Effect of ultrasound transmission gel on ultrasound-guided fine needle aspiration cytological specimens of thyroid Objective: To investigate prospectively the diagnostic impact of ultrasound coupling gel on thyroid specimens obtained under ultrasound guidance. Methods: Patients presenting for ultrasound-guided fine needle aspiration (USG-FNA) of the thyroid were invited to participate in the study. Four specimens per nodule were collected: two using chlorhexdine wash and two using sterile, colourless ultrasound gel as couplant according to routine protocol. All slides were analysed in a blinded fashion by two senior cytologists for the presence or absence of ultrasound gel-induced artefacts. The presence of gel-induced artefacts between the two groups was analyzed using Pearson s chi-square test. Kappa statistics were used to measure the inter-rater agreement between the cytologists. Results: Twenty thyroid nodules comprising 80 specimen slides were collected. On slides collected with gel, cytological artefacts were detected in 60-65 of cases compared with 10-15 of cases without gel (P <0.001). The inter-rater agreement between the two observers was very good (kappa = 0.84). Two of the 14 patients required repeat FNA due to non-diagnostic cytology results caused by inadequate sampling and gel-induced artefacts. Conclusions: Clinical cytopathologists, radiologists and sonographers should be aware of the potential for ultrasound gel to cause significant artefacts on cytological specimens. Our findings suggest that staff involved in USG-FNA cytology should remove the gel carefully before taking the aspirate.