Obtaining an adequate cell sample is a prerequisite to the success of thyroid cytology. Therefore, immediate microscopic assessment of the needle aspirate by a pathologist or a cytotechnologist is desirable. If the first sample is judged inadequate for cytological evaluation, the TN can be re-aspirated immediately. If a rapid evaluation is not available, multiple FNAs of different areas of the TN should be performed.
The range of inadequate or unsatisfactory specimens reported in the literature ranges from 2–21% (means
17%). Currently, criteria for specimen adequacy vary from institution to institution. Some investigators require that an adequate sample should contains five to six groups of well-preserved and well-visualized follicular cells with each group containing 10 or more cells. One group requires multiple punctures of the TN to be evaluated, with at least six properly prepared smears and a minimum of 8–10 tissue fragments of well-preserved follicular epithelium on each of two slides. Another group requires 10 clusters of follicular cells with at least 20 cells in each cluster. The Papanicolaou Society of Cytopathology Task Forces on Standard of Practice does not specify any numbers and groups of thyroid follicular epithelial cells for specimen adequacy. Two practical exceptions to these adequacy criteria are applied: (a) a benign colloid nodule may be suggested if a large amount of thick colloid material is present, regardless of the number of follicular epithelial cell clusters; or, (b) if a cell sample contains one or two small clusters of malignant or highly atypical cells, it should be reported as malignant or suspicious for malignancy and not as unsatisfactory or inadequate for cytodiagnosis. Thyroid FNA under ultrasonographic guidance achieved higher rates of adequate cell samples, in the range of 79–99.3%(mean, 91%). Ultrasound-guided thyroid FNA proved to be useful in sampling TNs smaller than 2 cm in greatest dimension, complex or solid-cystic TNs and abnormal thyroid beds.