Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (599 page)

   Low pH (<7.30) always means exudate, especially empyema, malignancy, rheumatoid pleurisy, SLE, TB, esophageal rupture; may also be caused by systemic acidosis, hemothorax, urinothorax, paragonimiasis.

   pH <6.0 is consistent with but not diagnostic of esophageal rupture.

   Collagen vascular disease is the only other cause of pH <7.0.

   In a parapneumonic effusion, a pH <7.20 indicates need for tube drainage; pH >7.30 suggests that resolution with only medical therapy is possible. A pH <7.0 indicates the presence of complicated parapneumonic effusion.

   pH may fall before glucose becomes decreased.

   
Proteus
infection may increase pH because of urea splitting.

   In a malignant effusion, pH <7.30 is associated with short survival time, poorer prognosis, and increased positive yield with cytology and pleural biopsy; tends to correlate with pleural fluid glucose <60 mg/dL.

Generally, low pH is associated with low glucose and high LD; if low pH with normal glucose and low LD, the pH is probably a lab error.

Amylase

   Increased pleural fluid-to-serum ratio >1.0 and may be >5 or pleural fluid ULN for serum; should be determined only for left pleural effusions.

   Acute pancreatitis—may be normal early with increase over time.

   Pancreatic pseudocyst—always increased, may be >1,000 IU/L.

   Also perforated esophageal rupture, peptic ulcer, necrosis of small intestine (e.g., mesenteric vascular occlusion); 10% of cases of metastatic cancer.

   Isoenzyme studies

   Pancreatic type of amylase in acute pancreatitis and pancreatic pseudocyst.