The cytospin technique improves the detection of calcium pyrophosphate crystals in synovial fluid samples with a low leukocyte count

Abstract

In synovial fluids (SF) with low leukocyte or/and crystal counts, important features may be missed, if exclusively smears are examined by polarized microscopy. That may be overcome by cytocentrifuges, which use low-speed centrifugal force to concentrate cells onto a glass slide and thus enhance the number of cells per high power field (HPF). We compared the calcium pyrophosphate (CPP) crystal counts in cytospin preparations with those in common smears of SF. The number of CPP crystals was counted in 50 SF samples by polarized microscopy, and statistical comparisons of the mean values of the cytospin and smear preparations were performed using the Wilcoxon test. The reproducibility within the slides of the cytocentrifuge and smear samples was determined by Spearman's rank correlation. The crystal counts were significantly higher in the cytospin than in the smear preparations (median 96/10 HPF vs. 2.5/10 HPF, p < 0.0001). The correlation in the crystal count between the slides 1 and 2 was significantly higher within the cytocentrifuge than in the smear group (0.97 vs. 0.73, p = 0.0004). CPP-negative cytospin preparations in initially smear-positive slides were not observed. We confirmed that the cytospin technique significantly enhances the number of examinable crystals per HPF, compared to common smears.

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

“…This is reinforced by the knowledge that CPP crystals are harder to detect than MSU crystals, because of their overall lower concentration and smaller, variable and often non-birefringent appearance [1][2][3]. Several previous studies by Robier et al supported our view that a form of centrifugation, either their cytospin (cytocentrifugation) or the technique we used, could be useful [6,7,9]. In an outpatient clinic setting, a conventional centrifuge would probably be more desirable than the cytospin because of its wide availability, lower cost, and lack of need for trained personal.…”

“…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]. This results in lower sensitivity of microscopic examination for CPP (12-67%) compared to MSU (78-79%) crystals [2,4,8].…”

“…However, in accordance with the recommendations regarding the cytospin used by Robier et al, it is likely that this technique is mainly beneficial in selected patients. As centrifugation is more expensive, time consuming, and labor intensive that usual care, it should be preserved for situations when the smear is (repeatedly) negative for crystals while the suspicion of crystal-induced arthritis remains high, the SF is hypocellular, microscopy is postponed to a later date, or when there is a low grade of experience by the physician [6,7,10].…”

“…With this technique (monolayers of), leukocytes and crystals are placed in increasing numbers onto a glass slide. As a result of this, crystals may be identified easier [6,7,[9][10][11]. However, an important disadvantage of this method is that it is more time consuming and expensive than polarized light microscopy of SF smears.…”

“…However, these techniques are very complex and impractical [4]. An alternative is to stain and cytocentrifuge SF (cytospin) as suggested by Robier et al [6,7,9,10]. With this technique (monolayers of), leukocytes and crystals are placed in increasing numbers onto a glass slide.…”