Pathogenesis of ceftriaxone-associated biliary sludge. In vitro studies of calcium-ceftriaxone binding and solubility. Ceftriaxone, a semisynthetic third-generation cephalosporin, has recently been associated with biliary sludge formation. Analysis of the biliary concretions induced by this agent shows a calcium salt of ceftriaxone. The present in vitro studies were undertaken to provide insight into the pathogenesis of ceftriaxone-associated biliary sludge formation by evaluating possible interactions that may exist between calcium, bile salts, and ceftriaxone. Ceftriaxone possessed high calcium-binding affinity. The formation constant for the calcium ceftriaxone salt at 37 degrees C was about 157.3 L/mol; stoichiometry of the salt was 1:1, i.e., calcium ceftriaxone. The calcium-binding property of ceftriaxone was observed to be additive to that of taurocholate in mixed taurocholate-ceftriaxone solutions. Although the solubility product constant for calcium ceftriaxone was only 1.62 x 10(-6) mol/L2, marked metastability was observed; neither visible nor microscopic precipitates developed until the [Ca2+] x [ceftriaxone] ion product exceeded the solubility product constant by a factor of 10.4. Metastability of the calcium ceftriaxone salt was also observed in human gallbladder bile in vitro. Estimates of human biliary calcium ceftriaxone solubility in vivo were than calculated from previously-reported values for biliary [Ca2+], [ceftriaxone], and from the solubility product constant as defined in this study. Calculated saturation indices for calcium-ceftriaxone in human bile generally increased (corresponding to a decrease in solubility) with increasing ceftriaxone dose. At doses less than or equal to 1 g, saturation index was well within the metastable range of this calcium-salt. However, at doses greater than or equal to 2 g, the saturation index surpassed the metastable limit. Under these conditions, precipitation of ceftriaxone could occur. It was concluded that the development of ceftriaxone-induced biliary sludge is a solubility problem that occurs in patients receiving high-dose treatment (greater than or equal to 2 g). This study proposes that the risk of developing ceftriaxone-associated biliary "pseudolithiasis" increases with increasing ceftriaxone dose and in patients with impaired gallbladder emptying. 