Serous Drainage: Understanding Wound Exudate

As a practitioner providing wound care, you understand how important it is to observe and document the presence of drainage. Even further, you want to be able to determine what type of drainage it is, as that can tells you more about how the wound is healing and any complications that may come up. There are four major types of exudate, with some indicating healing and others suggesting infection. But what does exudate, like serous drainage, tell practitioners? 

Ever find yourself asking questions like: what is serous drainage? What’s the most appropriate treatment plan when you see it? Knowing this information can better equip professionals in their practice, leading to improved patient outcomes and fewer costly errors. We’ll look below to understand how and why serous drainage occurs.

What Is Serous Drainage?

To fully understand serous drainage, it helps first to explain exudate, of which it’s a type. The term comes from the Latin exsudare, meaning “to sweat out.” 

While other names like ooze, drainage, or secretion might suggest something more insidious, exudate is simply defined as fluid that has seeped from blood vessels and deposited into nearby tissues. This leaking occurs in response to an infection or injury, and while often alarming to patients, it’s the body’s natural reaction to tissue damage and is a normal part of wound healing. But why exactly does this happen? 

Where Does Exudate Come from?  

To start, there would be no exudate without interstitial fluid, also referred to as bodily fluid. Interstitial fluid is comprised of substances that leak out of blood capillaries and are found in the spaces surrounding cells. Its role is to help transport oxygen and nutrients to cells and remove waste products from them. This process aids in maintaining homeostasis so that bodily fluid production isn’t greater than the drainage lymphatic system capacity. 

However, when an injury takes place, a chain reaction occurs. Blood vessels dilate to boost blood flow and deliver much-needed nutrients to the wounded area. Capillary permeability increases as well, allowing white blood cells, large molecules, and other solid matter to pass through capillary walls. With all of this going on, inevitably, interstitial fluid will leak into a wound cavity, forming the basis of exudate. 

How Much Exudate Is Produced? 

Interestingly, there is no widely accepted “normal” rate of exudate production. It depends on the type of wound, as some naturally produce more exudate than others. 

What we do know is that as the amount of bodily fluid maintained in wound tissue increases, so does the amount of exudate from the wound surface. Research shows that the primary factors responsible for encouraging increased interstitial fluid production are: 

• Elevated capillary hydrostatic pressure. This pertains to the amount of pressure created by fluid in the capillaries or tissues. More pressure drives fluid out of the capillaries and into the surrounding tissues, i.e. pushes fluid out.
• Reduced oncotic pressure, which refers to the pressure expended by proteins in the blood to help pull fluids back into the capillaries. When lowered, it leads to a decrease in plasma protein concentration, causing additional fluid to leave the capillaries and move into the interstitial space, i.e. pulls fluid in.
• Increased permeability of the capillary wall. This applies to the leakiness of the capillary wall. Capillaries are usually semi-permeable, so when permeability increases, more cells and other substances can move into the tissues. 

Note: Hydrostatic and oncotic pressure are supposed to have a delicate balance of push and pull. When there’s an imbalance in the two pressures mentioned above, capillary permeability increases and causes plasma to leak.  

There are additional factors that can affect exudate production, such as: 

• Wound healing stage. Normally, there’s more exudate during the inflammatory stage than in later stages. 
• Systemic factors, like age, chronic conditions, medications, malnutrition, fluid intake, and more. 
• Local factors, such as edema, foreign body in wound, infection, etc. 
• Other factors, like heat, patient compliance, and wound position.

What Is Exudate Composed of?

Wound exudate comes from bodily fluid that leaks out of blood vessels and, therefore, can be comprised of many things. It’s a protein-rich fluid that might include immune cells, like lymphocytes, that fight off infection and diseases. It might also have metabolic waste products, wound debris, microorganisms, and more. 

As previously mentioned, exudate is normal, so we want to see it. However, this fluid can cause issues if it’s not in the right location, in the accurate amount, or composed of the proper material. 

What Is Serous Drainage? 

Serous drainage is one of four main types of exudate and is a fairly common one for practitioners to encounter. It’s often seen during the inflammatory stage, which usually spans the first two to three days of wound healing. 

The usually odorless fluid has specific characteristics that distinguish it from the others, such as its thin, watery consistency and clear appearance. In fact, if a patient is wearing a bandage, this exudate type likely won’t leave behind much color. The color of serous drainage might occasionally be a pale yellowish hue, if colored. 

There are three other forms of exudate. 

• Sanguineous: This exudate form, with its bright red hue and almost syrupy consistency, occurs when fresh blood seeps from a wound following an injury.
• Serosanguineous: A combination of blood and serous fluid, this thin, watery secretion is typically light pink or pale red. It’s considered one of the most common types of wound drainage. 
• Purulent: Also called pus, this drainage form is usually tan, brown, opaque, or yellow in appearance. It’s the thickest form of drainage, may have a foul odor, and is the body’s inflammatory response to infection. 

Each has distinct features that tell a story about a wound’s status, including whether or not it’s healing correctly. 

What Is Serous Drainage Made of?

The Cleveland Clinic explains that serous drainage is plasma, which is the liquid part of blood without proteins. However, it’s important to clarify that this doesn’t mean there are absolutely no proteins in this exudate type. Serous fluid simply has a lower concentration of proteins because plasma is more than 90% water. The remaining 10% or so are solids, like protein. More specifically, it’s primarily made up of the proteins albumin and fibrinogen. 

Albumin is created by the liver and is the most abundant protein found in plasma. Its primary role is to monitor oncotic pressure and transport molecules called ligands. Some of those transported ligands are ions, fatty acids, and bilirubin. There is even exogenous albumin, often injected intravenously. Interestingly, methotrexate and warfarin are two drugs that bind to this plasma protein. 

Research supports that albumin is vital to wound healing, as it provides two important amino acids: arginine and glutamine. Arginine is necessary for collagen synthesis, which promotes tissue formation and the eventual closure of the wound. Glutamine is a primary fuel source for immune cells and thus necessary for wound repair. 

The liver produces fibrinogen, too, a protein that aids with the clotting of blood. Fibrin forms blood clots that help seal the damaged blood vessels and prevent additional blood loss, making them a key component of the first phase of wound healing: hemostasis. But fibrinogen’s role doesn’t stop there. Fibrinogen may behave as a reservoir for growth factors, direct immune system response, and even remove itself from the wound to ensure healing progress. 

How Much Serous Drainage Is Too Much Serous Drainage?

Wound care clinicians must monitor the production of serous drainage, as excess or insufficient amounts can interfere with wound healing. Although it’s normal for serous exudate to make a bandage damp or even wet, this should decrease over time. 

Overproduction of Serous Exudate

If serous production is ramping up, especially after the inflammatory stage has passed, harmful bacteria may be present. Soaked-through dressings, for example, might indicate that a large number of microorganisms are on the surface (aka high bioburden), which can slow healing and cause infection. 

Providers should also closely examine where the excess serous fluid comes from. If it comes from a post-operative incision line or leaks from areas where there’s no injury, it could indicate that a patient has an underlying medical condition. 

Declining Production of Serous Drainage

Typically, observing less serous exudate means that an infection is resolving and/or the wound is healing, which you want to see as your patient improves.

But sometimes, it might mean poor blood flow to the wound site. This can cause tissue damage since the tissue is likely not receiving the nutrients and oxygen it needs to thrive. Occasionally, it could even indicate the following: 

• Dehydration
• Improper intervention
• Wrong dressing choice
• Microangiopathy (small vessel disease) 
• Hypovolemic shock

Documenting the Amount of Serous Drainage

Practitioners understand that you can’t manage or improve what you don’t measure, even when it comes to healing wounds. That’s why it’s important to conduct wound checks frequently, with some health systems even doing so once per shift. 

During the assessment, wound care specialists should document the amount of exudate they see. Some may document it based on descriptions from the Bates Jensen Wound Assessment Tool below.

• None: The wound tissue is dry.
• Scant: The wound tissue is moist, but there is no measurable exudate on the dressing.
• Small/Minimal: The wound tissue is wet but evenly distributed within the wound. Drainage covers less than 25% of the dressing. 
• Moderate: The wound tissue is saturated but may or may not be evenly distributed. The exudate covers approximately 25% to 75% of the dressing. 
• Large/Copious: The wound tissue is filled with fluid that may or may not be evenly distributed. The exudate covers more than 75% of the dressing.

Is Serous Drainage Bad? 

For the most part, the presence of serous exudate isn’t cause for concern. Research suggests that in small amounts, it’s normal to observe during the inflammatory stage of wound healing and signals that the immune system is responding properly. However, as revealed earlier, it is possible to have too much of anything, even a potentially good one. 

Seroma: A Common Surgical Complication

Many patients develop a seroma following surgery, which is when excess serous fluid accumulates under the skin in an area where tissue was removed. It’s not entirely understood why they happen, but it might be due to the lymphatic and drainage systems being disrupted following tissue removal. The combination of serum and inflammatory exudate may trigger this potentially painful response. 

Their rate of incidence is nearly 18%, and they’re specifically common following breast cancer surgery, plastic surgery, and abdominal wall hernia repair. 

Usually, this type of surgical wound is harmless, but they can become infected. 

The Treatment of Serous Drainage

If a wound is healing normally, then serous drainage will stop on its own. Therefore, the best thing clinicians can do is continue following proper wound management protocols. 

However, providers may need to make treatment adjustments if there appears to be insufficient or excess production. 

If serous exudate production has increased, consider the following suggestions: 

• Use a foam, hydrocolloid, or alginate dressing for high-exudate wounds
• Change primary and/or secondary dressings more frequently 
• If appropriate, use compression
• Assess if a systemic intervention is needed 
• Elevate the area, if possible 

If serous drainage has trailed off significantly or sooner than what’s expected for a patient’s wound type, try the following: 

• Select a dressing type that maintains, conserves, or adds moisture (ex. for low-drainage injuries, consider using a film, hydrogel, or hydrocolloid dressing)
• Change dressing less frequently 
• Use a thinner version of the current dressing being used 
• Ensure the patient isn’t dehydrated or struggling with other local or systemic issues

Treating a Seroma

Currently, there is no single treatment option for seroma that has been proven to promise therapeutic resolution. Often, patients may require multiple outpatient visits to remove seroma fluid. Treatment may include one or more of the techniques below: 

• Percutaneous aspiration
• Applying sclerosing agents in the seroma cavity 
• Marsupialization
• Debridement of the seroma capsule
• Drainage

Serous Drainage: A Common Sign of Healing

Often overlooked but still holding much significance, serous drainage is a type of exudate that practitioners don’t mind seeing. Its presence during the initial stages of wound healing tells clinicians that things are heading in the right direction. The only time it becomes concerning is when it’s overstayed its welcome, there’s an overproduction of it, it has an odor, or an unusual color. 

We hope the information provided here helps clinicians provide wound care that leads to fewer complications, better patient outcomes, and healthier bottom lines.