Background and Aims: The red material occupying the larger portion of the acquired sample in EUS fine-needle biopsy (FNB) is seldom investigated. We aimed to evaluate the composition of the red material. Methods: Patients with a solid pancreatic mass who received EUS FNB from September 2020 to June 2021 were enrolled. The white or yellowish content with apparent bulk (white material) and the rest of pasta-like red content (red material) were separated immediately after puncture. Needle passes proceeded until 2 specimens with >4 mm of white material were obtained. An extra needle pass was conducted for DNA collection. The DNA amount, Kirsten rat sarcoma virus (K-ras) mutation type, and mutation allele frequency were compared between the white and red material. Results: Forty patients were enrolled with 68 paired white and red materials. The diagnostic accuracy was slightly higher in the white material (92.5% vs 82.5%, P = .219). On the histology slides, the area of the tumor gland was comparable in both materials, but the total tissue area was larger in the red material (9.74 mm2 and 10.74 mm2 larger according to generalized linear model and generalized estimating equation, respectively; both, P < .001). The amount of DNA was significantly higher in the red material (2.99 [interquartile range, 1.59-7.29] μg vs .70 [interquartile range, .27-1.24] μg; P < .001). Common pancreatic adenocarcinoma K-ras mutation was identified at a rate of 85% for the white material and 95% for the red material. Regardless of whether red or white material was used, there was a high concordance of K-ras mutation types (34 of 40 [85%]) and a high correlation of mutation allele frequency (ρ = .66, P < .001). Conclusions: In EUS FNB, the red material contains a higher amount of tumor DNA and can be an alternative source for tumor DNA analysis.
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