What condition is diagnosed by measuring ADA2 (adenosine deaminase isoenzyme 2) in pleural fluid?

ADA2 (DADA2) Enzyme Test for Diagnosis

The ADA2 (adenosine deaminase isoenzyme 2) test in pleural fluid is used to diagnose tuberculous pleural effusion, demonstrating superior diagnostic accuracy compared to total ADA with 97.2% sensitivity and 94.2% specificity at a cutoff >40.6 U/L. 1

Primary Clinical Application

ADA2 measurement in pleural fluid specifically identifies tuberculous pleurisy with the following performance characteristics:

• Sensitivity: 97.2% (95% CI: 95.0-98.7%) 1
• Specificity: 94.2% (95% CI: 91.8-96.0%) 1
• Positive predictive value: 92.2% (95% CI: 89.1-94.7%) 1
• Negative predictive value: 98.0% (95% CI: 96.3-99.0%) 1

The ADA2 isoenzyme is statistically superior to total ADA for diagnosing tuberculous pleuritis and should be the preferred test when available. 1, 2

Diagnostic Algorithm Based on TB Prevalence

High TB Prevalence Populations

• Measure pleural fluid ADA2 (or total ADA if ADA2 unavailable) alongside routine investigations including AFB smear, mycobacterial culture, and cell count analysis 3
• ADA2 >40.6 U/L in a lymphocytic exudate strongly suggests tuberculous pleurisy 1
• Consider empirical antitubercular treatment when ADA2 >40 U/L in compatible clinical context 4
• However, tissue sampling for culture and drug-susceptibility testing remains the preferred diagnostic approach (strong recommendation by consensus from the British Thoracic Society) 5, 3

Low TB Prevalence Populations

• Use ADA2 primarily as an exclusion test rather than a diagnostic test 5, 4
• ADA2 <40 U/L yields a negative predictive value of approximately 98%, effectively ruling out tuberculous pleural effusion 4, 3
• When ADA2 exceeds 40 U/L in low-prevalence settings, proceed to pleural biopsy for histology and culture (diagnostic yield approximately 90%) 3

Enhancing Diagnostic Accuracy

Combined Testing Strategies

• Combine ADA2 with lymphocyte/neutrophil ratio ≥0.75 to increase specificity from 81% to 95% while maintaining 88% sensitivity 6
• Add interferon-gamma (IFN-γ) testing when available for even higher diagnostic certainty: 95% sensitivity and 96% specificity 5, 4

Optimal Cutoff Values

Different studies report varying optimal cutoffs:

• >40.6 U/L (highest quality evidence): 97.2% sensitivity, 94.2% specificity 1
• >52.4 U/L: 93.7% sensitivity, 88.7% specificity for total ADA 1
• >55.8 U/L: 87.3% sensitivity, 91.8% specificity 7

The British Thoracic Society and ATS/IDSA/CDC guidelines recommend using 40-45 U/L as the practical threshold 5, 4

Critical Limitations and False Results

False-Positive Scenarios

ADA2 can be elevated in conditions other than tuberculosis, reducing specificity in low-prevalence regions:

• Empyema and parapneumonic effusions 3, 6
• Rheumatoid pleurisy 4, 3
• Some malignancies 4

False-Negative Scenarios

• HIV co-infection: Tuberculous pleural effusions may not exhibit elevated ADA2 levels in HIV-positive patients, creating significant false-negative risk 3
• This is a critical pitfall requiring tissue diagnosis in HIV patients regardless of ADA2 results 3

Practical Implementation

When to Order ADA2

• Request ADA2 in all lymphocytic exudative pleural effusions where tuberculosis is in the differential diagnosis 4
• The test is most valuable when tuberculosis prevalence is high or when tissue sampling is not immediately feasible 5

What NOT to Do

• Do not use ADA2 as a standalone diagnostic test—always interpret results within the complete clinical context 8
• Do not skip tissue sampling when ADA2 is elevated, as culture provides definitive diagnosis and enables drug-susceptibility testing 5, 3
• Do not rely on ADA2 alone in HIV-positive patients due to high false-negative risk 3

Specimen Handling

• Send 5-10 mL of pleural fluid in sterile containers for ADA2 measurement 9
• Simultaneously send fluid in blood culture bottles for mycobacterial culture 9
• Submit tissue for histology and culture when biopsy is performed 5, 4