Pigment Versus Cholesterol Cholelithiasis: Identification and Quantification by Infrared Spectroscopy

We previously reported that 27% of 92 cholecystectomized patients had pigment stones (Am J Dig Dis 19:585–590, 1974). Using standard biochemical methods, we found that cholesterol accounted for an average of 77% of the dry weight of cholesterol stones, but that unconjugated bilirubin represented a mean of only 7% of pigment stones. This quantitation of pigment stones was limited because approximately 66% of their weight was insoluble. To characterize pigment and cholesterol stone composition further, we used infrared spectroscopy — a technique requiring neither crystallinity nor solubilization — to quantitate pigment, carbonate, and cholesterol in gallstones. Other organic and inorganic components of stones were measured by standard methods. By infrared spectroscopy, two types of pigment stones were identified: carbonate-containing and noncarbonate pigment stones. Carbonate pigment stones contained significantly more calcium, carbonate, and phosphate, but less pigment than noncarbonate stones. Compared to our initial report, the total measured components of all pigment stones were increased 6-fold from 10 to 63%. Cholesterol was the major component of cholesterol stones by chemical assay or infrared spectroscopy. Among five cholesterol stones with limited solubility, 80% of the insoluble residue was identified as cholesterol by infrared spectroscopy. This study extends our knowledge of pigment stone and cholesterol stone composition by the use of quantitative infrared spectroscopy in conjunction with standard biochemical methods; furthermore, it confirms that pigment and cholesterol stones differ in composition and form by different mechanisms.