The National Healthcare Safety Network (NHSN) divides the types of surgical site infection (SSI) into three categories, based on the depth of the infection. A superficial incisional SSI is an infection that occurs within 30 days of an operative procedure, involves only the skin and subcutaneous tissues of the incision, and has at
least one of the following:
1. “Purulent drainage with or without laboratory confirmation, from the superficial incision.
2. Organisms isolated from an aseptically obtained culture of fluid or tissue from the superficial incision.
3. At least one of the following signs or symptoms of infection: pain or tenderness, localized swelling, redness, or heat and superficial incision is deliberately opened by surgeon, unless incision is culture negative.
4. Diagnosis of superficial incisional SSI made by a surgeon or attending physician.” 
The other two types of infection defined by the NHSN are deep incisional SSI and organ/space SSI. The definition of deep incisional SSI adds a period of up to 90 days after the procedure and includes purulent wound drainage or spontaneous dehiscence or required intentional opening of the wound, constitutional symptoms or positive imaging (e.g. abscess) or diagnosis by other testing or diagnosis by the surgeon or attending physician. Organ/space SSI includes all of the above signs and/or symptoms for deep incisional infection, but includes infection of any tissue manipulated during surgery. Despite improved practices in infection control, SSIs still cause morbidity, increased length of hospitalization, and mortality. With a goal of containing rising medical care costs, the Centers for Medicare & Medicaid Services selected conditions that are reasonably preventable and are costly to manage. These conditions are not reimbursed by Medicare and include SSI following certain orthopedic procedures as well as other surgical procedures.