Calcium bilirubinate at the Randall's plaque–kidney stone interface: first description

Background Randall's plaque formation is a very common initial step in the process of kidney stone formation. On morphological stereomicroscopic examination of kidney stones, red/orange deposits at the level of Randall's plaque or at its interface with the kidney stone can be observed in a minority of kidney stones. Methods We retrospectively reviewed our kidney stone database to determine the prevalence of red/orange deposits observed on morphological examination of kidney stones analysed between 1989 and 2023. When possible, analysis of the deposits was performed by scanning electron microscopy, energy-dispersive X-ray absorptiometry, and Fourier-transform infrared spectroscopy. Results Of 44 213 intact kidney stones upon which morpho-constitutional kidney stone analysis was performed between 1989 and 2023, 12 587 stones (28.5%) exhibited a Randall's plaque, of which 142 (1.1%) showed the presence of red/orange deposits at the Randall's plaque or at its interface with the kidney stone. Scanning electron microscopy of the deposits on two kidney stones displayed the deposits as sharp-edged, polygonal prismatic crystals, while Fourier-transform infrared spectroscopy of the deposits on one kidney stone revealed that their composition was calcium bilirubinate. Energy-dispersive X-ray absorptiometry of the deposits on one examined kidney stone demonstrated the absence of iron. Conclusions In a minority of Randall's plaques, red/orange deposits consisting of calcium bilirubinate can be detected. Further investigations are required to confirm whether their presence is a feature of an underlying hepatic or biliary disease.