Introduction: Thyroid fine-needle aspiration (FNA) is a well-established technique in the cytology literature. Through the introduction of rapid stains in cytology practice, the ever-increasing utility of rapid on-site evaluation (ROSE) has strengthened the place of FNA as a primary diagnostic method in patient management. There are few stain variants available in the market for ROSE, namely Diff-Quik (DQ), Toluidine blue, and ultrafast Papanicolaou stains. Recently, our group developed a new staining variant labeled as original “BlueStain®” technique that was not previously tested in this context. Methods: 40 FNA thyroid cases were studied. At least two slides were prepared from each patient: one stained by DQ and the other by BlueStain®. Simultaneously, a ROSE diagnosis was performed as the two staining methods were compared, evaluating the parameters of background, cellularity, details of colloid presence, cell morphology, nuclear details, cytoplasmic details, and overall staining, scored on a scale from 1 to 3, representing poor, average, and good, respectively. Results: The quality index was slightly better for BlueStain® (53% vs. 47%) but not significantly different between the two stains. BlueStain® provides better details in both the presence and type of colloid as well as nuclear details, which are regarded as very important for diagnosis in thyroid cytology. There were eight cases with discordant diagnosis when compared between two stains from the same patient. In five cases of indeterminate cases, BlueStain® allows to bring them to the benign category, probably because this staining method allows a clear observation of the colloid in the background of the smears. However, since we are observing two different slides, we cannot rule out that these differences are a question of sample collecting and/or smearing. Conclusions: Our data demonstrates that BlueStain® is suitable to provide good-quality slides for primary assessments of thyroid aspirates studied by ROSE. In fact, in some aspects, this new staining method shows better preservation of colloid and cell details, revealing itself as an alternative to the DQ stain variant, upholding performance level while being 10 times cheaper and simpler because it requires just one step of staining.

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