FAQs: Surgical Site Infections (SSI) Events

NHSN SSI Case Review

Q1: What can the user provide to NHSN for a complete case review request?

Please let NHSN know what your question(s) are and what your thoughts are regarding the case

Ex: This is a complicated case and our team is trying to figure out whether it meets criteria for a Deep Incisional SSI- can you help us confirm?

What NHSN needs from the user:

OR procedures and dates of all procedures including reoperations

  • Whether operative procedures are coded as NHSN operative procedures or not
  • If return to OR via same incision, was it within 24 hours of finish time of prior operative procedure?

Signs and symptoms?

Tissue levels involved- Superficial, Deep and/or Organ/Space?

Was any imaging testing performed and described?

Fluid collections or drainage?

  • CT guided drainage performed? Drainage from JP drain? Drainage from wound?
  • Purulent? How was the drainage described?

Culture Results

  • What site was the specimen collected from?
  • What tissue level (depth) was the specimen collected from? If you are unsure NHSN recommends consulting with the surgeon/physician to make that determination.

Other evidence of infection?

Surveillance Periods for SSI

Q2: Does each trip to the OR via the same site start a new surveillance period?

Yes. Each return trip to the OR via the same site ends the surveillance period from prior infection and resets the new surveillance period.

SSIs are normally attributed to the most recent trip to the OR.

Non-NHSN Operative Procedure

Q3: If a patient has a non-NHSN operative procedure, can I attribute an SSI to the procedure?

Always determine the ICD-10 PCS or CPT codes assigned to a procedure to determine if the procedure qualifies for SSI surveillance.  You cannot apply SSI criteria to a non-NHSN operative procedure.

Pathogen Assignment

Q4: Are common commensal organisms excluded from meeting SSI criteria?

No. Common commensal organisms are not excluded.  The only excluded organisms are found in Chapter 9 SSI protocol numerator reporting instruction #1 on page 9-14.

Broth Only Cultures

Q5: How do I interpret ‘broth only’ for the final culture report when reporting SSI events in NHSN?

Positive cultures from broth only are considered a positive culture result and treated as such for surveillance purposes. Such media can be enriched to identify organisms that might otherwise be missed.

Timeframe for SSI Elements

Q6: Is there a certain timeframe for all elements of SSI criteria to occur?

SSI guidelines don’t offer a strict timeframe for elements of criteria to occur other than to note that the date of event for an SSI must occur within the appropriate surveillance period, i.e. 30 or 90 days.  NHSN does train that all the elements required to meet a criteria must occur within the same general timeframe in the surveillance period following the NHSN operative procedure although this ‘general timeframe’ is not specified. NHSN certainly doesn’t want users to use an element that occurs on POD 1 or 2 and match it up with another element such as positive culture occurring 3 weeks later and attribute this to an SSI.  In our experience, all symptoms required to meet an SSI criteria usually occur within a 7-10 day timeframe with no more than 2-3 days between elements.  The elements must be relational to each other, meaning you should ensure the elements all associate to the SSI and this can only happen if elements occur in a relatively tight timeframe. Each case differs based on the individual elements occurring and the type of SSI.

Clarification of SSI Criterion

Q7: Does NHSN have a definition for purulence?

NHSN does not define purulent drainage as there is no standard, clinically agreed upon definition. Generally, thick/viscous, creamy/opaque fluid discharge with or without blood seen at the site or document of pus/purulence by a medical professional would be accepted evidence of purulent drainage.

At this time NHSN does not use any gram stain results such as WBCs or PMN’s to define purulence for the SSI protocol.

Procedure — Contaminated Procedure

Q8: If a patient has a NHSN procedure that was contaminated do I still have to report an SSI if one develops in the surveillance period?

POA definition does not apply to the SSI protocol. If there was evidence of infection at the time of the procedure and then later in the surveillance period the patient develops an infection that meets the NHSN SSI criteria it is attributed to the procedure (see PATOS below). A high wound class is not an exclusion for a patient later meeting criteria for an SSI.


Q9: Where can I find more information and examples of PATOS events?

Infection present at time of surgery (PATOS): PATOS denotes that there is evidence of an infection or abscess at the start of or during the index surgical procedure (in other words, it is present preoperatively). PATOS is a YES/NO field on the SSI Event form. The evidence of infection or abscess must be noted/documented intraoperatively in an intraoperative note (immediate postoperative note). Only select PATOS = YES if it applies to the depth of SSI that is being attributed to the procedures (for example, if a patient has evidence of an intraabdominal infection at the time of surgery and then later returns with an organ/space SSI the PATOS field would be selected as a YES. If the patient returned with a superficial or deep incisional SSI the PATOS field would be selected as a NO). The patient does not have to meet the NHSN definition of an SSI at the time of the primary procedure but there must be notation that there is evidence of an infection or abscess present at the time of surgery. PATOS is not diagnosis driven (e.g. diverticulitis, peritonitis, and appendicitis). Identification of an organism alone using culture or non-culture based microbiologic testing method or on a pathology report from a surgical specimen does not = PATOS. Additionally, the following verbiage alone without specific mention of infection does not meet the PATOS definition: colon perforation, necrosis, gangrene, fecal spillage, nicked bowel during procedure, or a note of inflammation. Fresh traumas that are contaminated cases do not necessarily meet PATOS. For example, a gunshot wound to the abdomen will be a trauma case with a high wound class but there would not have been time for infection to develop. PATOS can be met when an abscess is noted, there is mention of infection in the OR note, purulence or pus is noted, septic/feculent peritonitis is noted. An infected appendix that has ruptured will meet PATOS =Yes, if the patient has a subsequent intraabdominal organ space SSI.

See full SSI Protocol Cdc-pdf[PDF – 1 MB] for complete details.


Q10: If a patient has an SSI and it is found to meet criteria for a PATOS = Yes do I need to enter this SSI into NHSN?

An SSI that meets criteria for the PATOS should have the PATOS field selected as YES. This SSI is not excluded from reporting and it should be entered into NHSN if you are following this procedure in your monthly reporting plan.
NOTE: The PATOS field is only found on the SSI event form.

Denominator — Wound Class

Q11: Can NHSN tell me what wound class should be used for specific procedures?

NHSN does not make recommendations on wound class aside from the fact that the application does not offer clean as a choice for a small group of procedures. NHSN made the decision, regarding which NHSN operative procedures can never be classified as clean, based on feedback from external experts in the field of surgery.
The procedures that can never be entered as clean are: APPY, BILI, CHOL, COLO, REC, SB and VHYS. In the NHSN application clean is not listed as a choice on the drop down menu for these procedures.
Wound class should be recorded by someone who is part of the surgical team based on the findings of each specific case.

Denominator – Scope

Q12: Can you share more details on how to answer the Scope field question?

ICD-10-PCS codes can be helpful in answering this scope question. The fifth character indicates the approach to reach the procedure site:

  • Value of zero (0) = an open approach.
  • Value of four (4) = percutaneous endoscopic approach.
  • Value of F = via natural or artificial opening with endoscopic assistance approach.

If the fifth character of the ICD-10-PCS code is a four (4) or F then the field for scope should be YES.

Note:  If a procedure is coded as open and scope then the procedure should be entered into NHSN as Scope = NO. The open designation is considered a higher risk procedure.

Event Details — Gross Anatomical Exam

Q13: What is the meaning of “evidence of infection on gross anatomical exam” found in several of the NHSN infection criteria?

Gross Anatomical Exam: Evidence of infection elicited or visualized on physical examination or observed during an invasive procedure. Includes physical examination of a patient during admission or subsequent assessments of the patient, may include findings noted during a medical/invasive procedure dependent upon the location of the infection as well as the NHSN infection criterion.


  • An intraabdominal abscess will require an invasive procedure to actually visualize the abscess.
  • Visualization of pus or purulent drainage from drains within an abscess is acceptable.
  • Abdominal pain elicited on physical exam post CSEC or hysterectomy, is sufficient evidence of infection detected without an invasive procedure for the general Organ Space SSI criteria.

NOTE: Imaging test evidence of infection cannot be applied to meet gross anatomic evidence of infection.  Imaging test evidence has distinct findings in the HAI definitions. (For example, IAB 3b).

Clarification of IAB Criterion

Q14: If a patient is found to have for example stool or blood in the abdomen but there is no documentation of an abscess, purulence, or other infection can this meet the “other evidence of infection” for criterion 2 of the IAB – intraabdominal definition?

The presence of stool or blood in the intraabdominal cavity without evidence of actual infection such as pus or an abscess does not meet criterion 2 of the IAB definition. The leak or bleed may have been small or very recent and infection in the abdominal space has not developed. This case may have a high wound class which is a risk factor used for NHSN denominator data but it would not meet criterion 2. If a patient has no evidence of infection and a (+) blood culture criterion IAB 2b cannot be used, as this criterion requires evidence on gross anatomic or histopathology. Please note for IAB criterion 2b and 3b there are a limited number of allowable blood pathogens.

Event Detail — Aseptic Technique

Q15: What is an aseptically obtained culture?

NHSN defines aseptically obtained as “obtained in a manner to prevent introduction of organisms from the surrounding tissues into the specimen being collected”. Note that swabs collected at the bedside or in the OR can be aseptically obtained. There is a misconception by some that only cultures obtained in the OR are aseptic. An improperly collected or transported specimen from a surgical wound can make identification of true SSI difficult.

While there are general guidelines for the collection of optimum wound specimens, specific procedures for specimen collection and transport is institution dependent. Infection prevention professionals should review these practices with their nursing and laboratory colleagues to assure that aseptic technique is being used to obtain wound specimens. If you receive (+) culture results with pathogens identified one must assume it was properly collected unless evidence otherwise. Culture results of “Mixed flora” or “Mixed cutaneous flora” alone cannot be reported to NHSN as there is no such pathogen option in this list of pathogens. Wound culture results with this finding may require review for proper technique based on the facility/lab protocol for proper wound specimen collection.

Surgical Site — Infection at another Site

Q16: If a post-operative patient develops an infection which meets criteria for an SSI, but an infection was present in another site also, does this have to be reported as an SSI or can we attribute it to the other site of infection?  An example is a patient status post HPRO who has a UTI and develops a deep incisional SSI with the same organism causing the UTI. Must the SSI be reported?

Yes. An SSI can only be a primary infection and should be reported if criteria is met.  There may be additional primary infections present which would also be reportable.

Surgical Site — Invasive Manipulation

Q17: If a surgical patient develops an infection after the surgical site is invasively manipulated/accessed (e.g., breast implants are infused/enlarged; shunts have been accessed etc.) and there was no evidence of infection at the time of the manipulation, is this considered an SSI?

An SSI will not be attributed if the following 3 criteria are ALL met:

  • during the post-operative period the surgical site is without evidence of infection and,
  • an invasive manipulation/accession of the site is performed for diagnostic or therapeutic purposes (for example, needle aspiration, accession of ventricular shunts, accession of breast expanders) and,
  • an infection subsequently develops in a tissue level which was entered during the manipulation/accession.

Tissue levels that are BELOW the deepest entered level will be eligible for SSI.  For example, a superficial debridement following a COLO procedure, where the muscle/fascia and organ/space was not entered, a subsequent organ/space SSI following the debridement may be an SSI attributable to the index COLO procedure.  This reporting instruction does NOT apply to closed manipulation (for example, closed reduction of a dislocated hip after an orthopedic procedure). Invasive manipulation does not include wound packing, or changing of wound packing materials as part of postoperative care.

Denominator — Height & Weight

Q18: What do I do if I am missing a height or weight on a patient for the Denominator for Procedure Form?

First, confirm you have checked all data sources available, including the surgical records, in terms of trying to locate a height and weight.
In terms of our application the field must have a height and weight > 0.
If you cannot find a height and weight NHSN recommends a height of 1ft, 0 in and weight of 1 lb.
That will signal to NHSN that these data are definitely outliers due to unavailable data and should not be used in risk adjustment.
It is very important to have a system that is collecting this information so that the data will be used in risk adjustment and factored into the SIR analysis.

Surgical Site – Post-op Complications

Q19: Is an intraabdominal infection that meets SSI criteria which developed due to an anastomotic leak or postop complication still considered a Surgical Site Infection (SSI)?

Yes, these SSIs are reported if they meet SSI criteria in the SSI surveillance time period. This is addressed in the SSI protocol under numerator reporting instruction #12 on page 9-18.

Reporting instructions for specific post-operative infection scenarios:

An SSI that otherwise meets the NHSN definitions should be reported to NHSN without regard to post-operative accidents, falls, inappropriate showering or bathing practices, or other occurrences that may or may not be attributable to patients’ intentional or unintentional postoperative actions. SSI should also be reported regardless of the presence of certain skin conditions (for example, dermatitis, blister, impetigo) that occur near an incision, and regardless of the possible occurrence of a “seeding” event from an unrelated procedure (for example, dental work). This instruction concerning various postoperative circumstances is necessary to reduce subjectivity and data collection burden.

Surgical Site — Hematomas and Seromas

Q20: Is an “infected hematoma or seroma” after an NHSN operative procedure an SSI?

The fact that wounds can be labeled in various ways by different physicians is the reason that criteria rather than labels or diagnoses are used for determination of healthcare-associated infections in NHSN.  If a wound described as an infected hematoma or seroma meets an SSI criterion, it must be reported.

Denominator – Trauma

Q21: Is a fall considered “trauma” when completing the Denominator for Procedure form for surgical site infection surveillance?

Yes. Trauma is defined in NHSN as “blunt or penetrating traumatic injury.” Therefore, if the surgery was performed because of a recent fall, e.g., a hip arthroplasty following a fall, then indicate “yes” for the trauma field.


Q22: If an organ injury occurs during an operative procedure should the case be considered Trauma = Yes?

No – The trauma field is to be used to document trauma that occurred before the surgery and resulted in the need for the surgery.  For example, inadvertent organ injuries (e.g., colon nick) that occur during a procedure are not considered a trauma case.

Clinical Correlation

Q23: What is meant by the term clinical correlation in the IAB- intraabdominal definition?

Physician documentation of antimicrobial treatment for site-specific infection related to equivocal (not clearly identified as) findings for infection on imaging test.

For example, when applying intraabdominal infection (IAB) criterion 3b, the finding of ‘fluid collection seen in the lower abdominal cavity’ on an imaging test, may or may not represent an infection. This finding is not clearly identified as an infection and should be confirmed with clinical evidence that an infection is present. In the case of IAB criterion 3b, the clinical evidence required is physician documentation of antimicrobial therapy for treating the intraabdominal infection.

Event Detail — Level of SSI

Q24: Do I report “Superficial” SSIs or only “Deep Incisional” or “Organ/Space” SSIs?  What should I do if a superficial SSI progresses deeper within the allotted surveillance timeframe?

When following an NHSN procedure group in your monthly reporting plan the facility should follow the entire protocol and report all SSIs for that group. This includes superficial incisional, deep incisional and organs/space SSIs.

If a superficial SSI progresses to a deeper level of infection within the designated surveillance timeframe, e.g., the surveillance timeframe is 90-days and the infection progresses during this time,  the event and date of event should be edited to reflect the deepest level of tissue infected (remove superficial and replace with either deep incisional or organ/space).

See reporting instruction #4 on page 15 in the SSI Protocol Cdc-pdf[PDF – 1 MB] for full details.

Level of SSI After BRST Procedures

Q25: How do I determine level of infection after an NHSN BRST – breast procedure?

For SSI after a BRST procedure here is the guidance:

  • Apply the superficial incisional SSI criteria if the infection involves the skin or subcutaneous tissue
  • Apply the deep incisional SSI criteria if the infection involves the muscle/fascial level
  • Apply the organ space BRST criteria 1 or 2 if the infection is deeper than the muscle/fascial level

Level of SSI After Cardiac Procedures

Q26: How do I determine the level of infection for the sternal site after cardiac procedures?

Apply the superficial incisional SSI criteria if the infection involves the skin or subcutaneous tissue.

If the infection goes to the sternum but does not involve the bone apply the deep incisional criteria.

If the infection is of the sternal bone apply the organ/space BONE criteria.

If the infection is below the sternum in the mediastinal space apply the MED – Mediastinitis criteria. These cultures are often named mediastinal fluid or tissue.

NOTE: If a patient meets both BONE and MED you call it an organ/space MED – Mediastinitis infection.