Analysis of Crystals Leading to Joint Arthropathies by Raman Spectroscopy: Comparison with Compensated Polarized Imaging

Abstract

The current study assessed the feasibility of the application of Raman spectroscopy toward the diagnosis of gout and pseudogout. First, the lowest concentrations of monosodium urate monohydrate (MSUM) and calcium pyrophosphate dihydrate (CPPD) crystals detectable by Raman spectroscopy were investigated by mixing known amounts of synthetic crystals with synovial fluid in the concentration range of 1 to 100 μg/mL. Second, a digestion protocol was developed for clinical samples to improve crystal extraction. The ensuing centrifugation of the digest congregated crystals at a well-defined point and allowed for point-and-shoot Raman analysis without having to conduct an extensive search for individual crystals. Finally, synovial fluid samples obtained from patients ( n = 35) were cross-analyzed by polarized light microscopy (PLM) and the Raman method to compare and contrast the diagnoses of the two methods. It was found that Raman spectroscopy can detect MSUM and CPPD crystals with good sensitivity and specificity at concentrations as low as 5 μg/mL and 2.5 μg/mL, respectively, using the current method. This detection limit of Raman analysis is lower than that reported for PLM. Raman and PLM diagnoses of clinical samples agreed in 32 out of 35 samples in the entire sample pool. However, the rate of disagreement between PLM-based and Raman-based diagnoses was noteworthy within the subset of diseased samples (3 out of 10), indicating that PLM has limitations and that the confirmation by a secondary method is essential for a reliable outcome. The proposed protocol of sample preparation and Raman analysis ascribes baseline feasibility to the diagnosis of gout and pseudogout by Raman spectroscopy, thus justifying further studies using a larger clinical sample set for obtaining sensitivity and specificity.