Exudate versus transudate
An exudate is protein-rich fluid that leaks through vessel walls made leaky by inflammation; a transudate is protein-poor fluid from pressure imbalances across an intact vessel wall.
Remember
Check the protein: protein-rich fluid from leaky, inflamed vessels is an exudate, while protein-poor fluid from pressure changes across an intact wall is a transudate.
Memory aid
exudate means the wall is leaking protein; transudate means the wall is fine and the pressures are the problem
Ask whether the vessel wall is leaking protein or is intact with the pressures at fault.
An exudate is fluid, protein and cells that have escaped from the blood into the interstitium (the space between cells). It forms in inflammation, when mediators (inflammatory chemicals) such as histamine make the endothelial cells that line the vessel contract and pull apart, so that gaps open. Other routes to leakage include direct injury to vessels (for example in a burn) and injury caused by leucocytes (white blood cells), which damages endothelial cells and creates gaps.
Plasma proteins pass through these gaps, which is why the fluid is protein-rich. This rise in vascular permeability means that material crosses the vessel wall more easily. Proteins in the tissue draw water towards them, a pull called oncotic pressure, so more fluid is pulled out of the vessel and the tissue swells.
A transudate is the opposite kind of fluid: the vessel wall is intact, and the fluid is low in protein, with few or no cells. It forms because pressures across the wall are out of balance, not because the wall has become leaky. Heart failure and a low albumin level are the classic causes. In heart failure, raised hydrostatic pressure (the pressure that pushes fluid out of the vessel) forces fluid out through the intact wall. With a low albumin level, the pull holding fluid in the vessels is weaker, so fluid leaks out.
The distinction matters clinically. In a pleural effusion (fluid around the lung), a transudate can usually be treated without extensive investigation, but an exudate calls for a search for its cause, since inflammation, infection or cancer is behind it.
Key facts
- Exudate
- protein-rich fluid, often with cells, that leaks out of vessels because of increased permeability during inflammation.
- Transudate
- low in protein, with few or no cells, and formed without any increase in permeability.
- Causes of transudate
- heart failure and a low albumin level are the classic causes.
- Practical test
- protein content is high in an exudate and low in a transudate.
- Pressures
- hydrostatic pressure forces fluid out of the vessel, whereas oncotic pressure, exerted by plasma proteins, draws fluid into the vessel.
- Useful job
- an exudate delivers fibrin (a sticky clotting protein), mediators and immunoglobulins (antibodies) to the damaged site.
Exam traps
- Raised hydrostatic pressure on its own does not make an exudate: it pushes fluid out through an intact vessel wall, so the fluid is low in protein, which is a transudate.
- Gap formation is how many inflammatory mediators make an exudate, but clear fluid that forms through a wall that has not become leaky, so little protein escapes, is the picture of a transudate and does not describe an exudate.
- Arteriolar dilation (widening of the small arteries supplying the capillaries) does push more fluid out of the capillaries, but with an intact endothelium (the thin cell layer lining the vessel) that fluid is not protein-rich, because protein enters the tissue only when permeability rises.
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