A malignant pleural effusion (MPE) is the accumulation of fluid in the pleural space—the area between the lungs and the chest wall—caused by cancer. This buildup occurs when cancer cells disrupt the normal production and absorption of the liquid that lubricates the lungs. MPE is a complication of advanced cancer, with lung cancer, breast cancer, and lymphoma being the most common sources. Analyzing the collected fluid is the primary method for determining if an effusion is malignant.
Purpose of Fluid Analysis in Suspected Malignancy
The primary purpose of analyzing pleural fluid is to confirm an MPE diagnosis by identifying cancer cells. This confirmation is an important step, as the presence of an MPE indicates that a cancer has spread, which alters its stage and overall prognosis.
Fluid analysis can also help identify the cancer’s primary source if it is unknown. Cancers from different parts of the body, like the lung or breast, shed cells with distinct features. A pathologist examines these cells for clues about where the cancer started, which helps guide further tests and treatment.
The results of the analysis directly influence treatment. A confirmed MPE diagnosis shifts the focus of care toward palliative treatments aimed at relieving symptoms and preventing fluid recurrence. The findings help oncologists select appropriate therapies, such as chemotherapy or targeted treatments for the specific cancer type.
Fluid Collection and Gross Examination
The procedure to collect pleural fluid is called thoracentesis. A physician numbs a small area of skin and inserts a thin needle between the ribs into the pleural space to withdraw a fluid sample. The process is often guided by ultrasound imaging to ensure precise placement. This procedure relieves pressure from the fluid buildup while obtaining a sample for the laboratory.
Once in the laboratory, the first step is a gross examination, where a technician observes the fluid’s physical characteristics without a microscope. The appearance of the fluid can provide early clues about its cause. Healthy pleural fluid is clear or pale yellow, often described as “straw-colored.”
In cases of MPE, the fluid is often cloudy or turbid due to the high number of cells and proteins. Its color can also be informative. A reddish or pink hue suggests the presence of blood, which can be associated with cancer eroding into blood vessels. A milky-white appearance may indicate a chylothorax, where lymphatic fluid leaks into the pleural space, a condition sometimes linked to lymphomas.
Key Components of Laboratory Analysis
After the gross examination, the fluid undergoes several specific laboratory tests:
- Cytology: This is the microscopic examination of the fluid to detect cancer cells. A cytopathologist concentrates cells from the fluid onto a glass slide and uses stains to highlight cellular structures. This allows the pathologist to distinguish malignant cells from normal or inflammatory cells and is the most definitive test for diagnosing MPE.
- Biochemical Analysis: Technicians measure levels of protein and lactate dehydrogenase (LDH), an enzyme released during cell damage. These measurements are used in Light’s criteria to classify the fluid as an exudate. MPE is an exudate, characterized by high protein and LDH levels that indicate inflammation or cellular activity.
- Additional Markers: The fluid’s glucose level is often low in MPE because cancer cells consume it for energy. The pH of the fluid may also be low (acidic), often falling below 7.30, due to the metabolic activity of tumor cells. An acidic environment created by a tumor is a strong indicator of malignancy.
- Tumor Markers: The laboratory may test for specific tumor markers, which are substances produced in higher amounts by cancer cells. Examples include Carcinoembryonic Antigen (CEA) for lung cancers and Cancer Antigen 15-3 (CA 15-3) for breast cancer. These markers provide supportive evidence when cytology results are ambiguous.
Interpreting the Analysis Report
A result that is “positive for malignant cells” provides a definitive diagnosis of MPE. This means the cytopathologist identified cancerous cells in the fluid, confirming that the patient’s cancer has spread to the pleural space. This finding solidifies the medical team’s understanding of the cancer’s stage.
A “negative” report means no cancer cells were found, but this result does not completely rule out MPE. The diagnostic sensitivity of a single cytology test is not perfect and can miss cancer cells if they are few in number. A negative finding may prompt the physician to repeat the thoracentesis.
An “inconclusive” or “atypical” report indicates the pathologist observed abnormal cells that are suspicious but not definitively cancerous. This uncertainty requires further investigation. The physician might recommend a pleural biopsy, a procedure where a small piece of pleural tissue is removed for a clearer diagnosis.