How is pleural effusion diagnosed?
The patient's history and physical exam may indicate a presumptive diagnose of pleural effusion. For example, a patient with a history of congestive heart failure or cirrhosis with symptoms of cough, difficulty breathing, and pleuritic chest pain may have a pleural effusion. Findings from the physical exam, such as dullness to percussion of the lung area (when tapping the area of the lung with a finger, the percussion or sound is dull - if no fluid exists in the area the sound will be lighter - please see this for an informational video by the Stanford School of Medicine on percussion of the chest), decreased vibration (decreased tactile fremitus), and asymmetrical chest expansion (the lungs do not inflate or deflate equally - please see this for an informational YouTube video about asymmetrical chest expansion) may also be evidence of a pleural effusion. Other physical exam findings detected with a stethoscope may include reduced or inaudible breath sounds on the affected side, egophony (patient voices the letter "e," but when listening [auscultation] it sounds like "a"), and a friction rub (if there is fluid in the pleural area, the heart will rub against the inflamed or fluid filled space). To hear what a friction rub sounds like please see this informational YouTube video.
Chest X-ray can detect pleural effusions, as they usually appear as whitish areas at the lung base, and they may occur on only one side (unilateral) or on both sides (bilateral). If a person lies on their side for a few minutes, most pleural effusions will move and layer out along that side of the chest cavity which is positioned downward (because of the effects of gravity). This movement of the pleural effusion can be seen on an X-ray taken with the person lying on their side (a lateral decubitus X-ray).
Other imaging tests, such as CT scan, may be ordered to further identify the possible cause and the extent of the pleural effusion.
Diagnosing the cause(s) of a pleural effusion often begins with determining whether the fluid is transudate or exudate. This is important because the results of this fluid analysis may provide a diagnosis and determine the course of treatment. Thoracentesis (a procedure to remove the fluid from the pleural space) followed by laboratory analysis of the fluid can differentiate between transudate and exudate. The results from the fluid obtained from the thoracentesis are compared to certain blood tests (for example, LDH, glucose, protein, pH, cholesterol and others). Additional testing of the pleural fluid may also include a cell count, cytology, and cultures. Criteria are then used to differentiate exudate from transudate. Exudate has the following characteristics:
- Pleural fluid LDH > 0.45 of the upper limits of normal blood values
- Pleural fluid protein level > 2.9g/dL
- Pleural fluid cholesterol level > 45mg/dL
Other health care professionals may use different criteria to determine the presence of exudate, such as the ratio of pleural fluid to serum protein levels > 0.5, LDH ratio > 0.6 and LDH ratio > 2/3 the upper limits of normal. Other pleural fluid test results (cytology or amylase, for example) may also reveal the source of the effusion.