31 Flow cytometry of skin provides valuable diagnostic information while representing an under-recognized paucicellular specimen type

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Abstract Background Cutaneous lymphomas remain notoriously difficult to diagnose despite integration of multiple ancillary studies, including PCR analysis of the gamma chain of the T cell receptor (TRG) for clonality. Although widely available, flow cytometry on skin samples is infrequently used in the evaluation of possible cutaneous lymphoma, in part due to high rates of non-diagnostic laboratory results reported in small studies. Systematic evaluation of this in large cohorts is lacking, however, and the role of paucicellularity–which represents an analytic challenge for any flow cytometry sample–is poorly understood in skin. Objective This study aims to assess the performance of flow cytometry in analyzing skin biopsies with a focus on paucicellularity. Methods Records of flow cytometry studies of the skin from a single institution were categorized by diagnosis (negative, suspicious, abnormal, or non-diagnostic), and, if abnormal/suspicious, by the population involved (T cell, B cell, plasma cell, or myeloid). The percentages of records including disclaimers related to a paucicellular specimen, possible blood contamination, and/or non-specific background staining were then calculated. For a subset of samples, flow cytometry diagnosis was compared to concurrent TRG PCR and final clinical diagnosis. Results We identified 424 flow cytometry samples from 379 patients analyzed between May 2000 and September 2024. No abnormal populations were identified in 198 (46.7%) samples, while populations of abnormal T cells were identified in 102 (24.1%) samples, abnormal B cells in 64 (15.1%) samples, and abnormal myeloid cells in 20 (4.7%) samples. Twenty (4.7%) samples were non-diagnostic. The remaining diagnoses each comprised <2%. Overall, 26.7% of samples had one or more disclaimers in the comment; 21.7% were paucicellular (including the 4.7% non-diagnostic samples), 4.7% had possible blood contamination, and 4.3% had high non-specific staining. Paucicellularity was more often reported in suspicious samples (46.7%) compared to negative (28.3%) and abnormal (4.7%) samples. The language used to report cellularity was highly variable. In a subset of 38 samples that had concurrent flow cytometry and TRG analyses, 25 (66%) had concordant results, despite 20% of these flow samples being paucicellular. The remaining 13 (34%) had discordant results. Flow cytometry studies agreed with the final clinical diagnosis in 7 (78%) of the 9 discordant samples for which there was clinical follow up, and the remaining 2 were paucicellular. Conclusions The vast majority (>90%) of flow cytometry studies of skin provides diagnostic information that complements existing ancillary studies (e.g. molecular clonality), and its use should be considered more frequently in the evaluation of cutaneous lymphomas. Given that a substantial number of samples included disclaimers for paucicellularity, skin is an under-recognized paucicellular specimen type for flow cytometry that should be included in ongoing efforts for standardization of paucicellular specimen analysis and reporting.

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