Crystalluria: prevalence, different types of crystals and the role of infrared spectroscopy

Background: Studies on the frequency of the different types of urinary crystals and the role of Fourier transform infrared microspectroscopy (FTIRM) for identification are few. We describe the results of a retrospective study on the prevalence and typology of crystalluria and on the role of FTIRM. Methods: Urinary crystals were identified using the combined knowledge of crystal morphology, birefringence features and urine pH (combined approach). When this was inconclusive, FTIRM was performed. Results: Crystalluria was found in 807 out of 9834 samples (8.2%). In 793, the combined approach identified "typical'' crystals, while in 14 FTIRM was needed to identify "atypical'' crystals. Among "typical crystals'', calcium oxalate (75.9%), uric acid (25.9%) and amorphous urates (7.9%), alone or in combination, were the most frequent. Brushite, ammonium biurate and cystine were the most rare (0.1%-0.7%). FTIRM identified 12 of 14 atypical crystals: three crystals were due to a drug (amoxicillin, indinavir, doubtful phenytoloxamine); four were due to calcium oxalate mono-or bihydrate, uric acid bihydrate or struvite; five were due to calcium carbonate, Tamm-Horsfall glycoprotein, or rare salt combinations. Conclusions: Crystalluria is not rare and most crystals can be identified by the combined approach. Occasionally, identification of crystals will require FTIRM.