <0.0001) and decreased calcium ( P = 0.0475), [tm] ( P = 0.0045), number of stones passed per year ( P = 0.0016), and remedial procedures per year ( P <0.0001). Patients taking allopurinol in addition to K-Cit required higher doses ( P <0.0001) of K-Cit to control their disease, had lower pretreatment urine pH ( P = 0.0493), and showed greater increase in urine citrate ( P = 0.0092) than those on K-Cit alone. Those taking high-dose K-Cit were younger ( P = 0.0363) and showed greater decrease in SFA ( P = 0.0005) than those taking lower doses. A small group of 10 medication refractory patients, who retained (n = 9) or increased (n = 1) their stone burden during K-Cit-Rx, was identified. Compared with the medication-responsive group, the refractory patients were older ( P = 0.0124), and had greatly increased SFA ( P <0.0001) and higher ( P = 0.0347) urine pH before and during ( P = 0.0173) treatment (data not shown). Conclusions. The data confirm that [tm] can be used not only to verify previously documented stone formation rate but also to help evaluate the long-term effectiveness of therapy. In this report, changes in [tm] after K-Cit-Rx reflected decreased stone formation rate and decreased remedial procedures.