Thyroid fine-needle aspiration biopsy (FNAB) is widely used for thyroid nodule characterization, with approximately 2.7% of samples classified as "inadequate." Non-diagnostic samples pose limitations, resulting in repeated procedures, and unnecessary diagnostic thyroidectomies. Conventional smear (CS) is commonly the method of choice for cytologic preparation of thyroid FNAB. The cell block technique is an alternative that concentrates cells providing additional material for better evaluation and ancillary testing. While conventional smears are commonly used, introducing routine complementary cell blocks could potentially lower costs associated with repeat procedures and improve patient management.

The study aimed to investigate the diagnostic value of incorporating the cell block technique as adjunct to conventional smear technique in reducing nondiagnostic rates (Bethesda Category I) in thyroid-fine needle aspiration biopsies (FNAB) conducted in 2 private hospitals.

This is a multi-center, retrospective cross-sectional study with 701 samples from 528 adult patients, who underwent thyroid FNAB between January 2020 - September 2022. The primary outcome of interest is the reduction in non-diagnostic rates with the combined use of conventional smears and cell block.

The non-diagnostic rates were significantly higher with cell block technique (28.10%) as compared to conventional smears (16.26%), p-value < .01. The results show that conventional smears have lower non-diagnostic rates. With smear cytology alone, 114 (16.3%) of all samples were nondiagnostic. With the addition of cell block technique, 15 of these samples were reclassified as benign (n = 13), Bethesda III (n = 1) or Bethesda IV (n = 1). The rest of the non-diagnostic samples (n = 99) remained Bethesda I. Overall, the equivalent decrease in non-diagnostic rate was 2.1%.

The combined use of cell block and conventional smears did not significantly decrease nondiagnostic rates in thyroid FNAB. In general, conventional smears demonstrated superior diagnostic efficacy across all Bethesda categories, establishing it as the preferred sampling preparation method for thyroid FNAB. Cell blocks should be considered a supplementary technique, particularly in cases where ancillary methods like immunohistochemistry or molecular testing are needed.