A thyroid nodule is one of the commonly encountered lesions in the Ear Nose Throat–Outpatient Department, with a prevalence of 4–7% [2]. It is an important clinical entity for clinicians because of its malignancy potential, which is about 5% [10]. Works of literature have shown that a visual aid as ultrasonography added to guide the needle to aspirate cytology from the lesion site shows promising diagnostic ability. Studies have shown that it reduces inadequacies, and impalpable lesions are also easily aspirated [6,7,8, 11, 12].

A similar study supported female predilection in our study. We also found the age demographics to be like existing literature [7, 8].

As per TBSRTC, most lesions belonged to TBSRTC category VI as they are the most likely candidates for the surgery, which is a prerequisite for the final histopathology report.

Koo et al. also reported the higher prevalence of malignancy, notably TBSRTC VI. The referred cases reviewed by the hospital’s pathologist were included in the study [13].

Only 3% prevalence of TBSTRC III “grey zone” finding in our study could be because of imaging advantage while performing FNAC, as shown in the study by Kumari et al. [8] The low number in our study could be because of including only operated patients.

Our study showed a different percentage of ROM than the implied ROM for TBSRTC categories. The variations are probably because of the smaller sample size in our study. Categories III and IV showed an increased percentage of ROM in our study than the implied ROM percentage of the TBSRTC system. Thus, including TBSRTC categories III and IV in the malignancy category seems sensible. Kumari et al. reported similar ROM in categories III and IV. Our TBSRTC category VI showed a less risk of malignancy percentage. Since TBSRTC is a relatively adapted system for reporting thyroid pathology, institutional experience in using TBSRTC plays a role in proper diagnosis. So, some overlapping features between benign and malignant lesions, such as pseudo inclusions and nuclear grooves, might have misguided the cytopathologist in reporting cytology. The ROM in TBSRTC VI in the study by Kumari et al. was 100% [8].

The high sensitivity and high NPV of USG-FNAC of a thyroid nodule of our study are broadly in line with previous literature studies, and reporting of cytology report may or may not be based on TBSRTC [8, 13,14,15]. Similar to our study, the sensitivity was above 90% and better than specificity, mainly when cytological reporting was as per TBSRTC [8, 15, 16].

Thus, USG-FNAC finding as per TBSRTC could be an excellent tool for screening thyroid lesions as negative findings in USG-FNAC are more likely to be negative and rule out disease.

Our study’s findings differed from the study by Said et al. [17] probably because the study did not report cytological findings according to TBSRTC.

The specificity, PPV, and accuracy of our study were not satisfactorily high. Most of the false-positive findings were seen in small lesions. An explanation could be because all the lesions might have been aspirated that no pathology remained behind. Similarly, liberal diagnoses made by cytopathologist for lesions with overlapping features could have an increased number of false positives. USG-FNAC presents itself with the limitation of inability to distinguish between follicular adenoma and Hashimoto adenoma with their malignant counterpart, leading to high false positives in the current study. Also, including TBSRTC categories III and IV as malignant lesions could have accounted for high false positives.

Contrary to findings by existing studies, sensitivity did not increase or decrease with size [14, 18,19,20]. This study showed that USG-guided FNAC has high sensitivity in less than 2 cm or greater than 4 cm. In minor lesion, USG help in locating the nodule and thus accurate sampling. In a more significant lesion, it can accurately sample the part of the lesion with the suspicious feature, increasing the sensitivity.

Specificity, however, increased with size, which is similar to the study by Kim et al. but at odds with Aydogan et al. where specificity decreased with size [18, 19]. Zhong et al. proffered that size did not influence specificity with the highest specificity reported at size 1–2 cm [20]. Size did not affect accuracy in our study and is consistent with previous studies. Also, our study validated claims by Aydogan et al. that ROM decreased with size, but it may be because of selection bias as we included only suspicious small lesions in the study [14, 18,19,20].

When the Cohen Kappa inter-rater agreement was applied, the Kappa coefficient increased with size > 4cm, having an almost perfect agreement. However, lesion < 2cm also had a fair agreement; hence, for small thyroid lesion, USG–FNAC can be advocated if not for diagnostic but for screening as our study revealed high sensitivity.

Our study has a limitation of a smaller sample size and not having a comparison group. The data were collected at a tertiary care referral center, thus carrying a risk of sampling bias.

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