Diagnosis of atypical carcinoid can be made on biopsies > 4 mm 2 and is accurate.

In the 2021 WHO thoracic tumors, gradation of lung carcinoids in biopsies is discouraged. We hypothesized that atypical carcinoid (AC) could be reliably diagnosed in larger preoperative biopsies. Biopsy-resection paired specimens of carcinoid patients were included, and definitive diagnosis was based on the resection specimen according to the WHO 2021 classification. A total of 64 biopsy-resection pairs (26 typical carcinoid (TC) (41%) and 38 AC (59%)) were analyzed. In 35 patients (55%), tumor classification between the biopsy and resection specimen was concordant (26 TC, 9 AC). The discordance in the remaining 29 biopsies (45%, 29 TC, 0 AC) was caused by misclassification of AC as TC. In biopsies measuring < 4 mm 2 , 15/15 AC (100%) were misclassified compared to 14/23 AC (61%) of biopsies ≥ 4 mm 2 . Categorical concordance of Ki-67 in biopsy-resection pairs at threshold of 5% was 68%. Ki-67 in the biopsy was not of additional value to discriminate between TC and AC, irrespective of the biopsy size. Atypical carcinoid is frequently missed in small bronchial biopsies (< 4 mm 2 ). If the carcinoid classification is clinically relevant, a cumulative biopsy size of at least 4 mm 2 should be considered. Our study provides strong arguments to make the diagnosis of AC in case of sufficient mitosis for AC on a biopsy and keep the diagnosis "carcinoid NOS" for carcinoids with ≤ 1 mitosis per 2 mm 2 . Ki-67 has a good concordance but was not discriminative for definitive diagnosis.