Background/Purpose: The current golden standard in diagnosing gout and calcium pyrophosphate deposition is polarized light microscopy (PLM). However, small crystal sizes, the presence of other crystal types, glass artefacts and cellular uptake of MSU can pose uncertainties in the clinical analysis of synovial fluids. There are multiple methods to improve synovial fluid analysis, of which Raman spectroscopy is the most promising. In our approach, we have integrated an ordinary polarized light microscope with a Raman spectroscope (iRPolM, Hybriscan), allowing for simultaneous use of both techniques. In this abstract, we demonstrate the added value of this technique for the first 50 patients, including 23 diagnosed with gout.

Methods: 50 patients were punctured for diagnostic purposes when presenting with swollen joints. Patient characteristics are given in figure 1. Samples were analyzed by an experienced rheumatologist with polarization microscopy, who also gave an indication of certainty in diagnosis. The iRPolM analysis followed this. A visual representation of this method can be found in figure 2. Retrieved Raman spectra were analyzed by a trained Raman spectroscopist for the presence of MSU, calcium pyrophosphate dihydrate (CPPD) or other (deposited) materials.

Results: A summary of the results from this experiment can be found in figure 3. We demonstrate a high correspondence (95%) between a certain identification with PLM and iRPolM. While the rheumatologist is only able to identify MSU/CPPD, Raman can also discriminate other microparticles.

Hydroxyapatite crystals were found in 1 patient with RA and one patient with a degenerative shoulder syndrome. Calcite (calcium carbonate) could be identified in 17 patients, 4 of whom were diagnosed with gout, 4 with CPPD, 3 with Rheumatoid Arthritis, 2 with Osteoarthritis and 4 undefined.

Titanium dioxide crystals were found in 6 patients. Only one of 6 patients with titanium crystals had an orthopedic implant, there is no clear relation between the two.

Carotenoids, a precursor molecule for vitamin A, could be found in 4 patients, 1 of which was diagnosed with gout and 3 had an undefined joint disease. All thenardite (sodium sulphate) crystals could be found in patients with osteoarthritis. Microplastics could be found in 19 patients, but environmental pollution must be excluded.

Conclusion: Raman spectroscopy has a significant added value to PLM, as it can not only can discriminate between MSU and CPPD but also identify a whole range of other crystals which might be related to potentially novel crystallopathies. In the future crystal identification in rheumatology may well have to rely on Raman based diagnostics.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/raman-spectroscopy-integrated-with-polarized-light-microscopy-for-diagnosis-of-crystallopathies/