A pleural tap can be performed for diagnostic purpose or for therapeutic reasons to drain the fluid around the lungs. A needle or a canula is passed into the pleural space and a small quantity, about 30 to 50 ml, of the fluid is collected for analysis. In some patients, with a small pleural effusion, this diagnostic procedure is usually combined with the treatment. A pleural tap helps in the diagnosis of the cause of the pleural effusion in about 80% of the patients. It may help in excluding certain diseases in the remaining individuals, even if the procedure may not be diagnostic.
The procedure is not indicated in individuals with coagulation (blood clotting) disorders that cannot be controlled. It is also done with great deal of caution in patients on mechanical ventilation as in with emphysema, those with only one functional lung and other high risk conditions.
A pleural tap can be complicated at times by pneumothorax (air accumulation in pleural cavity) or hemorrhage. These complications can be minimized with use of ultrasound for guiding the needle used for the pleural tap. Some individuals can develop sudden hypotension during the procedure (vasovagal). Other complications include pain, surgical emphysema (accumulation of air in the skin and subcutaneous tissue), infection, and puncture of spleen or liver.
Percutaneous Pleural Biopsy
Patients with exudative effusion who remain undiagnosed after pleural tap may require a pleural biopsy for further evaluation. Patients having pleural fluid with a lymphocytic predominance may also benefit from a pleural biopsy. Most frequent diagnosis in these patients is cancer or tuberculosis.
Thoracoscopy may be performed in patients with undiagnosed pleural effusion. Pleural biopsy may be performed under direct visualization through thoracoscopy.
Tests on Fluid and Interpretation of Results
A wide variety of tests are done with the pleural fluid to find the cause of the pleural effusion. Pleural fluid is grouped as exudative and transudative effusion, based on the protein content of pleural fluid and its lactate dehydrogenase (LDH) concentration.
- In transudative effusion the ratio of total pleural fluid protein to serum total protein is less than 0.5 and ratio of LDH in pleural fluid to serum is less than 0.6. Transudative effusion is suggestive of an imbalance in the fluid balance in the pleural space (example heart failure or liver failure). Usually present on both sides.
- Exudative effusion results from the damage or disruption to pleural integrity or its lymphatic drainage (eg lung infections or malignancy). More often present on one side.
- The pH of the fluid is less that 7.2 in conditions like empyema (pus around the lungs), rheumatoid or tuberculous pleuritis.
- The glucose level of the pleural fluid is also tested and may be found to be low in patients with effusion due to conditions like rheumatoid disease or infections.
- Pleural fluid amylase levels are elevated in patients in whom pancreatic disease or esophageal rupture is the cause for the effusion.
Color of Effusion
- Transudative effusion is usually clear while exudative effusion is cloudy.
- Blood stained pleural fluid may be seen in malignancy and trauma. Presence of red blood cells is a confirmatory sign of hemorrhage. In frank bleeding, the fluid is dark or red colored (hemothorax).
- Pale effusion is seen commonly in patients with generalized edematous conditions like heart failure.
- Milky effusion may be due to a chylothorax.
Cell Count and Cytology
A cell count and cytology can be performed on the pleural fluid. Increase in number of white blood cells is suggestive of infection. A sample with predominant neutrophils is suggestive of bacterial infection and those with lymphocyte predominance may be of tuberculosis or lymphoma. Culture of the pleural fluid is also done to identify the organism and to find the antibiotic sensitivity.
Cytology is important in diagnosis of malignant conditions that can cause effusion. Cancerous cells can be found in the pleural fluid in lung cancer, malignant mesothelioma and metastatic cancer lesions from other sites. Presence of cancer cells in pleural fluid is diagnostic of cancer but their absence does not rule out cancer.
Fats in the Fluid
The cholesterol and triglyceride levels of the pleural fluid are elevated in individuals with leakage of lymph vessels into pleural cavity (chylothorax). Chylothorax is often seen in patients with rupture of thoracic duct due to trauma or cancer. The pleural fluid in these patients usually has a milky appearance.