Is an adenosine deaminase (ADA) level of 78 units per liter (U/L) possible in empyema?

Is an ADA Level of 78 U/L Possible in Empyema?

Yes, an ADA level of 78 U/L is absolutely possible in empyema, though it is uncommon and creates diagnostic confusion with tuberculous pleurisy. 1, 2

Understanding ADA Elevation in Empyema

While ADA is primarily elevated in tuberculous pleurisy, empyema represents one of the key conditions that can produce similarly high values:

• Empyema can elevate ADA above typical tuberculous thresholds (>40-47 U/L), making it a critical diagnostic pitfall 1, 2, 3
• In one study, 6 of 7 empyema cases had ADA levels exceeding 47 U/L, overlapping completely with the tuberculous range 3
• The British Thoracic Society explicitly warns that ADA levels are raised in empyema, rheumatoid pleurisy, and malignancy, which limits the test's utility in countries with low tuberculosis prevalence 1

Distinguishing Empyema from Tuberculous Pleurisy

When faced with an ADA of 78 U/L, use these clinical and laboratory features to differentiate:

Immediate Bedside Differentiation

• Perform bench centrifugation of the pleural fluid: empyema will leave a clear supernatant as cell debris separates, while tuberculous effusions remain milky 1
• Empyema fluid typically has a purulent appearance and may smell foul 1

Laboratory Parameters

• Glucose levels are critically different: purulent effusions with positive cultures have significantly lower fluid glucose (47.3 ± 25.3 mg/dL) compared to non-infectious effusions (102.5 ± 35.6 mg/dL) 1
• Neutrophil predominance: bacterial effusions have the highest proportions of neutrophils (69 ± 23%), while tuberculous effusions typically show lymphocyte predominance 1
• White blood cell count is highest in bacterial empyema compared to tuberculous pleurisy 1

Microbiological Confirmation

• Send at least three cultures for aerobes and anaerobes along with blood cultures for bacterial empyema 1
• For tuberculosis, order acid-fast bacilli staining, mycobacterial culture, and PCR analysis 1, 2
• PCR for tuberculosis is more specific (100%) than ADA estimation (78%) for tuberculous pericarditis, and this principle applies to pleural disease 1

Critical Clinical Pitfalls

• Never rely on ADA alone for diagnosis when the value is in the overlapping range (>40 U/L), as both empyema and tuberculosis can produce identical elevations 2, 4
• The American College of Chest Physicians emphasizes that ADA cannot definitively distinguish between tuberculous and bacterial empyema 4
• Do not delay drainage while awaiting culture results: both tuberculous empyema and bacterial empyema require immediate chest tube drainage 4

Practical Algorithm for ADA 78 U/L

1. Examine the fluid appearance and perform bench centrifugation 1
2. Check pleural fluid glucose and neutrophil percentage 1
3. Send cultures for bacteria AND tuberculosis simultaneously 1
4. Insert chest tube for drainage immediately regardless of etiology 4
5. If clinical suspicion for tuberculosis is high (lymphocyte predominance, chronic symptoms, endemic area), initiate empiric anti-tuberculous therapy while awaiting cultures 4
6. If purulent appearance with neutrophil predominance and low glucose, treat as bacterial empyema with appropriate antibiotics 1