Crystal identification of synovial fluid aspiration by polarized light microscopy. An online test suggesting that our traditional rheumatologic competence needs renewed attention and training

Abstract

Testing a reading exercise for identification of several typical crystal such as the negatively birefringent needle-shaped crystals that are under polarized light microscopy is the gold standard for diagnosing gout. The objective of this study was to assess current performance of crystal identification by professionals involved in examining synovial fluid in routine care. Rheumatologists, trainees, lab technicians, and other physicians with an interest in crystal arthritis completed an online test. The test consisted of 30 images: 8 monosodium urate (MSU) crystals, 5 calcium pyrophosphate (CPP), 4 cholesterol, 2 depot methylprednisolone, 2 calcium oxalate, 2 rice bodies, 1 hydroxyapatite, 1 liquid lipid, 1 fibrin, 1 Charcot-Leyden, and 5 different artifacts. Of the 22 non-MSU slides, a subset of 8 was pre-designated that were thought to be clinically important to be identified as non-MSU. The primary outcome was defined as the correct identification of all eight MSU slides plus the identification of all eight pre-defined non-MSU slides as non-MSU. The online test was completed by 110 participants. The primary outcome was achieved by 39%. Correct identification of all MSU images was achieved by 81%, correct identification of all 8 pre-defined non-MSU, CPP images, and all 22 non-MSU images as non-MSU by 68, 68, and 23%, respectively. MSU crystals were well identified, but incorrect identification of non-MSU crystals occurred frequently. This study suggests that there is room for improvement regarding crystal identification of particularly CPP and other non-MSU crystals even in this highly motivated group.

The added value of synovial fluid centrifugation for monosodium urate and calcium pyrophosphate crystal detection

“…This was shown by Berendsen et al using an online test to assess the performance of crystal identification by 110 professionals [5]. However, it is unlikely that a different degree of experience between both observers could have influenced our results significantly, as the interobserver agreement for crystal recognition between both observers was sufficiently high before (kappa 0.82, p value 0.049) and even higher after centrifugation (kappa 0.92, p value 0.032).…”

“…MSU and CPP images were identified correctly by 81 and 68% professionals, respectively. Incorrect identification of non-MSU crystals occurred frequently, instead MSU was well identified [5]. Fourth, in SF with a low leukocyte count, the crystal load is also frequently low [6,7].…”

“…Third, correct microscopic identification of MSU and CPP crystals by professionals requires experience and is therefore no sinecure. Berendsen et al used an online test to assess the performance of crystal identification by 110 professionals involved in examining synovial fluid in routine care [5]. The test consisted of numerous images comprising the whole spectrum of crystals which can be identified at polarized light microscopy.…”

Gouty arthritis: decision-making following dual-energy CT scan in clinical practice, a retrospective analysis

“…Microscopic demonstration of MSU crystals in SF during an acute arthritis attack has sensitivity of 0.84 (95% CI, 0.77–0.92) and specificity of 0.99 to 1.00 [14, 15]. However, correct identification of crystals using polarized light microscopy in SF can be challenging [16]. …”

The American College of Physicians and the 2017 guideline for the management of acute and recurrent gout: treat to avoiding symptoms versus treat to target

“…This ambiguity reflects to the setting of the potential threat of a septic arthritis, versus a nontricky situation of inflammatory osteoarthritis, rheumatoid arthritis, or calcium pyrophosphate (CPP) arthritis. Microscopical evaluation of synovial fluid is the gold standard once monosodium urate crystals are seen, but is not 100% perfect, particularly if no crystals are found in the acute phase: ill identifications occurs with false negatives due to ill recognition/microscopisation, or false positives due to artifacts [7]. Further aids for the clinician have been made by the rheumatological societies with updated gout classification criteria, or by GPs in cooperation with a rheumatology centre with a simplified gout calculator [8,9], the latter being specifically meant for GPs.…”