FAQs: Surgical Site Infections (SSI) Events

NHSN SSI Case Review

Q1: When sending an inquiry to NHSN, what should the NHSN user provide when requesting assistance with an SSI case review?

Please note it is very important to provide NHSN your thoughts regarding the case you are requesting for review. Please provide the elements of the criteria for which you are having difficulty making a determination, and please clearly outline your question(s). Our hope is that the case review process will be educational for you for making future determinations.

Please provide the following information when sending NHSN a question for SSI case review:

  • OR procedure(s) and date(s) of all OR 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?
  • Other procedures that access the surgical site during the SSI surveillance period and dates of these procedures (for example, CT-guided drainage, tap to knee). Please include findings from these procedures.
  • Patient signs and symptoms and dates of signs and symptoms.
  • Tissue level(s) that may be involved in the infection – superficial incisional, deep incisional and/or organ/space and dates of involvement.
  • Imaging tests performed and results of these tests – please include dates of these tests.
  • Other diagnostic testing performed – please include dates of these tests.
  • Culture or non-culture based microbiologic tests performed and the results – please include collection dates.
    • Include tissue level (If you are unsure NHSN recommends consulting with the surgeon/physician to make that determination).
  • Please provide any other evidence of infection – please includes dates.

SSI Surveillance Period

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

A visit to the OR for an NHSN operative procedure begins an SSI surveillance period. A return trip to the OR via the same surgical site ends the surveillance period for the prior NHSN operative procedure and begins a new SSI surveillance period if an NHSN operative procedure is performed.

Non-NHSN Operative Procedure

Q3: Can I attribute an SSI to a non-NHSN operative procedure?

For in-plan SSI reporting purposes, no. A procedure must meet the definition of an NHSN operative procedure in order to qualify for SSI surveillance.

Implant Presence

Q4. I’m looking for guidance related to implant presence and length of SSI surveillance period.

The surveillance period for an NHSN operative procedure is not determined by the presence or absence of an implant. The SSI surveillance period is determined by the NHSN operative procedure category. Table 2 on page 9-12 within the SSI protocol pdf icon[PDF – 1 MB] provides information related to surveillance periods for SSI for each of the NHSN operative procedure categories.

Pathogen Assignment

Q5: Are common commensal organisms excluded from meeting SSI criteria?

No. The only organisms excluded from SSI event reporting are found in the SSI protocol pdf icon[PDF -1 MB] on page 9-14 (SSI Event Reporting Instruction #1 Excluded Organisms).

Timeframe for SSI Elements

Q6: There is no Infection Window Period (IWP) in SSI surveillance. What is the timeframe for all elements used for citation of an SSI event to occur?

The Infection Window Period (IWP), Present on Admission (POA), Healthcare-associated Infection (HAI), and Repeat Infection Timeframe (RIT) definitions do not apply to SSI surveillance. Instead, SSI surveillance is based on a 30- or 90-day SSI surveillance period, which is determined by the NHSN operative procedure category and the tissue level of SSI event. SSI guidelines do not offer a strict timeframe for elements of criteria to occur (due to the variety of NHSN operative procedures and types of SSI events that can be assigned), but the date of event (DOE) for an SSI must occur within the appropriate 30- or 90-day SSI surveillance period.

Commonly, all elements used to meet a criterion occur within the same general timeframe. You would not want to use an element that occurs on POD 1 or 2 with another element that occurs three weeks later to cite an SSI. In NHSN’s experience, all elements required to meet an SSI criterion 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 – Purulence

Q7: Does NHSN have a definition for purulence?

NHSN does not define purulence as there is no standard, clinically agreed upon definition. The descriptors “pus” or “purulence” are sufficient evidence of infection. Documentation that includes descriptors such as milky, thick, viscous, creamy, opaque, yellow, or green may also be accepted evidence of purulence when used in combination with another eligible descriptor. For example, fluid only described as yellow, or only described as thick, is not sufficient. However, if the terms are combined, then they may be more representative of purulence (for example: fluid described as thick and yellow).

Gram stain results such as WBCs or PMNs cannot be used to define purulence within the SSI protocol.


Q8: 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

Q9: 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?

Contamination does not exclude a procedure from being reported in your denominator nor would it exclude an SSI event from being reported if SSI criteria is met within the surveillance period following the procedure.

Event Detail – PATOS

Q10. Where within the patient medical record can I find the documentation I need to answer the PATOS question on the SSI event form?

PATOS is a YES/NO question found on the SSI event form. PATOS is determined by evidence of infection documented in the operative procedure report. The evidence of infection must be noted intraoperatively and documented within the narrative portion of the operative note or report of surgery (the surgeon’s description of what is seen/done during procedure). 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.


Q11. Where can I find more information and examples of PATOS events?

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


Q12. Do I have to enter PATOS = YES SSI events into NHSN?

Yes. PATOS SSI events and the procedures to which they are linked are reported to NHSN. SSI events that are PATOS = YES are not excluded from SSI event reporting and must be entered into NHSN if you are following the procedure category in your monthly reporting plan.

NOTE: An SSI event must first be identified in order to assess for PATOS.

Denominator – Wound Class

Q13: Can NHSN tell me what wound class should be assigned for specific procedures?

No. 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).

Wound class should be assigned by a person involved in the surgical procedure (for example, surgeon, circulating nurse, etc.). The wound class must be applied according to the wound class schema that is adopted within each organization.

Event Detail – Gross Anatomical Exam

Q14: 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 or tenderness post Cesarean section (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 the documentation of abdominal pain or tenderness as gross anatomic evidence of infection to meet general Organ/Space SSI criterion “c” allows the SSI determination to move on to meet 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 NHSN definitions (for example, IAB 3b).

Evidence of Infection

Q15: Is the finding of stool and/or blood in the abdomen without documentation of an abscess, purulence, or infection, considered to be gross anatomic evidence of infection?

No. The presence of stool and/or blood in the intraabdominal space without purulence or abscess is considered contamination but not infection, therefore cannot be used as gross anatomic evidence of infection.

Event Detail – Aseptic Technique

Q16: 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”.  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.  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 clinical staff and laboratory colleagues to assure that aseptic technique is being used to obtain wound specimens.

For purposes of NHSN surveillance, it is the expectation that proper technique for specimen collection is being performed.  If a wound specimen was collected and sent to the lab for culture, the findings are eligible for use in SSI criteria (no exclusions).  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

Q17: If a patient meets criteria for an SSI, but an infection is also present at another site, does the SSI get reported or can we attribute the SSI to the other site of infection?  Example: A patient status post HPRO meets criteria for a UTI and develops a Deep Incisional SSI with the same organism that is identified in the urine. Must the SSI be reported?

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

Surgical Site – Invasive Manipulation

Q18: 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) 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 still be eligible for SSI. For example, if within an SSI surveillance period a superficial debridement of the surgical site is performed, and the deep incisional level and organ/space level are not entered, a subsequent deep incisional or organ/space SSI following the superficial 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 Event Reporting Instruction #10, SSI Protocol pdf icon[PDF – 1 MB], p.9-17.).

Denominator – Height & Weight

Q19: 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

Q20: Is an SSI that may be caused by an anastomotic leak or other post-operative occurrence excluded from SSI reporting?

No. Post-op occurrences and complications are not an exclusion for monitoring for an SSI and reporting an SSI if SSI criteria are met. If SSI criteria are met within the appropriate SSI surveillance period, an SSI is reported. This is addressed in the SSI protocol pdf icon[PDF – 1 MB] under SSI Event Reporting Instruction #11 on page 9-18.

Surgical Site – Hematomas and Seromas

Q21: 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

Q22: 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, for example, a hip arthroplasty following a fall, then indicate “yes” for the trauma field.


Q23: 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 (colon nick) that occur during a procedure are not considered a trauma case.

Event Detail – Clinical Correlation

Q24: What is meant by the term “clinical correlation” that is used in some site-specific (chapter 17) definitions that are available for Organ/Space SSI attribution?

Clinical correlation refers to physician documentation of antimicrobial treatment for site-specific infection related to equivocal (not clearly identified as) findings of 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

Q25: When monitoring 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 monitoring an NHSN operative procedure category in your monthly reporting plan, the facility must follow the entire SSI protocol and report all SSIs for that procedure category. This includes superficial incisional, deep incisional and organs/space SSI events.

If a superficial incisional SSI progresses to meet criteria at a deeper tissue level within the designated surveillance period, the event type and date of event must be edited to reflect the deepest tissue level where SSI criteria are met during the surveillance period. See SSI Event Reporting Instruction #4 on p. 9-16 in the SSI Protocol pdf icon[PDF – 1 MB] for full details.

Event Detail – Level of SSI After BRST Procedures

Q26: How do I determine level of infection after an NHSN BRST – breast procedure?

  • 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

Q27: 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.