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Similar calcium pyrophosphate crystal detection rate found between electron, polarized light microscopy
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In patients with knee osteoarthritis, there was a high concordance between scanning electron microscopy and compensated polarized light microscopy in detecting calcium pyrophosphate crystals in synovial fluid, according to study findings. However, there was a low concordance between scanning electron microscopy and alizarin red staining in detecting basic calcium phosphate crystals.
“The [synovial fluid] SF analysis is one of the most important diagnostic tests in rheumatology,” Paola Frallonardo, MD, PhD, from the rheumatology unit in the Department of Medicine at University of Padua in Italy, and colleagues wrote. “It allows [us] to determine the degree of inflammation and permits, through the identification of monosodium urate and [calcium pyrophosphate] CPP crystals, an immediate diagnosis of gout and CPP crystal deposition disease, respectively.”
The most common type of calcium-containing crystals in osteoarthritis (OA) are CPP— which has been detected in up to 60% of SF samples in patients with knee OA — and basic calcium phosphate (BCP) — which has been detected in 20% of samples, the researchers wrote. Compensated polarized light microscopy (CPLM) is the routine method of detection for CPP crystals, while alizarin red staining is the routine method for BCP.
In this study, the researchers compared these two methods with scanning electron microscopy (SEM) coupled with X-ray energy dispersive spectroscopy (ESD).
They included 74 patients with knee OA. Researchers collected synovial fluid samples after arthrocentesis and examined for each crystal type using CPLM and alizarin red. Afterward, they assessed all samples through SEM. They determined concordance by the Cohen agreement coefficient.
Through SEM, they found CPP crystals in 31.1% of OA patients and found BCP crystals in 17.5% of patients. They found CPP crystals in 31.1% of patients through CPLM and BCP crystals in 36.4% of patients through alizarin red. According to the Cohen coefficient, there was a 0.83 concordance for CPP identification and a 0.68 concordance for BCP identification.
The researchers concluded SEM should not be used in the clinical setting to identify BCP.
“The use of SEM for the identification of BCP crystals in SF remains, at least for the moment, restricted to the research environment,” the researchers wrote. “On the other hand, some efforts could be made to improve the reliability of alizarin red staining through the study of red precipitates’ physical and birefringent properties on a large number of OA SF samples.” – by Will Offit
Reference: The researchers report no relevant financial disclosures.
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