Point of service testing may improve accuracy, reduce future hospital admissions in gout, pseudogout patients

Gout is a chronic disease that involves painful joint swelling and redness that often strikes joints in the lower extremities including ankles, knees and feet, especially the big toe. Gout occurs when excess uric acid (a normal waste product) collects in the body, and needle‐like urate crystals deposit in the joints. This may happen because uric acid production increases or excretion is reduced, most often if the kidneys cannot remove uric acid from the body well enough.

"The objective of this study was to demonstrate the usefulness of a shoebox sized point of service clinical grade RS instrument in speeding the time to clinical diagnosis of gout and pseudogout while maintaining the accuracy comparable to that attained by microscopic analysis if synovial aspirates," said Nora Singer, MD, professor of rheumatology at Case Western Reserve University School of Medicine.

The diagnosis of gout or pseudogout hinges upon the identification of monosodium urate (MSU) crystals or calcium pyrophosphate dihydrate (CPPD) crystals, respectively, in synovial fluid aspirates from affected joints. Prompt recognition of synovial fluid crystals using polarizing light microscopy (PLM) depends upon a skilled observer's identifying negatively and/or positively birefringent crystals. Rheumatologists and pathologists routinely examine fluid for crystals using PLM-, however this task is complex, and appropriate personnel to perform and interpret PLM are not always available in urgent cares, ERs and community healthcare settings where patient may present acutely. Failure to promptly diagnose crystalline arthritis results in delay in treatment and even potential hospital admission due to uncertainty about diagnosis.

Investigators developed a desktop instrument specifically for identifying MSU and CPPD crystals to perform RS on synovial fluids. The instrument was used to analyze eighty synovial fluid samples.

MSU crystals were detected by PLM in 19/80 and CPPD crystals in 10/80 samples. Initial measurement using RS detected crystals in 26/29 samples with a sensitivity of 89.7 percent and a specificity of 100 percent. In 80 samples, 2 MSU and 1 CPPD sample were identified by PLM but missed by RS and 3 CPPD containing samples were identified by RS but missed by PLM.

"The value in benchtop RS lies in the ability to quickly and accurately detect the presence of SF MSU and CPPD crystals. Identification of SF MSU as evidence of gout flare facilitates prompt treatment with outpatient follow-up. Limitations of RS include this technique cannot confirm CPPD crystals are intracellular yet, as is required for the diagnosis of pseudogout. However in our hands, RS was at least as sensitive as PLM for crystal detection. Our results suggest that RS used at POS for synovial fluid crystal detection could help guide initiation of targeted outpatient therapy and potentially reduce the need for inpatient admission in patients with joint effusion, in whom diagnosis might otherwise be uncertain. This could potentially improve use of inpatient resources and the overall quality of patient care and should now be tested in a clinical trial" said Dr. Singer.