Most recommend that the pH be measured anaerobically, keeping the specimen on ice when delay of any kind is unavoidable. One study, however, showed that pleural fluid pH preserved anaerobically at room temperature is stable during the first hour following thoracentesis.

Pleural, peritoneal, or pericardial fluid pH levels have been used where appropriate in the evaluation of pneumonia, rheumatoid pleuritis, malignancy, uremia, empyema, tuberculosis, hemothorax, esophageal rupture, and cirrhosis with spontaneous bacterial peritonitis.

Pleural fluid: Low pleural fluid pH levels (<7.2-7.3) identify patients with effusions due to pneumonia, empyema or lung abscess; such conditions require aggressive treatment. Low pleural fluid pH is accompanied by low pleural fluid glucose in these patients.
pH of pleural fluid <6.0 is highly suggestive of rupture of esophagus.6 Pleural fluid amylase is very helpful for this diagnosis as well.

The pleural fluid pH is often <7.2 and consistently <7.3 in rheumatoid pleural effusion, which is characterized by low glucose, high LD, and high rheumatoid factor. Effusions related to lupus erythematosus generally have pleural fluid pH levels >7.35.6

Low pleural fluid pH with negative cytologic examination may occur in a malignant effusion as well as tuberculosis or rheumatoid disease. Differences in survival have been shown between patients with malignant pleural effusions having low pleural fluid pH (<7.30) compared to those having pH ≥7.30.9 On the basis of these observations, recommendations exist for selecting patients for pleurodesis.10 However, the use of pleural fluid pH for this purpose is controversial.
 

A low, transudative, pleural fluid pH (<7.30) accompanied by a pleural fluid creatinine to serum creatinine ratio >1 is consistent with urinothorax, a condition found in some patients with obstructive uropathy.

Peritoneal (ascitic) fluid: The most reliable diagnostic criteria for spontaneous bacterial peritonitis have been reported to include ascitic fluid pH <7.35 (with a mean of 7.24), ascitic fluid PMN >500/mm3, and an arterial-ascitic fluid pH gradient >0.10.15 Reduction of ascitic fluid pH and in some, but not all, series increments of ascitic fluid PMN counts may be found with peritoneal metastases.15

Pericardial fluid: Pericardial fluid pH measurements are not widely used, though decreased (<7.30) levels are observed in rheumatic and purulent disorders, malignancy, uremia, tuberculosis, and other conditions.