You've no doubt seen exudate but probably didn't know what it was called. It's that sticky fluid that seeps from a skin injury. This fluid is particularly noticeable with an open wound, like a scrape, that results in loss of the superficial skin layer. Wound exudate consists primarily of water but also contains proteins and other important substances that normally promote wound healing. In fact, a scab primarily consists of exudate after the water has evaporated. With large or persistent wounds, tracking the characteristics of the exudate -- including the color, consistency, quantity and odor -- helps identify potential problems with the normal healing process.
Serous exudate refers to thin, clear, watery, colorless to straw-colored fluid drainage from a wound. When you sustain a skin wound, the area becomes inflamed. Small blood vessels in the area leak fluid in response to this inflammation, which seeps from the surface of the wound as serous exudate.
In addition to water, serous exudate contains proteins that promote blood clotting and scab formation. Drainage of serous exudate normally occurs during the first stage of skin healing. However, excess amounts or prolonged seepage of serous fluid might signal a problem with wound healing.
Serosanguineous exudate is similar to serous drainage but appears slightly pink to light red due to the presence of a small amount of blood in the fluid. Serosanguineous exudate is considered normal during the early, inflammatory stage of the skin healing process. Drainage of serosanguineous exudate slowly diminishes before stopping altogether, which usually occurs within 1 to 5 days after sustaining a superficial skin wound.
With certain types of surgery, a tube called drain is left in the incision site for a few days after the procedure to allow exudate to escape. Exudate from a surgical site is often serosanguineous. A drain is removed once the exudate decreases to minimal level.
Sanguineous exudate is thin and watery but bright red due to a larger volume of blood than present in serosanguineous fluid. Sanguineous exudate represents fresh bleeding from the wound and is uncommon once initial bleeding after an injury stops. Sanguineous drainage occasionally occurs if the wound is accidentally bumped. It can also occur for a short period if the wound is reopened while changing the bandage or cleaning the wound.
Hemorrhagic exudate contains an even greater amount of blood than sanguineous exudate. It appears very similar to blood and is thicker than sanguineous fluid. The blood present in the fluid typically comes from minute holes in tiny blood vessels called capillaries. Sanguineous exudate is unusual and typically signals reinjury to the wound site or infection. Prolonged sanguineous drainage from a surgical drain might signal a problem, such as leakage from a blood vessel cut during the surgery. Large volumes of hemorrhagic exudate may require emergency care to stop the bleeding.
Seropurulent exudate is usually slightly thicker than serous drainage due to the presence of large number of white blood cells (WBCs). While a small number of WBCs are present in normal serous exudate, large numbers typically signal an infected wound. Seropurulent exudate appears milky, tan or slightly more yellow than normal serous fluid. The fluid might also have an unusual odor due to the infection.
Purulent exudate is essentially what most people call pus. It resembles seropurulent drainage but is thicker due to a high concentration of WBCs and bacteria. This thick yellow, gray or green drainage almost always signals a significant wound infection requiring medical treatment. Purulent exudate often emits a distinctly foul odor caused by bacteria, and degrading WBCs and skin tissue. An infected wound draining purulent exudate is usually tender and often surrounded by an expanding area of red, warm skin.
Warnings and Precautions
A small amount of serous or serosangineous exudate from a relatively minor skin wound is normal for the first few days. However, prolonged or abnormal drainage from a skin wound requires immediate medical evaluation. This is particularly true if you have diabetes or a weakened immune system, or experience any warning signs or symptoms, including:
-- fever or chills
-- increasing wound pain
-- expanding redness around the wound
-- red streaks radiating from the wound
-- purulent or foul drainage
-- heavy or persistent bleeding
Reviewed and revised by: Tina M. St. John, M.D.