What is the recommended diagnostic work‑up and management for a patient with atypical cells identified in pleural fluid?

Management of Atypical Cells in Pleural Fluid

When atypical cells are identified in pleural fluid, you must proceed directly to pleural tissue biopsy—preferably via thoracoscopy—because cytology alone cannot reliably distinguish reactive mesothelial cells from malignancy, and a definitive tissue diagnosis is essential for staging, prognosis, and treatment planning. 1, 2

Why Cytology Alone Is Insufficient

• Atypical cells are diagnostically indeterminate: The morphologic overlap between reactive benign mesothelial cells and malignant mesothelioma cells makes cytologic diagnosis unreliable, particularly for mesothelioma 1, 2
• Cytology sensitivity is limited: Pleural fluid cytology detects only approximately 60% of malignant effusions overall; the first specimen identifies 65% of malignancies, a second adds 27%, and a third contributes only 5% additional yield 3, 2
• Mesothelioma has particularly poor cytologic yield: Even when malignant cells are present, sarcomatoid and biphasic mesothelioma subtypes are rarely detected in fluid specimens 2
• Tissue architecture is required: Recognition of tissue invasion is mandatory for definitive diagnosis of malignant pleural mesothelioma, which cannot be assessed on fluid cytology 1

Immediate Next Steps: Obtain Tissue Biopsy

Proceed to thoracoscopic pleural biopsy as the gold standard, which achieves >92–95% diagnostic sensitivity and consistently outperforms cytologic examination 1, 2

Biopsy Options (in order of preference):

1. Thoracoscopy (medical or surgical):

   • Diagnostic yield >92–95% for malignancy 1
   • Allows direct visualization, multiple biopsies from different pleural sites, and simultaneous pleurodesis if indicated 1
   • Provides sufficient tissue for immunohistochemistry, molecular profiling, and receptor status testing needed for targeted therapies 1

2. Image-guided (ultrasound or CT) core needle biopsy:

   • Alternative when thoracoscopy is contraindicated or unavailable 1, 3
   • Must obtain adequate depth to assess for tissue invasion 2
   • Never perform blind (non-image-guided) pleural biopsies 1

3. Specimen handling: Place tissue in both saline (for culture) and formalin (for histology) 1, 3

Specialized Testing on Tissue Specimens

Once tissue is obtained, perform comprehensive analysis:

For Suspected Mesothelioma:

• BAP1 loss of expression and homozygous p16 deletion (by FISH) are highly specific for mesothelioma, though negative results do not exclude the diagnosis 1, 2
• Standard mesothelial markers (calretinin, CK5/6, D2-40, WT-1) help confirm mesothelial origin 1
• Histologic subtype (epithelioid, biphasic, sarcomatoid) must be documented, as it carries prognostic significance 1

For Suspected Metastatic Adenocarcinoma:

• Immunohistochemical panels differentiate primary site: CEA, B27.3, BerEP4, MOC-31 for adenocarcinoma 1
• Organ-specific markers guide primary tumor identification (TTF-1/Napsin A for lung, mammaglobin/ER/PR/GATA3 for breast, CDX-2/CK20 for GI, PAX-8 for gynecologic/renal) 1

For Molecular Profiling:

• Tissue specimens allow gene expression profiling and receptor status testing to assess eligibility for molecular targeted therapies 1
• Cell blocks from pleural fluid rarely provide sufficient material for comprehensive molecular testing 1

Additional Fluid Studies to Order Concurrently

While awaiting tissue biopsy, maximize diagnostic information from the pleural fluid:

• Send adequate volume: Obtain 25–50 mL (minimum 25 mL) for optimal cytologic yield 1, 3
• Cell block preparation: Use both direct smear and cell block techniques to facilitate immunohistochemistry 1
• Flow cytometry: Useful adjunct if lymphoma is suspected (sensitivity 50–94% for non-lymphoma malignancies) 1
• Microbiological studies: Send 5–10 mL in blood culture bottles (aerobic and anaerobic) plus sterile containers for Gram stain, AFB stain, and culture to exclude infection 1, 3

Critical Pitfalls to Avoid

• Do not diagnose mesothelioma on cytology alone: The International Mesothelioma Interest Group recommends diagnosis always be based on tissue biopsy 1
• Do not delay biopsy for repeat cytology: If the first cytology shows atypical cells and clinical suspicion for malignancy is high (especially mesothelioma), proceed directly to biopsy rather than repeating fluid sampling 1, 2
• Do not perform diagnostic bronchoscopy unless the patient has hemoptysis or features of bronchial obstruction; bronchoscopy is not indicated for undiagnosed pleural effusion workup 1, 3
• Beware of false-negative biopsies: Up to 15% of patients with initial biopsies showing nonspecific pleuritis are subsequently diagnosed with pleural malignancy (most frequently mesothelioma) 1

Management of Persistent Uncertainty After Initial Biopsy

If the first biopsy shows only nonspecific pleuritis but clinical suspicion remains high:

• Consider repeat biopsy via a different approach based on clinical suspicion and patient suitability 1
• Implement long-term radiological monitoring to ensure malignancy is not missed 1
• Reconsider alternative diagnoses: Tuberculosis and pulmonary embolism are treatable conditions that may present with atypical cells and should be systematically excluded 1, 3, 4