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 specifics as to the criteria you have considered and which elements of the criteria you have determined the patient does or does not meet.  If you are unable to make a determination, please let us know specifically why you are unable to decide and 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/surgical space, 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.


NHSN SSI Reporting

Q2. What procedures am I required to report to NHSN?

NHSN does not mandate any reporting.  A facility selects reporting and indicates this via the NHSN Monthly Reporting Plan (MRP). A facility is required to report (procedure and SSI event) data that represents what is included in their MRP.  What a facility includes in their monthly reporting plan may depend on their own internal facility risk assessment, any state required reporting, and any CMS required reporting.


Q3: Following an NHSN operative procedure a patient is readmitted to a facility other than where the procedure was performed with evidence of infection BUT was subsequently readmitted back to the facility where the procedure was performed. An SSI is identified to be reported. How should the ‘Detected’ field be answered on the SSI event form?

‘Detected’ should be answered as RF.

Per the Instructions for Completion of Surgical Site Infection (SSI) Form (CDC 57.120) definition for ‘Event Details: Detected’: Check RF if SSI was identified due to patient readmission to the facility where the procedure was originally performed. Check RF regardless of readmission to a prior facility.

See the SSI event Table of Instructions [PDF – 500 KB] ‘Event Details: Detected’ for complete guidance.


SSI Surveillance Period and SSI attribution

Q4. How are Surveillance periods and SSI attribution determined?

Those procedures identified by ICD-10-PCS or CPT operative procedure codes as an NHSN operative procedure will begin an SSI surveillance period; only NHSN operative procedures are eligible for SSI attribution.  Non-NHSN operative procedures do not set a surveillance period nor are they eligible for SSI attribution.  Following a non-NHSN operative procedure, site-specific infection criteria [found in chapter 17] may be reviewed to determine if HAI criteria can be applied.  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.


Implant Presence

Q5. 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 found on page 9-16 within the SSI protocol provides information related to surveillance periods for SSI for each of the NHSN operative procedure categories.

Pathogen Assignment

Q6. 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 – 1 MB] on page 9-18 (SSI Event Reporting Instruction #1 Excluded Organisms).


Timeframe for SSI Elements

Q7. 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.  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 but in NHSN’s experience, all elements required to meet an SSI criterion usually occur within a 7-10 day timeframe with typically no more than 2-3 days between elements. To ensure that all elements associate to the SSI, the elements must occur in a relatively tight timeframe. For example, an element that occurs on day 2 of the surveillance period with another element that occurs three weeks later should not be used to cite an SSI. Each case differs based on the individual elements occurring and the type of SSI but the DOE for an SSI must occur within the appropriate 30- or 90-day SSI surveillance period.


Clarification of SSI Criterion – Purulence

Q8. 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 define purulence within the SSI protocol.


Q9. If a patient reports purulence from their surgical site, is this acceptable to meet an SSI criterion?

Yes.  Patient-reported signs and symptoms are an important aspect of SSI surveillance and are useable within the SSI definitions if documented within the patient medical record.  Patient reported purulence documented in the medical record by a healthcare professional 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

Q10. If a patient has an NHSN operative procedure that was contaminated do I still have to report an SSI if criteria are met in the surveillance period?

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


Event Detail – PATOS

Q11. 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 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. Pre and/or post-op diagnoses, ‘indications for procedure/surgery’ and other headings routinely included in the operative note cannot be used to answer the PATOS question.


Q12. Where can I find more information and examples of PATOS events?

SSI Event Reporting Instruction #3 addresses PATOS.  See SSI Protocol [PDF – 1 MB] for complete details. Additionally, a Quick Learn “Surgical Site Infection (SSI) Event PATOS – Infection Present at Time of Surgery” can be found here: PATOS – Quick Learn [Video – 6 min].


Q13. 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 within the surveillance period following an NHSN operative procedure to answer the PATOS question that is found on the SSI event form.

Denominator – Wound Class

Q14. Can NHSN tell me what wound class should be assigned for specific procedures?

The wound class should be assigned by someone participating in the surgical case. NHSN does not make recommendations on wound class aside from the fact that the application does not offer Clean (C) as a choice for the following procedure categories: APPY, BILI, CHOL, COLO, REC, SB and VHYS. Procedures in these categories that are assigned a Clean (C) wound class do not meet the definition of an NHSN operative procedure and should not be included in denominator reporting for NHSN SSI surveillance. Excluding a procedure from the denominator for procedure data should be made on a case-by-case basis with knowledge of the procedure and after review of the operative report. We recommend that you document the rationale for excluding a procedure in your facility’s surveillance records in the event an explanation is needed in the future.

The IP should not modify the wound class based on their interpretation of the operative note. If it is determined that an error was made in wound class assignment, the IP should work with the OR to validate this error with the OR making any such change in the medical record.


Event Detail – Gross Anatomical Exam

Q15. 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 gross anatomic evidence of infection without an invasive procedure to meet general Organ/Space SSI criterion “c” when OREP or EMET is met.  Allowing the documentation of abdominal pain or tenderness as gross anatomic evidence of infection to meet general Organ/Space SSI criterion “c” enables the user to report an SSI-OREP or SSI-EMET.  Note that abdominal pain or tenderness cannot be applied as ‘other evidence of infection on gross anatomic exam’ to meet Deep Incisional SSI criterion ‘c’ or to meet any Chapter 17 site-specific criterion (for example, OREP ‘2’)

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

Q16. Is the finding of stool/feces 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/feces 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

Q17. What is an aseptically obtained specimen?

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 ensure 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

Q18. 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

Q19. If a surgical patient develops an infection after the surgical site is invasively manipulated/accessed and there was no evidence of infection at the time of the manipulation/accession, is this considered an SSI?

This will depend on the individual details of the case, keeping in mind the invasive manipulation reporting instruction must be met in its entirety before the exclusion can be applied.  Note that tissue levels that are not entered are still 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 – 1 MB], p. 9-22)


Denominator – Height & Weight

Q20. What do I do if I am missing a height or weight on a patient for the Denominator for Procedure Form?

First, confirm that all available data sources have been reviewed, including the surgical records, in order to locate a height and weight.  Within the application, the height and weight field must have a value > 0.

If a height and/or weight is not available, NHSN recommends a height of 1ft, 0 in and weight of 1 lb. This will signal to NHSN that these data are outliers due to unavailable data and will not be used in risk adjustment.

It is very important to have a system or process in place for collecting this information, so that the data used in risk adjustment and factored into the SIR analysis is accurate. For information on how to report height and weight in NHSN, including its uses in SSI analysis, please see: Reporting Height and Weight for Procedures in NHSN [PDF – 500 KB]


Surgical Site – Post op Complications

Q21. 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 – 1 MB] under SSI Event Reporting Instruction #11 on page 9-22.

Surgical Site – Hematomas and Seromas

Q22. 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

Q23. 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.


Q24. 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

Q25. 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 – Other Recognized Cause

Q26. What is meant by “with no other recognized cause” * seen next to some signs and symptoms used in various site-specific (chapter 17) definitions that are available for Organ/Space SSI attribution?

“With no other recognized cause” means the sign/symptom is eligible for use in meeting the SSI criteria unless there is physician documentation within the medical record that specifically states the sign/symptom is due to something other than an SSI. The local facility must make this determination based on the documentation available in the medical record.


Event Detail – Level of SSI

Q27. When monitoring an NHSN operative procedure category in my monthly reporting plan, do I have to report superficial incisional SSI events or can I just report deep incisional and organ/space SSI events?  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. 9-20 in the SSI Protocol [PDF – 1 MB] for full details.


Event Detail – Level of SSI After BRST Procedures

Q28. 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

Q29. 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.