SHORT COMMUNICATION The cytospin technique improves the detection of calcium pyrophosphate crystals in synovial fluid samples with a low leukocyte count Christoph Robier • Franz Quehenberger • Manfred Neubauer • Mariana Stettin • Franz Rainer Received: 8 December 2012 / Accepted: 28 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013 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 Wilco- xon 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 enhan- ces the number of examinable crystals per HPF, compared to common smears. Keywords Calcium pyrophosphate Á Synovial fluid Á Cytocentrifuge Á Cytospin Introduction The diagnosis of crystal-related arthropathies is based on the detection of crystals in the synovial fluids (SF) of the affected joints. Thereby, polarized microscopy of fresh SF smears is still regarded as the gold standard by many lab- oratories [1]. However, in several situations, such as in SF with low leukocyte or/and crystal counts, important mor- phological features, which are crucial for the correct diagnosis, may be missed if exclusively smears are examined. In particular, in calcium pyrophosphate (CPP) crystal-related disorders [2], in which a varying and often low concentration of CPP crystals is frequently observed [1], the crystals may be easier overseen than monosodium urate (MSU) crystals in patients with gout [1, 3]. This limitation may be overcome by cytoconcentration using the cytospin technique. Cytocentrifuges (CC) use low-speed centrifugal force to place a monolayer of cells onto a defined area of a glass slide and thus enable the analyst to examine a higher number of cells per high power field (HPF), compared to common smears. Cytocentrifuges spin at lower speeds and show a more gradual way of acceler- ation and deceleration than normal centrifuges. The tech- nique provides thin-layer cell preparations from any liquid materials, especially from hypocellular fluids such as SF, cerebrospinal fluid, aszites, or pleural effusions. Based on the method’s technical background, we hypothesized that CPP crystals, which are mostly found within the cytoplasm of SF macrophages, but also extracellular [4], should be easier detected in cytospin preparations. The aim of this study was to compare the CPP crystal counts in CC C. Robier (&) Á M. Neubauer Á M. Stettin Á F. Rainer Central Laboratory, Department of Internal Medicine, Hospital Barmherzige Brueder Graz-Eggenberg, Bergstrasse 27, 8020 Graz, Austria e-mail: christoph.robier@bbegg.at F. Quehenberger Institute of Medical Informatics, Statistics and Documentation, Medical University of Graz, Graz, Austria 123 Rheumatol Int DOI 10.1007/s00296-013-2689-0