What does pleural fluid arterial blood gas (ABG) indicate?

Pleural Fluid pH Analysis (Not ABG)

Pleural fluid pH measurement—not a full arterial blood gas panel—is the critical diagnostic test that primarily identifies pleural infection requiring chest tube drainage, with a pH <7.2 indicating the need for immediate intervention. 1

Primary Clinical Indication: Identifying Pleural Infection

The main clinical use of pleural fluid pH is identifying pleural infection that requires chest tube drainage. 1 The pH measurement serves as the most useful biochemical index for predicting drainage needs in parapneumonic effusions. 1

Key pH Thresholds for Management Decisions

• pH <7.2 with normal blood pH mandates chest tube drainage in non-purulent parapneumonic effusions 1
• pH 7.2-7.3 represents a borderline zone requiring serial measurements and close clinical observation to determine if drainage becomes necessary 2
• pH >7.3 typically allows treatment with antibiotics alone if clinical progress is satisfactory 1, 2

The normal pleural pH is approximately 7.6 due to bicarbonate accumulation in the pleural cavity, so a pH <7.2 represents substantial hydrogen ion accumulation. 1

Other Diagnostic Applications Beyond Infection

Malignant Effusions

• **pH <7.3 in malignant effusions predicts poor prognosis** (median survival 2.1 months vs 9.8 months for pH >7.3) and poor response to sclerosing agents 1, 2
• Lower pH typically indicates effusions present for several months with higher LDH, lower glucose, and higher white cell counts 2

Rheumatoid Arthritis

• Rheumatoid pleural effusions most often have pH <7.3, frequently accompanied by very low glucose (<1.6 mmol/L or 29 mg/dL) 1, 2
• pH <7.0 occurs in rheumatoid effusions, empyema, and esophageal rupture 2

Esophageal Rupture

• pH <6.0 is seen almost exclusively in esophageal rupture, though rarely with empyemas 2

Tuberculous Effusions

• All reported tuberculous pleural effusions have pH <7.4, which may help distinguish them from recent malignant effusions that tend to have higher pH 2

Critical Technical Requirements

Proper Collection and Measurement Technique

Pleural fluid for pH must be collected anaerobically with heparin and measured in a blood gas analyzer—not with pH paper or a pH meter. 1

• pH paper is unreliable with 72% discordance in classification for parapneumonic effusions and should never be used 3
• pH meters are also unreliable compared to blood gas analyzers 3, 4
• Aerobic storage causes clinically significant pH overestimation of 0.14-0.16 pH units 4
• Blood gas analyzers measure pleural pH with negligible bias (0.01±0.01 pH units) 4

Common Pitfalls to Avoid

Local anesthetic (lidocaine/lignocaine) is acidic and can falsely depress measured pH, especially when given in large volumes or left in the same syringe used for fluid collection. 1, 5 In small effusions (<15% of hemithorax), local anesthetic caused 80% discordance in classification and wrongly predicted chest tube need in 100% of cases. 5

Do not put frank pus through a blood gas analyzer—purulent fluid already indicates need for drainage without pH measurement. 1 However, turbid/cloudy fluid of uncertain infection status can be safely measured. 1

Relationship to Other Pleural Fluid Parameters

Pleural fluid pH is more useful than glucose or LDH alone for predicting drainage needs. 1 However, the combination of pH 7.2-7.3 with glucose <60 mg/dL and LDH >1,000 IU/dL indicates impending empyema. 2

pH correlates with the biochemical criteria for pleural infection: pleural fluid acidosis develops alongside rising LDH and falling glucose levels. 1