What parameters should be checked in pleural fluid analysis in an adult patient with a history of respiratory or cardiovascular issues presenting with pleural effusion?

Pleural Fluid Analysis Parameters

All pleural fluid samples should be analyzed for protein, lactate dehydrogenase (LDH), pH, glucose, cytology, Gram stain, acid-fast bacilli (AAFB) stain, and sent for microbiological culture in both sterile vials and blood culture bottles. 1

Essential Initial Parameters

Routine Analysis for All Effusions

• Gross appearance and odor must be documented in every case, as this provides immediate diagnostic clues (e.g., purulent appearance suggests empyema, unpleasant odor indicates anaerobic infection) 1

• Protein and LDH levels (both pleural fluid and serum) to apply Light's criteria for transudate vs. exudate differentiation 1, 2

  • Light's criteria: exudate if pleural fluid/serum protein ratio >0.5, OR pleural fluid/serum LDH ratio >0.6, OR pleural fluid LDH >0.67 upper limit of normal 2
  • Sensitivity 98% but specificity only 72% for exudates 2

• Cytology should be sent on all exudative effusions, though it only diagnoses 60% of malignant effusions 1, 3

  • If first cytology is non-diagnostic, obtain a second sample to increase diagnostic yield 1

• Differential cell count to identify predominant cell type (neutrophils suggest infection, lymphocytes suggest tuberculosis or malignancy, eosinophils >10% suggest drug reaction, malignancy, or prior instrumentation) 1, 4

Microbiological Studies

• Gram stain and bacterial culture in sterile tubes for all exudates 1, 4

• Blood culture bottles in addition to sterile vials significantly increase diagnostic yield for infection 1

• AAFB stain and TB culture for all exudates, particularly when lymphocyte-predominant 1

Context-Specific Parameters

For Suspected Parapneumonic Effusion or Infection

• pH measurement is mandatory and should be performed immediately in all non-purulent effusions when infection is suspected 1

  • pH ≤7.2 indicates high risk of complicated parapneumonic effusion/empyema requiring chest tube drainage 1
  • pH >7.2 and <7.4 indicates intermediate risk; measure LDH 1
  • pH ≥7.4 indicates low risk, no immediate drainage needed 1
  • Critical pitfall: Avoid contamination with local anesthetic or heparin, eliminate air from syringe, and analyze immediately as delays falsely elevate pH 1

• Glucose level if pH unavailable: glucose <3.3 mmol/L (60 mg/dL) suggests high probability of complicated parapneumonic effusion/empyema 1, 2

• LDH >900 IU/L in intermediate-risk cases (pH 7.2-7.4) warrants chest tube consideration, especially with fever, high fluid volume, low glucose, or septations on ultrasound 1

• Extremely low pH <6.5 mandates immediate evaluation for esophageal perforation 5

For Suspected Tuberculosis

• Adenosine deaminase (ADA) level >45 IU/L strongly suggests tuberculous pleuritis 6, 4

• Gamma-interferon level >3.7 U/mL supports TB diagnosis 6

• Lymphocyte-predominant exudate with positive tuberculin skin test justifies empirical anti-TB therapy 1

For Suspected Malignancy

• Pleural fluid pH <7.3 in malignant effusions correlates with higher cytology yield, worse survival, and poorer pleurodesis response 5

• Cytology has only 60% sensitivity; if negative and malignancy suspected, proceed to image-guided pleural biopsy or thoracoscopy 1, 3

For Suspected Heart Failure

• Serum NT-proBNP (not pleural fluid) should be measured for unilateral effusions suspected of cardiac origin 1
  • Pleural fluid NT-proBNP offers no advantage over serum levels 1

For Other Specific Conditions

• Amylase (pleural/serum ratio >1.0) suggests pancreatitis, esophageal rupture, or malignancy (particularly adenocarcinoma) 5, 2

• Pleural fluid ANA may support lupus pleuritis diagnosis, though 10% of non-SLE effusions are ANA-positive 1

• Haematocrit if grossly bloody to diagnose hemothorax 1

Critical Clinical Pitfalls

• Do not aspirate bilateral effusions in clinically obvious transudates (heart failure, cirrhosis) unless atypical features present or failure to respond to therapy 1

• Reconsider pulmonary embolism and tuberculosis in persistently undiagnosed effusions, as these have specific treatments 1

• Perform contrast-enhanced CT with fluid present before complete drainage to optimize pleural visualization and identify biopsy targets 1, 3

• Avoid blind pleural biopsies; use image-guided techniques or thoracoscopy 1