FAQs: Surgical Site Infections (SSI) Events

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).  ONLY the following descriptors are eligible for use to meet the definition of purulence [there must be at least one color descriptor and at least one consistency descriptor]:  

Color 

Green 

Yellow 

Consistency 

Milky  

Thick 

Creamy 

Opaque 

Viscous 

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. 

Yes.  Patient/caregiver-reported signs and symptoms are an important aspect of SSI surveillance and are useable within the SSI definitions    when identified in the appropriate surveillance period as part of the SSI investigation.  

The intent of ‘new/worsening’ pain is to capture pain that is outside of what may be ‘typical’ post-operative pain within the SSI surveillance period.  Documentation used to determine new or worsening pain is based on facility policy and may include documentation of pain level, change in type and/or character of pain, frequency/dose/type of analgesic administration, pain impact on mobility or other life quality indicators, and other healthcare provider and physician documentation.   Examples of new/ worsening pain include [but are not limited to] the following: 

• New pain is experienced in a patient in which ‘typical’ post-operative pain was resolved/not present/reduced and then resumes or develops 

• If pain is assessed on a pain scale, an increase in the pain scale [for example, pain that was previously documented at a level of 2 and then is documented at a level of 5] 

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 ‘Event Details: Detected’ for complete guidance. 

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

The ICD-10-PCS and/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 – 686 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.

Yes – if an operative procedure is assigned ICD-10-PCS and/or CPT code(s) that are included in NHSN operative procedure code mapping and otherwise meets the definition of an NHSN operative procedure (as defined in the SSI Protocol, pg. 9-4), it is considered an NHSN operative procedure for NHSN SSI surveillance purposes and sets an SSI surveillance period, regardless of whether the NHSN procedure category is included in your facility’s MRP.  

Procedures that are not assigned ICD-10-PCS and/or CPT code(s) included in the NHSN operative procedure code mapping (or do not fully meet the definition of an NHSN operative procedure) are considered non-NHSN operative procedures and do not set SSI surveillance periods. 

Example: A facility monitors only COLO and HYST NHSN operative procedures in the MRP. A patient in the facility had an NHSN qualifying COLO procedure on 9/1, which sets a 30-day SSI surveillance period. The patient returned to the OR on 9/15 for an NHSN qualifying XLAP. Since the XLAP is still considered an NHSN operative procedure, the SSI surveillance period for the 9/1 COLO ends at the conclusion of the 9/15 XLAP and a new 30-day SSI surveillance period begins. Since the facility does not include XLAP procedures in its MRP, SSI surveillance would not be required after the conclusion of the 9/15 XLAP.  

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).

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 historically, all elements used to meet an SSI criterion generally occur within a 7-10 day timeframe. To ensure that all elements associate to the SSI, the elements must be relational to one another.  

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.

SSI Event Reporting Instruction #3 found on page 9-18 of the SSI protocol addresses PATOS.  Additionally, a Quick Learn “Surgical Site Infection (SSI) Event PATOS – Infection Present at Time of Surgery” can be found here: PATOS-Quick Learn. 

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).   

NOTE: An SSI event must first be identified within the surveillance period following an NHSN operative procedure to review the PATOS question that is found on the SSI event form. SSI procedures that are PATOS = YES are not excluded from SSI surveillance. An SSI associated to this procedure is reportable and must be entered into NHSN if you are monitoring the procedure category in your MRP.   

The intent of the PATOS field is to capture what gross anatomic evidence of infection is seen/done by the surgeon during the index operative procedure at the same tissue level as the subsequent SSI, and documented. If at the time of surgery, the surgeon identifies infection, it must be reflected within the narrative of the operative report.  

The NHSN SSI protocol outlines that evidence supporting PATOS must be documented in the narrative portion of the operative note. Pre- and post-operative diagnosis, and operative indication may not be used for PATOS. Aside from this guidance the facility may decide what constitutes the narrative of the operative note.  

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; wound class validation/modification should be done with OR staff involved in the case.  

The application does not offer Wound Class 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.  

Missing Wound Class: Wound class is a required field on the Denominator for Procedure form and therefore this information is necessary in order to report a procedure in your denominator.   If the wound class is missing, you are correct that the IP should not interpret the wound class based on their interpretation of the operative note.   We would recommend collaborating with your OR team to ensure that they determine a wound class and this is documented in the medical record.     

Missing ASA: ASA is a required field on the Denominator for Procedure form and therefore this information is necessary in order to report a procedure in your denominator.  Only patients with an ASA score of 1-5 are eligible for SSI surveillance. If you are not able to find this documentation in the medical record, consult with the surgical team that performed this procedure to verify that an ASA score was or was not assigned. If an ASA score was not assigned, the procedure would not be eligible for SSI surveillance and would not be reported as a denominator.   

Gross anatomic evidence of infection is evidence of infection elicited or visualized on physical examination or observed often 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.  See additional guidance on page 9-15 of theSSI protocol [PDF – 913 KB] under 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 NHSN definitions (for example, IAB ‘3b’). 

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 are 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 the NHSN Terminology Browser.  However, wound culture results with this finding may require review for proper technique based on the facility/lab protocol for proper wound specimen collection. 

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.

Invasive Manipulation exclusion is case specific and 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.   SSI Event Reporting Instruction #10 is found on page 9-22 of the SSI protocol. For additional information on invasive manipulation, see the SSI protocol on page 9-22 (SSI Event Reporting Instruction #10). 

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]

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.

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.

Yes.  Trauma is defined in NHSN as “blunt or penetrating traumatic injury.” Therefore, if the operative procedure was performed because of a fall [resulting in a direct trip to the OR], 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 operative procedure 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.

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.  For more information on CMS reporting requirements, visit https://www.cdc.gov/nhsn/pdfs/cms/cms-reporting-requirements-deadlines.pdf. 

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.

SSI following an NHSN qualifying operative procedure depends on the specific case details.  Apply superficial and/or deep incisional SSI as outlined in the SSI protocol. Apply general organ/space criteria and an eligible site-specific definition if the infection is deeper than the muscle/fascial level. 

NOTE:   

• For NHSN qualifying BRST procedures, apply the organ space BRST criteria 1 or 2 if the infection is deeper than the muscle/fascial level [such as the chest wall]. 

• For NHSN qualifying CRAN procedures, apply deep incisional SSI criteria if the infection involves the fascial layers or the subgaleal space. 

• For NHSN qualifying CRAN procedures, apply general organ/space criteria and IC as the site-specific definition if the if the infection involves the epidural area [as this is considered part of the dura].  

• For spinal operative procedures that involve the thecal sac, apply organ/space criteria. 

Organisms identified from implanted devices by culture or non-culture based microbiologic testing methods are eligible for use to meet applicable definitions.   

Examples include:  

• Vascular graft explanted from a vessel and culture positive can be used to meet VASC 1.   

• Screws or other fixation devices removed from bone and culture positive can be used to meet BONE 1.   

• Explanted ventricular shunt device and culture positive may be used to meet MEN 1.   

• Explanted joint prosthesis that is culture positive may be used to meet PJI element. 

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

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

What criteria or definition have you considered for your surveillance activities? 

What are your specific questions?  

• 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.