Pleural Fluid Cytology (also called pleural cytology or lung fluid cytology) is a laboratory test in which a specialist examines fluid taken from around the lungs under a microscope. The fluid comes from the pleural space, a thin area between the lung and the chest wall. When extra fluid builds up there, it is called a pleural effusion.

In this test, the lab looks closely at the cells within the pleural fluid to check for:

• Malignant (cancer) cells that may indicate cancer involving the pleura or cancer that has spread (metastasized) to the pleural space
• Inflammatory cells that suggest ongoing infection, irritation, or chronic inflammatory conditions
• Reactive or atypical cells that are not clearly cancer but may require follow-up testing or repeat sampling

Results are usually reported in categories such as:

• Negative for malignant cells: No cancer cells were seen. This does not completely rule out cancer, because cancer cells may be absent in the sample or present in very low numbers.
• Suspicious/atypical: Some cells look abnormal but are not definitive for cancer. Your clinician may recommend repeat cytology, additional imaging, or a pleural biopsy.
• Positive for malignant cells: Cancer cells were identified. This helps confirm a malignant pleural effusion and can guide next diagnostic and treatment steps.

Pleural fluid cytology is often performed along with other pleural fluid tests (such as protein, LDH, glucose, pH, Gram stain/culture) to better determine the cause of a pleural effusion.

Doctors order Pleural Fluid Cytology when someone has a new or unexplained pleural effusion or when imaging (like a chest X-ray, ultrasound, or CT scan) shows fluid around the lungs that needs evaluation. The goal is to determine whether the pleural effusion is related to cancer, infection, or another inflammatory condition.

This test is commonly considered if you have symptoms such as:

• Shortness of breath or trouble breathing, especially when lying down
• Chest pain (often sharp and worse with deep breathing)
• Persistent cough
• Fever, chills, or signs of infection
• Unexplained weight loss, fatigue, or night sweats

It may also be ordered when there are risk factors or clinical concerns such as:

• Known cancer (for example, lung cancer, breast cancer, lymphoma) to check for spread to the pleura
• Suspicion of mesothelioma, particularly with a history of asbestos exposure
• Recurrent pleural effusion or pleural effusion that does not respond as expected to treatment
• Possible tuberculosis or other chronic infections that can involve the pleura

The fluid sample is typically obtained through thoracentesis, a procedure where a clinician uses a needle to remove pleural fluid. Besides helping diagnose the cause, removing fluid can also relieve breathing symptoms caused by the effusion.

• Malignant pleural effusion (pleural effusion due to cancer)
• Lung cancer (including adenocarcinoma and other subtypes)
• Breast cancer with pleural involvement
• Lymphoma or leukemia with pleural involvement
• Malignant mesothelioma (often associated with asbestos exposure)
• Metastatic cancers to the pleura (e.g., ovarian, gastrointestinal cancers)
• Pneumonia with parapneumonic effusion or empyema (infection-related pleural fluid)
• Tuberculous pleuritis (TB-related pleural effusion)
• Chronic inflammatory pleuritis (non-infectious inflammation of the pleura)

• Identifying the cause of a pleural effusion to guide targeted treatment
• Evaluating possible cancer when fluid around the lungs is present (supporting early detection and staging)
• Monitoring known malignancy for pleural spread or recurrence
• Assessing persistent or recurrent shortness of breath related to fluid buildup
• Supporting diagnosis of infection or chronic inflammation affecting the lungs/pleura
• Helping clinicians choose next steps (repeat drainage, additional lab tests, imaging, or pleural biopsy)