FAQs: Surgical Site Infections (SSI) Events

NHSN 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 include dates.

NHSN Procedure and SSI Event Reporting

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

Appendix B in the SSI protocol [PDF – 586 KB] provides clarity and support for Denominator Reporting Instruction #6 (which addresses the same NHSN operative procedure category performed via separate incisions). Appendix B provides guidance on correct procedure reporting when multiple procedures of the same category are performed per patient per calendar day via separate incisions (with an explanation for each of the 12 procedure categories found within this instruction). Denominator Reporting Instruction #6 is found on page 9-25 of the SSI protocol [PDF – 586 KB] and Appendix B begins on page 9-40 of the SSI protocol [PDF – 586 KB].

‘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

The ICD-10-PCS or CPT operative procedure codes are required to identify NHSN operative procedures. Only NHSN operative procedures begin an SSI surveillance period and 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 [PDF – 621 KB] – CDC/NHSN Surveillance Definitions for Specific Types of Infections) 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

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 [PDF – 586 KB] provides guidance related to surveillance periods for SSI for each of the NHSN operative procedure categories.

Pathogen Assignment

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

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

There is no standard, clinically agreed upon definition for purulence.  For NHSN surveillance purposes, the descriptors “pus” or “purulence” are sufficient gross anatomic evidence of infection.  When the terms ‘pus’ or ‘purulence’ are not written in the medical record, NHSN has allowed determinations for purulence based off descriptors. Documentation that uses a color descriptor and a consistency descriptor (from the list below) in combination is acceptable to indicate ‘purulence’.  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).











NOTE: The following descriptors cannot be used to define purulence/infection: ‘Cloudy’, ‘turbid’, ‘murky’ or the odor of a wound.

Gram stain results such as WBCs or PMNs cannot be used to define purulence within the SSI protocol [PDF – 1 MB].

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 (even if the provider doesn’t document purulence).  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

Contamination does not exclude a procedure from being reported in your denominator if the procedure is one selected for surveillance in your 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 – Infection present at time of surgery (PATOS)

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 the procedure).  Traditionally, NHSN uses the part of the operative note labeled ‘description of procedure’ or ‘procedure details’ as the surgical narrative. The language/verbiage in the surgical narrative must clearly reflect infection is ‘seen’ during the operative procedure and should additionally include reference to the tissue level where the infection is seen.  There is not a ‘standard’ format used with operative notes so within SSI Event Reporting Instruction #3 NHSN provides examples of headings not considered eligible as the surgical narrative for use with PATOS: procedure title, indication for surgery, and/or a pre/post op diagnosis. A ‘Findings’ section within the operative report, if a reflection of what the surgeon ‘sees’ present at time of surgery, can be used as a surgical narrative with a PATOS determination.

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].

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

NHSN does not make recommendations about wound class. The wound class is assigned by someone participating in the surgical case. 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.

The application does not offer Wound Class Clean (C) 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.

Event Detail – Gross Anatomical Exam

Gross anatomic evidence of infection is 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 Organ/Space SSI criterion ‘c’ when a Chapter 17 [PDF – 621 KB] Reproductive Tract Infection criteria 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, SSI-EMET or SSI-VCUF event.  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 [PDF – 621 KB] 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

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

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

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

This will depend on the individual details of the case. SSI Event Reporting Instruction #10 identifies three bullet points that must be met in entirety before the exclusion can be considered.  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. SSI Event Reporting Instruction #10 is found on page 9-22 of the SSI protocol [PDF – 1 MB].

Denominator – Height & Weight

First, confirm that all available data sources have been reviewed, including the surgical records, 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

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 on page 9-22 of the SSI protocol [PDF – 1 MB] under SSI Event Reporting Instruction #11.

Surgical Site – Hematomas and Seromas

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

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 prosthesis (HPRO) following a fall, then indicate Trauma = Yes on the SSI event form.

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 (such as an organ nick) that occur during a procedure are not considered a trauma case.

Event Detail - Clinical Correlation

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

“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 a condition other than an SSI. The determination to exclude a sign/symptom with no other recognized cause must be made by the local facility based on the documentation available in the medical record. Clinical judgment determination needs to be defended and backed up by physician documentation and there should be clear rationale in the event the case is validated.

Event Detail – Level of SSI

When monitoring an NHSN operative procedure category in your MRP, the entire SSI protocol [PDF – 586 KB] must be followed and you must report all SSIs for that procedure category. This includes superficial incisional, deep incisional and organ/space SSI events.

If a superficial incisional SSI progresses to meet SSI criteria at a deeper tissue level within the designated surveillance period following an NHSN operative procedure, 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 page 9-20 in the SSI protocol [PDF – 586 KB] for full details.

Event Detail – Level of SSI After BRST 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 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

  • 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 – Osteomyelitis 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.

Event Detail – Superficial Incisional SSI criterion ‘d ’

No. An ‘antimicrobial indication’ is not the same as a diagnosis and cannot be used to meet Superficial Incisional SSI criterion ‘d’ (diagnosis of a superficial incisional SSI by a physician* or physician designee).