Introduction:

The goal of this observational study is to learn whether point-of-care ultrasound (POCUS) can help detect early fluid build-up under the skin after open abdominal surgery (laparotomy) and whether these findings can predict surgical site infection. The study includes adults who recently had emergency or planned open abdominal surgery.

Hypothesis:

The main questions it aims to answer are:

1. Does ultrasound identify subcutaneous fluid collections early after surgery?
2. Do these fluid collections help predict which participants will develop a surgical site infection?

Intervention:

Participants will:

1. Have a superficial wound swab taken once at the start of the study (for research only).
2. Receive three ultrasound scans of their surgical wound during days 1-3, 4-6, and 7-9 after surgery.
3. Have their wound checked for redness, swelling, tenderness, or discharge before each scan.
4. Be followed for 7 and 30 days after surgery (and up to 90 days if they have implanted mesh) to see whether an infection develops.

Ultrasound is non-invasive and safe. According to the protocol, "no serious adverse effects are expected," though some participants may feel brief discomfort from pressure on a tender wound.

This study will help researchers understand whether routine bedside ultrasound can support earlier detection of wound infections and improve postoperative care in the future.

Background:

Surgical site infections (SSIs) remain a major concern in postoperative care, particularly following major emergency abdominal surgery (MEAS), where infection rates range from 8% to 23%, depending on a range of patient related (endogenous) and surgical related (exogenous) risk factors. SSIs are associated with significant clinical and economic burdens, including a 2- to 11-fold increase in mortality (absolute 7.4%), prolonged hospital stays, additional surgical procedures and readmissions. Furthermore, associated healthcare costs estimated $ 3.3 billion in United States and € 1.47-19.1 billion in Europe.

Subcutaneous fluid accumulation, such as seromas and hematomas, may serve as a nidus for bacterial growth and impair wound healing, thereby predisposing patients to SSIs. However, early detection of such fluid collections is challenging using routine postoperative clinical assessment.

Traditional clinical examination and wound inspection often in accordance to CDC criteria for SSIs, makes it challenging to accurately diagnose and report true SSIs, potentially missing deep or small fluid collections. As a result, SSIs may go undetected until after discharge, with reported rate ranging from 12% to 84%.

In comparison to CDC- guidelines for identifying SSI, which is subjective to interpretation due to the arbiter factor of 'diagnosis by attending physician'. In contrast, the ASEPSIS wound scoring system offers a more objective assessment and has shown more reliability in identifying SSIs, though it still depends on observable clinical signs.

Point of care ultrasonography (POCUS) has gained increasing attention as a non-invasive and real-time imaging modality that can enhance the early detection of subcutaneous fluid accumulation. Several studies have demonstrated the utility of POCUS in identifying fluid pockets, allowing for timely drainage and targeted management, potentially reducing the risk of SSI. However, dependent on operator's level of expertise conflicting findings have also been reported in literature regarding the accuracy of POCUS in evaluating fluid collection resulting SSIs. Some evidence suggests that POCUS combined with physical examination may improve diagnostic precision.

Despite growing interest in POCUS for postoperative wound evaluation, standardized protocols for the routine use of ultrasound in postoperative wound assessment are lacking, and the association between ultrasound-detected fluid and SSI development remains inadequately studied. Furthermore, to the best of our knowledge there is no prior study evaluating that POCUS adds diagnostic value when used in combination with ASEPSIS in identifying SSI.

Purpose of the study:

The aim of this prospective study is to evaluate the diagnostic utility of bedside ultrasound in early detection of subcutaneous fluid accumulation following laparotomy and its correlation with development of SSIs. Additionally, the study will assess whether POCUS added with the ASEPSIS scoring system improves the diagnostic accuracy for identifying SSIs compared to using either method alone, with the goal of enhancing early detection and postoperative infection surveillance.

Secondarily we wish to describe in detail the phenotype of laparotomy wounds and their microbiological environment.

Hypothesis:

Primary Hypothesis:

POCUS can detect subclinical subcutaneous fluid accumulation earlier than clinical signs of SSI following laparotomy, and the combination of POCUS with the ASEPSIS scoring system improves the diagnostic accuracy for identifying SSIs compared to either method alone.

Secondary Hypothesis:

Larger subcutaneous fluid collections detected by POCUS are associated with an increased risk of developing SSIs following laparotomy.

Microbiological profiles correlated with SSI risk and may serve as an early independent indicator of infection susceptibility.

Methodology

Point-of-Care Ultrasound Procedure and Bias Control Measures.

Pre-POCUS Assessment:

Following informed consent, each patient's laparotomy incision and surrounding area will undergo an initial evaluation for SSI through photographs, objective assessment and microbiological sampling.

Between postoperative day (POD) 1 and day 3, and prior to the first ultrasound (POCUS) examination, as a part of intervention a microbiological swab will be obtained once from the closed incision and surrounding skin. Each wound will be photographed, and an objective wound assessment will be done using the ASEPSIS scoring system.

This sequence-microbiological sampling followed by ASEPSIS scoring before any POCUS examination is designed to prevent contamination of cultures and to minimize assessment bias. Conducting the objective evaluation before the ultrasound ensures that POCUS findings do not influence ASEPSIS scoring or the clinical impression of infection.

POCUS Protocol:

To monitor subcutaneous fluid accumulation and detect early signs of surgical site infection (SSI), each patient will undergo a total of three standardized point-of-care ultrasound (POCUS) examinations during the postoperative course. These assessments will be performed at predefined intervals: POD 1-3, POD 4-6, and POD 7-9. This interval-based imaging schedule was selected to provide structured temporal coverage of the critical early postoperative period while ensuring clinical feasibility and minimizing patient burden. Daily or fixed days for ultrasonography was deemed impractical due to resource constraints and the potential for limited additional diagnostic value. The selected time points aim to capture the dynamic evolution of postoperative fluid collections and facilitate correlation with subsequent clinical outcomes.

Examinations will be conducted with sterile technique: the ultrasound probe will be covered using a sterile probe cover, and the patient's skin will be cleaned with sterilizing wipes before and after each scan.

The examination will be performed with the patient in a supine position. A high-frequency linear probe (1-12 MHz) is positioned on the incision, orienting the probe toward the Linea alba. Fluid collections will be evaluated in both longitudinal and transverse planes to accurately characterize their maximal length and depth. A structured ultrasound reporting template (Appendix 1) will be used and documented with corresponding electronic images. The template was developed for this study, informed by the ASEPSIS criteria, CDC wound classification, and prior published work (5,6,11,21). It is designed for point-of-care ultrasound by clinicians with varying levels of ultrasound training and therefore aims to combine methodological rigor with generalizability and practical use in real-world clinical settings.

Training and Quality Control:

The principal investigator (TM) will undergo formal ultrasonography training under the supervision of a consultant radiologist and subsequently provide structured training to all participating investigators across sites. This centralized approach aims to ensure consistent examination technique and adherence to standardized protocols. Although, due to local scheduling and workflow constraints, it cannot be guaranteed that the same investigator will always perform all serial examinations on a given patient, the standardized training is intended to minimize inter-rater variability and thereby strengthen the overall reliability of the collected data.

To minimize assessment bias, all photographic and ultrasound imaging data will be prospectively collected following a standardized acquisition protocol across all participating centers. These data will be reviewed by 2 clinicians formally trained in point-of-care ultrasound (POCUS), who will perform the evaluations according to ASEPSIS and proposed SPOTLIGHT SSI risk score independently and individually. Reviewers will be blinded to patient identifiers, treatment allocation, and clinical outcomes. To ensure consistency across study sites, the same training criteria and evaluation framework will be used for all raters. Inter-rater reliability will be assessed using appropriate statistical methods (e.g., Cohen's kappa for categorical variables or intraclass correlation coefficient [ICC] for continuous measures). In cases of significant disagreement, a third expert reviewer may be consulted to reach consensus. This process is designed to ensure objective image interpretation and enhance the methodological rigor of the study.

Integrated Assessment and Follow-Up:

Each POCUS session will be accompanied by photography and repeated objective assessments using the ASEPSIS scoring system, with ongoing classification as superficial, deep or organ space based on CDC SSI criteria. All data, including clinical scores and imaging, will be systematically documented in a secure REDCap database.

As a part of the 7- and 30- day follow up process, enrolled patients will be contacted via telephone and electronic health registry will be reviewed following each enrolled patients' initial surgery to note wound outcomes, all cause and SSI related readmissions.

Data collection:

Data will be registered prospectively in the hospital's surgical log (Sundhedsplatformen, Epic Systems Corp, Verona, USA) and subsequently collected in the dedicated REDCap database. Demographic characteristics (age, sex, body mass index), diagnosis, nutritional status (serum albumin level), anemia status (hemoglobin level), comorbidities (cardiovascular disease, pulmonary disease, diabetes), WHO Performance status (World Health Organization), American Society of Anesthesiologists (ASA) score and intraoperative findings, CDC surgical wound classification will be recorded for each patient.

Sample size estimation:

The responsible lead investigator will screen the most recent 24 hours of abdominal surgeries daily to identify potential participants for the project. All available eligible patients during the recruitment period will be contacted and asked to participate.

The study is explorative in nature, and all cases are intended included. A prospective study is chosen over RCT to provide real-world insights and the highest degree of external validity and with respect to the study design. The primary analysis is based on detecting a difference in 30-day surgical site infection (SSI) rates between patients with and without postoperative subcutaneous fluid collections on point-of-care ultrasound. Prior literature has reported clinically relevant differences in SSI risk by ultrasound fluid status, including markedly higher SSI rates among patients with fluid collections (7). To avoid relying on the most extreme published estimates, we used conservative assumptions for power planning. We assumed an SSI rate of 35 percent among patients with fluid collections and 10 percent among patients without fluid collections. With a two-sided alpha of 0.05 and 80 percent power, approximately 86 patients are required under equal group sizes. Since the prevalence of fluid collections and therefore the group split is uncertain, we considered imbalanced scenarios (for example 40:60 or 30:70), which increase the required total sample size. We therefore aim to recruit 100 patients to ensure adequate power for the primary analysis and to allow for incomplete imaging and loss to follow-up.

Secondary objectives, including the diagnostic performance of ASEPSIS, POCUS, and their combination (AUROC-based comparisons), are considered exploratory and will be interpreted accordingly.

Statistical methods:

The primary objective of the statistical analysis is to evaluate the prognostic performance of the ASEPSIS score, point-of-care ultrasound (POCUS) findings, and their combination in the early detection of surgical site infections (SSI). Discrimination will be assessed using the area under the receiver operating characteristic curve (AUROC), which serves as the primary analytic metric for prediction accuracy. Separate logistic regression models will be constructed for ASEPSIS and POCUS, as well as a combined model incorporating both parameters. Comparisons between AUROCs will be conducted using the DeLong test to assess statistically significant differences in discriminatory performance between models.

Categorical variables will be summarized as frequencies and percentages using chi-squared or the two-tailed Fisher's exact test, as appropriate. Continuous variables will be assessed visually for normality using histograms and Q-Q plots. Based on distribution, these will be presented as either mean ± standard deviation (SD) or median with interquartile range (IQR) and compared using the t-test or Mann-Whitney U test.

Clinically relevant variables and those identified from prior literature will be selected as a priori for inclusion in univariate logistic regression to identify potential risk factors for SSI. In addition, the relationship between subcutaneous fluid volume on POCUS and the ASEPSIS score will be explored using Pearson or Spearman correlation coefficients, depending on distributional assumptions. Diagnostic performance of ASEPSIS, POCUS, and their combination will be assessed through calculation of sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and AUROC.

All statistical tests will be two-tailed, with p-values ≤ 0.05 considered statistically significant.

All statistical analyses will be conducted using SPSS software (version 21.0; SPSS Inc, Chicago, IL, USA).

Ethical Considerations:

The study will be conducted in accordance with the principles of the Declaration of Helsinki & General Data Protection Regulation (GDPR). The protocol will be submitted to the local Research Ethics Committees and the Danish Data Protection Agency for approval. The investigator will also inform the Research Ethics Committee and the Danish Data Protection Agency of any significant or major changes in the protocol. The study will be registered in the international database www.clinicaltrials.gov and OSF before initiation.

All patients taking part in the study must provide their oral and written informed consent before they are included in the study. It is the investigator's duty to inform patients so that they are fully aware of all aspects of the survey. Related to patient information, the investigator is also obligated by patient doctor confidentiality obligations.

Patients may at any time withdraw their consent to participation in the survey. If the patient decides to do this, this will not impair the patient's relationship with the investigator, or the hospital and the patient will continue to receive the best treatment the hospital can offer.

Patients showing symptoms of SSI or other illnesses during the study period will be treated according to the department's clinical practice.

Use of medical records:

Medical records will be accessed both prior to and after obtaining informed consent. Prior to consent, medical records will be reviewed solely for the purpose of assessing eligibility according to the predefined inclusion and exclusion criteria. After informed consent has been obtained, relevant medical record data will be accessed and used for study conduct, follow-up, data collection, monitoring, and analysis.

Information about and consent from patients:

All patients undergoing laparotomy at the participating centres, who are eligible according to previously described criteria will be invited to participate in the study.

Information about the study will be be provided after the operation as part of the postoperative care and follow-up. Written information about the study will also be given at this point (Appendix). The patient will be informed about the possibility of having a next of kin present. If necessary, the information will wait until the next of kin is present. The patient and their next of kin will also be informed verbally about the study.

The information will be given by the investigators or project nurse. The information will be conducted as uninterrupted as possible and preferably be given in a separate room. The patient and next of kin will be given time to consider the request.

Relevant provisions of the Research Ethics Committee regarding informed consent will be followed. The information and consent declaration for patients is attached.

Risks, side effects, disadvantages, etc.:

This study does not involve risks or side effects for the patients. The care provided in the entire admission period or during the outpatient period will not be affected negatively by our study. Instead, the increased focus on monitoring of postoperative SSI may lead to better care of patients in the future. Any unforeseen risks and side effects will be monitored and documented accordingly for patients' safety.

Compensation for injury:

Compensation for injury will be provided through ''Patienterstatningsordningen'' for eligible paitents. If any request occurs, it will be the investigator's responsibility to guide and council the participants in the application process.

Relevance/Impact:

This study aims to provide real-world insights into the utility of POCUS along with ASEPSIS as an extension of CDC-guidelines for the early detection of SSI, thereby reducing patient morbidity, mortality and economic burden on the healthcare system.

Additionally, the study seeks to establish an optimal subcutaneous fluid collection size cut-off to aid in early treatment decisions, with or without drainage.

Inclusion Criteria:

• Adults >18 years.
• Undergoing emergency or elective laparotomy.
• Primary or delayed primary wound closure (DPC).
• Wound classification: clean, clean-contaminated, contaminated, or dirty.
• Informed consent obtained.

Exclusion Criteria:

• Laparoscopic surgery
• Wounds healing by secondary intention.
• Ongoing wound dehiscence treatment with Negative Pressure Wound Therapy (NPWT).
• Non-Danish residents

IPD will not be share as per Danish Law and The Data Data Protection Agency.