Pleural Fluid Glucose Analysis and pH Correlation
Pleural fluid glucose levels below 3.3 mmol/l (60 mg/dl) indicate complicated exudative effusions, and while glucose correlates with pH, there is no clinically useful "ratio" between pleural fluid glucose and pH—both are independent markers that should be measured separately to assess disease severity. 1
Key Glucose Thresholds and Clinical Significance
Low pleural fluid glucose (<3.3 mmol/l or <60 mg/dl) indicates:
- Empyema (with lowest concentrations alongside rheumatoid effusions) 1
- Rheumatoid disease (glucose >1.6 mmol/l essentially excludes rheumatoid arthritis as the cause) 1
- Tuberculosis 1
- Malignancy 1
- Lupus 1
- Oesophageal rupture 1
The Glucose-pH Relationship: Not a Ratio, But a Correlation
Despite a strong statistical correlation (r = 0.81), pleural fluid glucose and pH cannot reliably predict each other in individual patients and should both be measured. 2, 3
Why Both Tests Matter:
- In parapneumonic effusions: pH has superior diagnostic accuracy (AUC = 0.92) compared to glucose (AUC = 0.84) for identifying complicated effusions requiring drainage 4
- Concordance is high but imperfect: 91.9% of samples show concordant results at cutoffs of pH 7.2 and glucose 3.3 mmol/L, meaning 8.1% show discordant results that would be missed if only one test is performed 3
- Using pH alone captures 95% of infection-related effusions, while glucose alone identifies only 91.7% 3
Clinical Decision-Making Algorithm
For Parapneumonic Effusions:
- pH <7.2: Requires chest tube drainage regardless of glucose level 1, 5
- pH 7.2-7.3 with glucose <60 mg/dl and LDH >1,000 IU/dl: Indicates impending empyema, requires drainage 5
- pH >7.3: Generally does not require drainage 5
For Malignant Effusions:
- Both low glucose (<60 mg/dl) and low pH (<7.3): Associated with 90% positive cytology yield, extensive disease, poor pleurodesis response, and median survival of only 2.1 months 1, 6, 5
- Both normal glucose and pH: Only 50% positive cytology yield, better prognosis (median survival 9.8 months with pH >7.3) 1, 6
Common Pitfalls to Avoid
Do not rely on glucose alone in pleural infection—pH discriminates better than glucose for identifying complicated parapneumonic effusions. 1, 4
Extremely low pH (<6.5) should trigger immediate evaluation for esophageal perforation, as this is the only condition producing such profoundly low pH values. 7
When both glucose and pH are low together, this indicates severe pleural inflammation with interrelated mechanisms—these effusions are invariably complicated (loculated or empyemas) and require aggressive management. 2