pleural fluid analysis

When fluid is drawn off the chest, it is not simply discarded — it is interrogated like evidence. Pleural fluid analysis is the laboratory study of that aspirated fluid, reading its appearance, chemistry, cells and microbes to work out where it came from and why.

Several questions are asked at once. Color and clarity give immediate clues: straw-colored and clear suggests a simple effusion, bloody suggests trauma or cancer, milky suggests chyle, and frank pus means empyema. Chemistry (protein, LDH, glucose, pH) separates transudate from exudate and flags infection. A cell count and differential shows whether neutrophils, lymphocytes or eosinophils dominate, and cytology hunts for cancer cells, while cultures and special stains look for bacteria or tuberculosis.

No single number gives the answer; the panel is read together and against the clinical story. A lymphocyte-rich exudate may point toward tuberculosis or malignancy, a neutrophil-rich one toward acute infection. When the fluid is inconclusive but suspicion of cancer or tuberculosis remains, a tissue biopsy of the pleura is the usual next step.