Correspondence Extraneous epithelial cells from thromboplastin in cell blocks DOI:10.1111/cyt.12129 Dear Editor, In Australia, the most common method for making cell blocks is the thrombin–plasma clot technique. 1 The source of the thrombin reagent is variable between laboratories. We investigated the source of extraneous epithelial cells discovered in a cell block of an index case. The thromboplastin Sim- plastin (Biomerieux Cat. No. 235124; bioM erieux - Australia Pty. Ltd., Baulkham Hills, NSW, Australia) contains well-preserved nucleated epithelial cells; it is derived from rabbit brain and lung. Previously, we used thromboplastin Simplastin Excel S (Biomerieux Cat. No. 43-02325), which is derived from rabbit brain only, and is acellular. Although cells in the thromboplastin do not affect the haematology coag- ulation assays for which it is manufactured, these extraneous cells are a potential source of interpreta- tion error in thrombin cell blocks. The index case which alerted us to investigate extraneous cells in a cell block was a cerebrospinal fluid (CSF) from a 17-year-old girl who had had a pleomorphic xanthoastrocytoma resected 1 month earlier. Cytospin slides were of moderate cellularity and showed no malignant cells (Figure 1a). As a result of the large volume of CSF, a cell block was prepared from centrifuged material in order to enable immunohistochemical staining if required. The cell block showed abundant benign epithelial cells (Figure 1b). This type and quantity of cell was unexpected in CSF and was discordant with the cy- tospin slides, which did not contain epithelial cells. The cells were nucleated and abundant, which was inconsistent with incidentally shed skin squames as the source. The thromboplastin and plasma controls used for cell block preparation were expired stock donated by our haematology department, used for prothrombin time determinations: Simplastin (Biomerieux Cat. No. 235124), Simplastin Excel S (Biomerieux Cat. No. 43- 02325) and Biomerieux Verify 1 (Figure 2). All of the commercial lyophilized reagents were reconstituted using sterile distilled water. Blank cell blocks were prepared by mixing three drops of plasma with three drops of thromboplastin in a disposable 10-ml centri- fuge tube. Samples of the reconstituted reagents were cytocentrifuged, air dried and stained with Diff-Quik ® as a quality control procedure. 2 Cell blocks prepared using Simplastin Excel S gave sections that appeared homogeneously eosinophilic with a suggestion of thin fibres. Cell blocks prepared from Simplastin had a background similar to Simpla- stin Excel S, but also contained nucleated epithelial cells (Figure 2). These were positive for cytokeratin (AE1/AE3) 3 and thyroid transcription factor 1 (TTF1) on immunostaining. Cytospin preparations of Simplastin showed aggregates of nucleated epithelial cells. Simplastin Excel S and plasma were acellular. Data sheets were obtained for the thrombin reagents. Simplastin Excel S (Biomerieux Cat. No. 43-02325) contained tissue thromboplastin reagent from rabbit brain, but Simplastin (Biomerieux Cat. No. 235124) contained tissue thromboplastin from rabbit brain and lung. In the Royal College of Pathologists of Australasia Quality Assurance Program 2009 questionnaire (a) (b) Figure 1. (a) Cytospin slide from cerebrospinal fluid of index case: benign monocytes (Diff-Quik 9400). (b) Cell block from the same case showing nucleated epithelial cells (haematoxylin and eosin 9400). © 2014 John Wiley & Sons Ltd Cytopathology 2014, 25, 412–421 Correspondence: A. F. Henwood, Histopathology Department, The Children’s Hospital at Westmead, Locked Bag 4001, Westmead 2145, NSW, Australia Tel.: +612 9845 3306; Fax: +612 9845 3318; E-mail: tony.henwood@health.nsw.gov.au 412