Tubular proteinuria in renal calcium stone formers

The incidence of tubular proteinuria measured as urinary excretion of beta 2-microglobulin was evaluated in 68 consecutive renal calcium stone formers, 14 of whom had impaired urinary acidification capacity. During normal conditions 13% of these stone formers had tubular proteinuria. There was no statistical difference in the incidence of tubular proteinuria between subgroups with proximal or distal acidification defects. During transient metabolic acidosis induced by an ammonium chloride load a pronounced increase in beta 2-microglobulin excretion was noticed in all patient groups but especially in those with urinary acidification defects. In stone formers with normal acidification capacity the degree of acidosis-induced beta 2-microglobulinuria was moderate but a small group of patients exhibited a large urine excretion of beta 2-microglobulin. This finding may reflect a latent tubular defect of importance for stone genesis.