LACK OF INCREASED URINARY CALCIUM-OXALATE SUPERSATURATION IN LONG-TERM KIDNEY-TRANSPLANT RECIPIENTS

Nephrolithiasis is uncommon after kidney transplantation. However, cal cium (Ca) supplementation, which has been proposed as a treatment of p ost-transplant ostropenia, might increase calcium and bolster Ca stone formation. Therefore, in 24-hour urine of 82 normocalcemic long-term renal transplant recipients (RT) and in 82 healthy subjects (HS). we a ssessed some Cn nephrolithiasis risk factors and the Ca-salt saturatio n estimated by the ion-activity product index (AP) and relative supers aturation (RS). In RT, calciuria was lower (mean +/- SD, 3.20 +/- 2.25 vs. 4.61 +/- 1.71 mmol/day: P < 0.001), urinary volume higher (2.41 /- 0.83 vs. 1.39 +/- 0.53 liter/day: P < 0.001), oxaluria higher (419 +/- 191 vs. 311 +/- 79 mu mol/day; P < 0.001) and citraturia lower (1. 40 +/- 1.36 vs. 3.77 +/- 1.36 mmol/day; P < 0.001) than in HS. As a re sult, Ca-oxalate supersaturation was lower in RT than HS (AP, 1.07 +/- 0.69 vs. 2.07 +/- 1.13, P < 0.001; and RS, 0.62 +/- 0.26 vs. 0.94 +/- 0.21, P < 0.001), and was similar in subgroups of RT (N = 37) and HS (N = 37) matched for urinary volume, demonstrating that even without a np larger urinary volume, Ca-oxalate saturation was not higher in RT t han HS, and suggesting that opposite changes in Ca and oxalate in RT l ikely canceled their effects on lithogenic risk. In RT which had simil ar urinary pH and phosphate (P) than HS, Ca-P supersaturation was lowe r than in HS fur brushite (AP, 3.25 +/- 6.67 vs. 6.01 +/- 4.85, P < 0. 001; RS, -0.33 +/- 0.76 vs. 0.48 +/- 0.53, P < 0.001) and octacalcium phosphate (RS, -0.95 +/- 0.72 vs. 0.21 +/- 0.85, P < 0.001), and simil ar for apatite. Finally, fasting calciuria and calciuric response to a single oral Ca load were similar in RT (N = 19) and HS (N = 8). Toget her, these results argue strongly against a higher risk of Ca stone fo rmation in RT than HS, even in case of Ca supplementation.