Indications for Thoracentesis Diagnostic thoracentesis Indicated for almost all patients who have pleural fluid that is new or of uncertain etiology and is ≥ 10 mm in thickness on computed tomography (CT) scan, ultrasonography, or lateral decubitus x-ray (see figure Diagnosis of Pleural Effusion) Diagnostic thoracentesis is usually not needed when the etiology of the pleural fluid is apparent (eg, viral pleuritis, typical heart failure).