Introduction: Fine needle aspiration biopsy (FNAB) is the main examination in diagnostics of thyroid nodules. Yet, it does not allow the differentiation of follicular lesions (FL). A new classification of thyroid biopsy results (newC) might improve pre-operative diagnostics mainly due to the introduction of a new subcategory of ‘follicular lesion of undetermined significance’, as well as the clarification of definitions of other categories. The aim of this study was to analyse the impact of the newC on the distribution of the cytological results among main diagnostic categories and on the values of the parameters describing the effectiveness of FNAB in detecting malignancy.

Material and methods: 34,371 FNABs outcomes and 3,557 results of surgical follow up were analysed. The frequencies of five main diagnostic categories in two periods: before (years: 2000–2009, 24,271 FNAB) and after the newC had been introduced (years: 2010–2012, 10,100 FNAB) were compared, as well as parameters describing the biopsy effectiveness, with a special focus on FL.

Results: In the second analysed period, the frequency of the ‘suspicious for malignancy’ results was three times lower, while diagnoses of ‘malignant neoplasm’ were 33% more frequent and the results corresponding to FL decreased by 10%. In respect to FL, specificity of FNAB increased (22.6% vs. 57.4%), but sensitivity decreased (78.8% vs. 50.0%). In respect to other diagnostic categories considered jointly, the sensitivity of FNAB (61.2% vs. 85.7%) and PPV (57.4% vs. 77.8%) increased, and the specificity was similar (95-97%) for both periods. NPV exceeded 95% irrespective of applied classification.

Conclusions: Separation of FL with low risk of malignancy affects the values of the parameters describing the effectiveness of FNAB. The improved diagnostic effectiveness of FNAB is also a consequence of epidemiological changes and better selection of thyroid lesions for biopsy. (Endokrynol Pol 2014; 65 (6): 431–437)