Pleural pH in Empyema

A pleural fluid pH ≤7.2 indicates a high risk of complicated parapneumonic effusion (CPPE) or empyema requiring immediate chest tube drainage. 1

Diagnostic Criteria for Empyema

Frank pus in the pleural space definitively indicates empyema and requires immediate drainage without need for pH testing 1
When pleural fluid is not frankly purulent but infection is suspected, pH is the most useful biochemical marker for determining the need for drainage 1
Pleural fluid pH ≤7.15 indicates a high risk of complicated parapneumonic effusion or empyema 1
A pH <7.00 almost invariably indicates a complicated parapneumonic effusion that will not resolve without drainage 2

pH ≤7.2: High risk of CPPE/empyema - insert intercostal drain if ultrasound shows sufficient accessible fluid 1
pH between 7.2-7.4: Intermediate risk - measure pleural LDH; if >900 IU/L, consider drainage, especially with other supporting features (fever, high fluid volume, low glucose, CT contrast enhancement, or ultrasound septations) 1
pH ≥7.4: Low risk - no immediate drainage indicated 1
Historical studies suggested a lower threshold (pH <7.00) for definitive drainage, but current guidelines recommend a more aggressive approach with drainage at pH ≤7.2 1, 2

Pleural fluid pH and glucose are highly correlated; when immediate pH measurement is not available, glucose <3.3 mmol/L (<60 mg/dL) indicates high probability of CPPE/empyema 1, 3
Pleural fluid LDH >900 IU/L with intermediate pH (7.2-7.4) suggests need for drainage 1
The presence of loculations on imaging is associated with poorer outcomes and may be an additional indication for early drainage 1

Pleural fluid for pH should be collected anaerobically with heparin and measured in a blood gas analyzer 1
Avoid contamination with local anesthetic or excessive heparin as this can artificially lower pH 1
Delays in processing the sample can falsely increase the measured pH 1
pH measurement using litmus paper or a pH meter is unreliable and should not be used 1

Not recognizing that pH is a better discriminator than glucose or LDH for pleural infection 3, 4
Failing to consider alternative diagnoses that can present with low pleural fluid pH (rheumatoid effusion, advanced malignancy) 1
Delaying chest tube drainage when indicated by pH criteria, which may increase morbidity and hospital stay 1
Not involving a respiratory specialist or thoracic surgeon in cases requiring chest tube drainage 1

Extremely low pH values (<6.5) should prompt evaluation for esophageal perforation 4
In patients with intermediate pH values where drainage is initially deferred, regular clinical reviews and repeat thoracentesis should be performed to ensure CPPE is not missed 1
Poor clinical progress during treatment with antibiotics alone should lead to prompt patient review and consideration of chest tube drainage even if initial pH was >7.2 1