Exudate

Exudate is a protein-rich fluid containing cellular elements that escapes from small blood vessels into surrounding tissues or body cavities during inflammation. Exudate formation is one of the key features of the inflammatory response.

Once this fluid appears, it marks increased vascular wall permeability — the principal mechanism of inflammation. Unlike transudate (a non-inflammatory edema), exudate signifies that the capillary walls become permeable not only to water but also to large protein molecules (such as fibrinogen) and blood cells (leukocytes).

Pathophysiology and Types

Based on its composition, which is determined by the cause and stage of inflammation, several major types of exudate are distinguished:

• Serous. Clear, watery fluid with few cells. Typical of the early stages of inflammation or viral infections (e.g., the fluid inside a burn blister).
• Fibrinous. Contains a large amount of fibrinogen, which precipitates as fibrin threads, forming films on organ surfaces (e.g., in fibrinous pericarditis).
• Purulent. Cloudy, viscous fluid composed of numerous dead and living neutrophils, microorganisms, and tissue breakdown products. Characteristic of bacterial infections.
• Hemorrhagic. Exudate with admixture of erythrocytes, indicating significant vascular damage.

Clinical significance

Accumulation of exudate leads to edema at the site of inflammation (e.g., swelling after an insect bite) or fluid collection in body cavities (e.g., pleuritis in pneumonia). The exudate nature is an important diagnostic indicator.

Exudate obtained by puncture has considerable clinical value. Laboratory analysis (protein concentration, cellular composition, microbiological culture) helps categorize the inflammatory process, identify the causative agent, and guide appropriate therapy.

Differential Diagnosis

It is essential to distinguish exudate from transudate. Unlike exudate, transudate is an edematous fluid of non-inflammatory origin, formed due to disturbances in hydrostatic or oncotic pressure (e.g., in cardiac or renal failure). Moreover, transudate is not rich in protein and cells.