To increase diagnostic accuracy of detection of pleural effusion sonographically, markers such as boomerang and VIP signs can be utilized. Thoracentesis
Once a pleural effusion is diagnosed, its cause must be determined. Pleural fluid is drawn out of the pleural space in a process called thoracentesis, and it should be done in almost all patients who have pleural fluid that is at least 10 mm in thickness on CT, ultrasonography, or lateral decubitus X-ray and that is new or of uncertain etiology. In general, the only patients who do not require thoracentesis are those who have heart failure with symmetric pleural effusions and no chest pain or fever; in these patients, diuresis can be tried, and thoracentesis is avoided unless effusions persist for more than 3 days. In a thoracentesis, a needle is inserted through the back of the chest wall in the sixth, seventh, or eighth intercostal space on the midaxillary line, into the pleural space. The use of ultrasound to guide the procedure is now standard of care as it increases accuracy and decreases complications.
Pulmonary emboli were once thought to cause transudative effusions, but have been recently shown to be exudative. The mechanism for the exudative pleural effusion in pulmonary thromboembolism is probably related to increased permeability of the capillaries in the lung, which results from the release of cytokines or inflammatory mediators (e.g. vascular endothelial growth factor) from the platelet-rich blood clots. The excessive interstitial lung fluid traverses the visceral pleura and accumulates in the pleural space.Conditions associated with transudative pleural effusions include:
Congestive heart failure
Liver cirrhosis
Severe hypoalbuminemia
Nephrotic syndrome
Acute atelectasis
Myxedema
Peritoneal dialysis
Meigss syndrome
Obstructive uropathy
End-stage kidney disease

Exudative
When a pleural effusion has been determined to be exudative, additional evaluation is needed to determine its cause, and amylase, glucose, pH and cell counts should be measured. Red blood cell counts are elevated in cases of bloody effusions (for example after heart surgery or hemothorax from incomplete evacuation of blood). Amylase levels are elevated in cases of esophageal rupture, pancreatic pleural effusion, or cancer. Glucose is decreased with cancer, bacterial infections, or rheumatoid pleuritis. pH is low in empyema (<7.2) and maybe low in cancer. If cancer is suspected, the pleural fluid is sent for cytology. If cytology is negative, and cancer is still suspected, either a thoracoscopy, or needle biopsy of the pleura may be performed. Gram staining and culture should also be done. If tuberculosis is possible, examination for Mycobacterium tuberculosis (either a Ziehl–Neelsen or Kinyoun stain, and mycobacterial cultures) should be done.