| Adenosine deaminase (ADA) | > 40 U per L (667 nkat per L) | Tuberculosis (> 90 percent), empyema (60 percent), complicated parapneumonic effusion (30 percent), malignancy (5 percent), rheumatoid arthritis5 | In the United States, ADA is not routinely requested because of the low prevalence of tuberculous pleurisy. |
| Cytology | Present | Malignancy | Actively dividing mesothelial cells can mimic an adenocarcinoma. |
| Glucose | < 60 mg per dL (3.3 mmol per L) | Complicated parapneumonic effusion or empyema, tuberculosis (20 percent), malignancy (< 10 percent), rheumatoid arthritis5 | In general, pleural fluids with a low glucose level also have low pH and high LDH levels. |
| Lactate dehydrogenase (LDH) | > Two thirds of upper limits of normal for serum LDH | Any condition causing an exudate | Very high levels of pleural fluid LDH (> 1,000 U per L) typically are found in patients with complicated parapneumonic pleural effusion and in about 40 percent of those with tuberculous pleurisy.5 |
| LDH fluid toserum ratio | > 0.6 | Any condition causing an exudate | Most patients who meet the criteria for an exudative effusion with LDH but not with protein levels have either parapneumonic effusions or malignancy.3 |
| Protein fluid to serum ratio | > 0.5 | Any condition causing an exudate | A pleural fluid protein level > 3 mg per dL suggests an exudate, but when taken alone this parameter misclassifies more than 10 percent of exudates and 15 percent of transudates.13 |
| Red blood cell count | > 100,000 per mm3 (100 × 106 per L) | Malignancy, trauma, parapneumonic effusion, pulmonary embolism | A fluid hematocrit < 1 percent is nonsignificant.13 |
| White blood cell count and differential | > 10,000 per mm3 (10 ×3 109 per L) | Empyema, other exudates (uncommon) | In purulent fluids, leukocyte count is commonly much lower than expected because dead cells or other debris account for much of the turbidity. |
| Eosinophils | > 10 percent | Not diagnostic | The presence of air or blood in the pleural space is a common cause. No diagnosis is ever obtained in as many as one third of patients with eosinophilic pleural effusion.3 |
| Lymphocytes | > 50 percent | Malignancy, tuberculosis, pulmonary embolism, coronary artery bypass surgery | Pleural fluid lymphocytosis > 90 percent suggests tuberculosis or lymphoma. |
| Neutrophils | > 50 percent | Parapneumonic effusion, pulmonary embolism, abdominal diseases | In about 7 percent of acute tuberculous pleurisy and 20 percent of malignant pleural effusions, a neutrophilic fluid predominance can be seen.5 |