FAQs: Surgical Site Infections (SSI) Events

NHSN SSI Case Review

Q1: What can the user provide to NHSN for a complete SSI 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 procedure(s) and date(s) of all procedures including reoperations:

  • Whether the operative procedures are coded as NHSN operative procedures (if so, provide the NHSN operative procedure code(s) and category(s))
  • If a return to OR via same incision, was the start time of the return to OR procedure within 24 hours of finish time of the prior operative procedure?

Signs and symptoms? Please include dates of signs and symptoms.

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

Was any imaging testing performed and described? Please include dates of any imaging performed.

Fluid collections or drainage?

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

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?

Generally, 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: Can I attribute an SSI to a non-NHSN operative procedure?

No. 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.

Timeframe for SSI Elements

Q5: 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, specifically, 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

Q6: Does NHSN have a definition for purulence?

NHSN does not define purulence as there is no standard, clinically agreed upon definition. Documentation that includes descriptors such as thick, viscous, creamy, opaque, or pus/purulence would be accepted evidence of purulence.

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


Q7: If a patient reports purulence from their surgical site, is this acceptable to meet an SSI criterion?

Yes – patient reported purulence documented in any healthcare provider note is acceptable for use with SSI criterion.  A specific date/timeframe must be documented in the patient medical record to indicate the purulence occurred within the SSI surveillance period.

Procedure — Contaminated Procedure

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

Yes. If there was evidence of contamination at the time of the procedure and then later in the surveillance period the patient develops an infection that meets 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?

SSI Event Reporting Instruction #3 addresses PATOS. See full SSI Protocol Cdc-pdf[PDF – 1 MB] for complete details. Additionally, view the Quick Learn for “Surgical Site Infections (SSI) Event form for PATOS” [Video – 6 min].


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 must be entered into NHSN if you are following this procedure in your monthly reporting plan.

NOTE: An SSI must be identified in order to assess for PATOS (the PATOS field is only found on the SSI event form).


Q11: Where within the patient medical record can I find the documentation I need to answer the PATOS question on the SSI event form?

The PATOS response (YES/NO) is determined by evidence of infection documented in the operative procedure report.  This documentation is commonly noted in the narrative of the operative procedure report.  The language/verbiage in the operative procedure report must clearly reflect infection is ‘seen’ during the operative procedure and should additionally include reference to the tissue level where the infection is seen.

Denominator — Wound Class

Q12: 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 (APPY, BILI, CHOL, COLO, REC, SB and VHYS). 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.

Wound class should be documented by someone who is part of the surgical team based on the findings of each specific case according to the wound class schema that they have adapted within their organization.

Event Detail — Gross Anatomical Exam

Q13: What is acceptable evidence of infection found on gross anatomic exam?  

Evidence of infection elicited or visualized on physical examination or observed during an invasive procedure.  This includes findings elicited on physical examination of a patient during admission or subsequent assessments of the patient and 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 (includes from a drain).
  • SSI only: Abdominal pain elicited on physical exam post CSEC or hysterectomy (HYST or VHYS) is sufficient evidence of infection detected without an invasive procedure to meet general Organ Space SSI criterion C.  Allowing abdominal pain elicited on physical exam as gross anatomic evidence of infection to meet general Organ Space SSI criterion C allows you to move on to meeting a site-specific organ/space infection 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.

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.

For purposes of NHSN surveillance, if you receive (+) culture results with pathogens identified one must assume it was properly collected and the report is eligible for use in meeting SSI criteria.  Culture results such as “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 are met.  There may be additional primary infections present which may also be reportable.

Surgical Site — Invasive Manipulation

Q17: If a surgical patient develops an infection after the surgical site is invasively manipulated/accessed (for example, 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?

No.  Note that 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. See SSI Protocol Cdc-pdf[PDF – 1 MB] for complete details.

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.
This 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 SSI that may be caused by an anastomotic leak or other post-operative occurrence excluded from SSI reporting?

No, if SSI criteria is met within the appropriate SSI surveillance period, an SSI is reported. This is addressed in the SSI protocol under numerator reporting instruction #11 on page 9-18.

Surgical Site — Hematomas and Seromas

Q20: Is a hematoma or seroma that is identified following an NHSN operative procedure considered 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 SSI determinations. If a wound described as a hematoma or seroma meets an SSI criterion (for example, an organism is identified from a hematoma) it must be reported as an SSI.

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: When following an NHSN operative procedure category in my monthly reporting plan, do I have to report superficial incisional SSIs or can I just report deep incisional and organ/space SSIs?  What should I do if a superficial incisional SSI progresses deeper within the designated surveillance period?

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

If a superficial incisional SSI progresses to meet criteria at a deeper tissue level within the designated surveillance period, the event and date of event should be edited to reflect the deepest tissue level where SSI criteria are met during the surveillance period.

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

Event Detail — 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

Event Detail — 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.
  • Apply the deep incisional SSI criteria if the infection goes to the sternum but does not involve the bone.
  • Apply the organ/space BONE criteria if the infection is of the sternal bone.
  • Apply the organ/space MED – Mediastinitis criteria if the infection is below the sternum in the mediastinal space.

  NOTE:  If a patient meets both organ/space BONE and MED criteria report the SSI event as organ/space MED – Mediastinitis.