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.