Cell block cellularity: A comparison of two fixatives and their impact on cellularity

Background Ancillary testing including immunohistochemistry and molecular diagnostics has become an increasingly important component for the evaluation of cytologic specimens. Ancillary testing is important not only for diagnosis but also for predictive and prognostic evaluation. While a number of substrates are appropriate for ancillary testing, cell block specimens are commonly utilized and the success of ancillary testing depends on cell‐block cellularity. Methods Forty‐six pairs of cases each fixed in both formalin and CytoLyt were each analyzed by two evaluators for overall cellularity. Linear regression was used to assess inter‐rater reliability of cell counts for each method. Cellularity scores for each case were obtained by averaging the scores for each rater and cellularity was compared between the methods. Results Inter‐rater agreement was very good for both methods. The coefficient of determination was 1.0 and 0.99 for the CytoLyt and formalin methods respectively. Cell blocks using the CytoLyt method have lower levels of cellularity than cell blocks performed by the formalin method. Conclusions Cell blocks prepared using a formalin fixative yield significantly greater cellularity than those produced by the CytoLyt method. Formalin fixation appears to optimize cellularity of cell blocks useful for ancillary testing.