This study evaluates the prognostic value of fecal lactate and the fecal-to-serum lactate gradient as early biomarkers of tissue hypoperfusion in critically ill patients. While serum lactate is widely used, it may not accurately reflect splanchnic perfusion. This prospective observational study aims to determine whether fecal lactate levels obtained within the first 12-24 hours can predict poor response to resuscitation at 24 hours. The primary outcome is a composite of increased vasopressor requirements, persistent hyperlactatemia, worsening organ dysfunction, or death.

Tissue hypoperfusion is a central mechanism in the development of organ dysfunction in critically ill patients. Although serum lactate is commonly used as a marker of hypoxia and a target for resuscitation, it may not adequately reflect regional perfusion, particularly in the splanchnic circulation. Persistent splanchnic hypoperfusion contributes to intestinal barrier dysfunction, bacterial translocation, and progression to multiple organ failure.

This study is based on the hypothesis that, under ischemic conditions, the intestinal mucosa behaves as a semipermeable membrane, allowing equilibration of lactate produced in the intestinal wall into the lumen. Therefore, fecal lactate may serve as a direct and early marker of intestinal hypoperfusion.

This is a prospective, observational, single-center cohort study conducted in an intensive care unit. Adult patients (≥18 years) with evidence of tissue hypoperfusion will be included. Fecal samples will be collected within the first 12-24 hours and processed through dilution, homogenization, centrifugation, and colorimetric analysis of the supernatant to quantify L-lactate levels. Simultaneously, serum lactate will be measured, and the fecal-to-serum lactate gradient will be calculated.

The primary outcome is poor response to resuscitation at 24 hours, defined as a composite endpoint including increased vasopressor requirements, serum lactate clearance <10% or persistent lactate >2 mmol/L, worsening organ dysfunction measured by SOFA score (increase ≥1 point), or death.

Secondary objectives include evaluating the correlation between fecal lactate and organ dysfunction severity, as well as determining the optimal cut-off value for fecal lactate to predict adverse outcomes using receiver operating characteristic (ROC) curve analysis.

This study aims to identify fecal lactate as a non-invasive, early, and specific biomarker of splanchnic hypoperfusion, potentially improving clinical decision-making and guiding resuscitation strategies in critically ill patients.

Inclusion Criteria:

• Adult patients ≥18 years admitted to the intensive care unit (ICU)
• Evidence of tissue hypoperfusion defined by at least ONE of the following: Arterial serum lactate ≥2.0 mmol/L or Hypotension requiring vasopressors to maintain mean arterial pressure (MAP) ≥65 mmHg
• Clinical signs of hypoperfusion (capillary refill time >3 seconds or mottling score ≥2)
• Availability of fecal sample within the first 24 hours of ICU admission

Exclusion Criteria:

• Active gastrointestinal bleeding
• Recent abdominal surgery (<48 hours) with intestinal resection or stoma
• Confirmed Clostridioides difficile infection
• Do-not-resuscitate (DNR) orders at ICU admission

Adult critically ill patients admitted to the intensive care unit with evidence of tissue hypoperfusion. Patients are identified at ICU admission or within the first 24 hours of shock onset and must have availability of a fecal sample for analysis. All patients receive standard of care management according to institutional protocols, and no interventions are assigned as part of the study.

Data sharing plans are currently undecided. Individual participant data may be shared in the future upon reasonable request, after study completion and publication of results, in accordance with institutional policies and ethical regulations.