The management of nondiagnostic fine-needle as-piration biopsy (FNAB) of thyroid nodules has not been determined because the significance of persistent nondiagnostic FNAB has been underestimated. The purposes of the present study were to estimate the likelihood of malignancy in patients with nondiagnostic FNAB results and to evaluate whether tumor factors could affect the nondiagnostic FNAB results.

2,400 FNABs were performed for thyroid nodules at our institution from 2001 to 2005. A total 294 patients who had initial nondiagnostic results were the subjects of this study. We retrospectively reviewed the age, gender, tumor size, the sonographic findings, the FNAB results and the pathologic reports.

The initial nondiagnostic rate was 12.3% (294/2,400). Among the 294 initial nondiagnosted patients, FNAB was secondarily performed in only 99 patients. Seventy patients (70.7%) were diagnostic and 29 (29.3%) remained nondiagnosed. Twenty of seventy diagnosed patients had malignant FNAB results, including atypical cells, and the other 50 patients were benign. The causes of nondiagnostic FNAB results by pathologic descriptions were 43.1%: scanty cellularity, 29.2%: blood, 13.2%: fluid or colloid, 11.1%: inconclusive, and 3.5%: dry artifact. There are no differences in the nondiagnostic rate according to tumor size (P=0.2) and calcification (P=0.7). When the sonographic results could predict the pathologic results, no difference was noted according to the sonographic findings that determined malignancy (P=0.4). Ten percent of the initial nondiagnostic FNAB results were finally reported as malignancy.

Scanty cellularity and blood aspiration were the major causes of nondiagnostic FNAB results (43.1% and 29.2%, respectively). Tumor characteristics such as tumor size, the presence of calcification and sonographic findings did not predict nondiagnostic FNAB results. Reaspiration biopsy for the initially nondiagnostic FNAB in the thyroid nodules had a high probability of achieving a nondiagnositc result. Because nondiagnostic FNAB of the thyroid nodules may be associated with a relatively high probability of thyroid malignancy, a nondiagnostic FNAB should not be considered as benign. So, if reaspiration biopsy is nondiagnostic, it should be the subject of concern or the patient might be considered for surgery with taking into account the other characteristics, in particularily malignant sonographic findings.