Introduction: In the evaluation of molecular markers in washouts from fine-needle aspiration biopsy (FNAB) the extremely small amount of material can be a major problem. Some authors tried to use washouts from core-needle aspiration biopsy (CNABs) to gain more material from larger needles. However, according to some studies, CNAB samples are commonly contaminated with blood. The aim of our study was to evaluate the proportion of nucleic acids from thyroid cells in washouts from FNAB and CNAB by measuring the relative expression of cytokeratin 17 (KRT17) on the mRNA level.

Material and methods: Relative expression of KRT17 and GADPH (reference gene) in washouts from FNAB and CNAB was measured using real-time PCR technique and compared to the results from surgical specimens.

Results: Surgical specimens form 22 nodules, FNAB samples from 20 lesions and CNAB samples from 24 lesions were analysed. The median difference in cycle threshold (Ct) between FNAB samples and surgical specimens was 3.3 (p = 0.047). In CNAB samples KRT17 was undetectable in most cases (median incalculable; proportion of samples with undetectable KRT17 significantly higher than in FNAB samples).

Conclusions: Samples obtained with different biopsy techniques had different proportions of contents. The proportionally low content of epithelial cells in CNAB can result in underestimated expression of molecular markers of malignancy. Consequently, the risk of malignancy or unfavourable prognosis can also be underestimated. To conclude, results obtained from samples gained with one biopsy technique cannot be directly related to thresholds, and generally with experiences gained with other techniques, because it can lead to incorrect clinical interpretation of the results. (Endokrynol Pol 2016; 67 (6): 550–553)