What are the fluid analysis studies for thoracentesis (pleural fluid aspiration)?

From the Guidelines

Thoracentesis fluid studies should include submitting 25-50 mL of pleural fluid for cytological analysis in patients with suspected malignant pleural effusion, as well as sending samples in both plain and blood culture bottle tubes in patients with suspected pleural infection. This approach is based on the most recent and highest quality evidence from the British Thoracic Society guideline for pleural disease 1. The guideline recommends that at least 25 mL, and where possible 50 mL, of pleural fluid should be sent for initial cytological examination, and that pleural fluid samples should be processed by direct smear and cell block preparation.

In terms of handling specimens, the guideline suggests that pleural fluid samples should be sent in both white top containers and volumes of 5-10 mL inoculated into (aerobic and anaerobic) blood culture bottles in patients with suspected pleural infection. If the volume of fluid sample available does not allow 5-10 mL inoculation, volumes of 2-5 mL should be prioritized to blood culture bottles rather than a plain, sterile container.

While other studies have suggested additional tests, such as amylase for pancreatic disease or esophageal rupture, triglycerides for chylothorax, and adenosine deaminase and interferon-gamma release assays for tuberculosis, these are not universally recommended and should be ordered based on clinical suspicion. The use of Light's criteria to differentiate transudates from exudates is also important, as transudates typically result from systemic processes, while exudates suggest localized pleural pathology requiring further investigation.

Proper handling of specimens is essential, with fluid collected in appropriate containers and transported promptly to the laboratory to ensure accurate results. The older study from 2000 1 provides some additional context, but its recommendations are largely superseded by the more recent guidelines from the British Thoracic Society.

Key points to consider when evaluating pleural effusions include:

• Submitting sufficient pleural fluid for cytological analysis
• Sending samples in appropriate containers for microbiology and other tests
• Using Light's criteria to differentiate transudates from exudates
• Ordering additional specialized tests based on clinical suspicion
• Ensuring proper handling and transport of specimens to the laboratory.

From the Research

Fluid Studies for Thoracentesis

• The routine pleural fluid (PF) evaluation usually includes cell count and differential, tests for protein, LDH, glucose, adenosine deaminase, cytology, and if infection is a concern, pH and bacterial and mycobacterial cultures 2.
• Distinguishing transudates from exudates with Light's criteria is a pragmatic first step in the diagnostic approach to an undiagnosed pleural effusion 2, 3.
• Various PF tests have proven diagnostic utility, such as:
  • Adenosine deaminase levels >35 IU/L usually indicate tuberculosis in lymphocyte-predominant PF 2.
  • pH < 7.2 or glucose less than 60 mg/dL allow the clinician to identify complicated parapneumonic effusions 2, 3.
  • Conventional cytology may reveal malignant cells in 60% of the patients with malignant effusions 2.
• Optional PF tests may complement the diagnostic approach to an undiagnosed pleural effusion, including:
  • Natriuretic peptide assays to improve the accuracy of a diagnosis of cardiac pleural effusion 2.
  • PF mesothelin levels greater than 20 nmol/L are highly suggestive of mesothelioma 2.
• In patients with bilateral pleural effusions, the main cellular and biochemical features of the pleural fluid on the right side tend to be similar to those on the left side, suggesting that diagnostic thoracentesis may not need to be performed on both sides unless there is a specific clinical indication 4.

Diagnostic Approach

• The patient's history and physical examination should guide the evaluation of pleural effusion 3.
• Small bilateral effusions in patients with decompensated heart failure, cirrhosis, or kidney failure are likely transudative and do not require diagnostic thoracentesis 3.
• Point-of-care ultrasound should guide thoracentesis to reduce complications and can detect small effusions and features that could indicate complicated effusion or malignancy 3, 5.
• Computed tomography of the chest can exclude other causes of dyspnea and suggest complicated parapneumonic or malignant effusion 3.