We designed and developed a novel dual-function semirigid thoracoscope (UE FET-680; UE Corporation, Zhejiang, China) with a straight channel adapted to flexible and rigid forceps for pleural biopsy. Our preclinical trial has proven that the dual-function semirigid thoracoscope has larger and deeper sampling while retaining the entire thoracic accessibility and flexible bronchoscopic maneuvering habits of the semirigid thoracoscope. We designed this study to directly compare the diagnostic yield, quality of sampling, and safety between the dual-function semirigid thoracoscope and semirigid thoracoscope.

Inclusion Criteria:

• Age ≥ 18 years
• Unilateral pleural effusion of unclear etiology after a less invasive diagnosis
• Fully informed of the purpose and method of the study, agreed to participate in the study, and signed an informed consent form.

Exclusion Criteria:

• PaO2/FiO2 < 300
• Tendency for uncontrolled bleeding, unstable cardiovascular status, or severe heart failure
• A complete pleural symphysis, where it is not possible to create a pneumothorax
• Refractory cough
• Eastern Cooperative Oncology Group performance status 4
• Disagreements regarding study participation
• Participation in other studies within three months without withdrawal or termination.