196 / CHAPTER 11
Figure 11–5. Pleural effusion as seen on CT scan (arrow). This effusion may be too small to see on a plain chest x-ray but is clearly evident on this CT scan.
Specific lab tests also may include pH, total protein (TP), lactate dehydrogenase (LDH), glucose, cell count, and differential. Pleural fluid pH should be performed with a blood gas analyzer. Normal pleural fluid pH is around 7.6. Pleural fluid acidosis may result from lactic acid production in the pleural space or tissues. A pH of less than 7.2 can be found in parapneumonic effusions, esophageal rupture, urinothorax, rheumatoid pleuritis, tuberculous pleuritis, hemothorax, systemic acidosis, lupus pleuritis, paragonimiasis, and malignancy. An elevation in LDH is a nonspecific marker of inflammation that is used along with TP to differentiate exudates from transudates (see below). Although the glucose level in the pleural fluid approximates the blood glucose, pleural fluid glucose levels less than 30 mg/dL are observed in tuberculosis, empyema, rheumatoid disease, or malignancy. The white blood cell count is fairly nonspecific, but the differential count may help in determining the diagnosis. Neutrophilic predominance generally indicates acute infection or pancreatitis. Lymphocyte predominance is more common in chronic diseases (such as TB) and malignancy. Pleural fluid adenosinedeaminase (ADA) level may be helpful in the diagnosis of TB. Eosinophils may indicate blood or air in the pleural space; additionally, pleural fluid eosinophils may be present in drug reactions, malignancy, benign asbestos pleural effusion, or parasitic infections. Depending on these initial findings, further testing may be warranted when an effusion proves to be exudative. CLASSIFICATION OF TRANSUDATES VERSUS EXUDATES
The goal in evaluating pleural fluid is to determine the etiology of the effusion. In order to do this, the effusion is first classified as either a transudate or an exudate.