Classification of gallstones using Fourier-transform infrared spectroscopy and photography

Gallstones have conventionally been classified by gross inspection into 4 categories: cholesterol gallstones, black pigment (calcium bilirubinate) gallstones, brown gallstones, and mixed gallstones that contain both cholesterol and calcium bilirubinate. Classification using Fourier-transform infrared (FT-IR) spectroscopy supplements gross inspection; however, the issue of ambiguity in gallstone classification has not been fully addressed to date.

Twenty-six gallstones obtained after surgical gallbladder removal were examined using FT-IR spectroscopy and digital photography, and classified into 6 gallstone groups according to characteristic FT-IR absorption bands.

FT-IR spectra of nine gallstones matched well with that of pure cholesterol, and the gallstones were thus classified as cholesterol stones. Twelve gallstones were classified as calcium bilirubinate stones as they showed characteristic absorption bands of calcium bilirubinate. However, the FT-IR spectra of these gallstones always showed a broad absorption band of bound water at 3600-2400 cm^- 1. The other five gallstones were classified as mixed stones with combinations of cholesterol, calcium bilirubinate, and calcium carbonate.

FT-IR spectroscopy is a powerful and convenient method for gallstone classification. Nevertheless, one should take serious note of the superposition of FT-IR absorption bands of different chemical components of gallstones including that of bound water.