Mechanical bowel preparation has traditionally been used before elective colorectal surgery with the objectives of reducing fecal load, facilitating operative manipulation and potentially reducing contamination-related complications.¹ The contemporary role of bowel preparation, however, is more complex than the historical assumption that more complete bowel cleansing necessarily produces better surgical outcomes.²

Within this broader strategy, an important clinical question is whether every component of mechanical preparation is necessary. An intervention may improve bowel evacuation while providing little additional benefit for outcomes that matter to patients.

Mannitol is an osmotic cathartic that has historically been used for preoperative bowel cleansing. Its low cost and availability have contributed to its use in many settings. Evidence concerning its effect on postoperative infection has nevertheless been inconsistent. The original study protocol cited markedly different infection rates in previous investigations, including 36.1% versus 16.9% in one study and 3.33% versus 6.66% in another. ^{1, 2, 3}

The clinically relevant question is therefore not simply whether mannitol changes the observed infection rate, but whether adding oral mannitol to an existing rectal preparation produces sufficient incremental benefit to justify routine use.

This manuscript presents a secondary exploratory analysis of the randomized cohort, focusing on the magnitude and consistency of the observed effect rather than reproducing the primary publication's outcome-focused presentation.

Study design and population The underlying study was a randomized controlled trial conducted in the Department of General Surgery, Lady Reading Hospital, Peshawar. A total of 164 patients aged 18–60 years undergoing elective colorectal surgery were included. Eligible procedures included colostomy closure and colorectal resections for malignant and inflammatory bowel disease. Patients were limited to ASA physical status I or II. Randomization and intervention Patients were allocated by block randomization into two groups of 82. The mannitol group received 10% mannitol at 10 mL/kg orally mixed with juice in addition to a normal-saline enema at 10 mL/kg. The comparison group received a normal-saline enema at 10 mL/kg without oral mannitol. Variables The available dataset included treatment allocation, age, sex, weight, ASA grade, duration of surgery and postoperative wound infection. The original study recorded wound infection according to its operational definition one week after surgery. Statistical approach The secondary analysis emphasizes absolute risk difference and relative risk in addition to the originally reported chi-square comparison. The observed treatment effect was also examined across the available predefined categories of age, sex, ASA status, weight and operative duration. Because only 18 wound-infection events were recorded, the analysis was treated as exploratory and no overfitted multivariable prediction model was constructed

Study population

The cohort comprised 164 patients, with 82 patients in each group. The mean age was approximately 43.3 years, with an age range of 18–60 years. The overall male-to-female ratio was 1.34:1. Age distribution between groups was not statistically different (reported p=0.531 in the age-distribution table).

Primary clinical outcome

Wound infection occurred in 8/82 patients (9.8%) in the mannitol group and 10/82 (12.2%) in the no-mannitol group. The absolute risk difference was −2.4 percentage points, favoring mannitol. The relative risk was approximately 0.80. The original comparison was not statistically significant (p=0.402).

Abbreviations: RD, risk difference; pp, percentage points; RR, relative risk.

Subgroup patterns

Age-specific analyses did not demonstrate statistically significant differences between treatment groups. Infection rates in the mannitol and no-mannitol groups respectively were 8.3% and 0% among patients aged ≤30 years; 5.6% and 7.7% among those aged 31–40 years; 13.6% and 10.3% among those aged 41–50 years; and 10.0% and 23.1% among those aged >50 years.

The principal finding of this secondary analysis is that adding oral mannitol to saline rectal preparation produced only a modest numerical difference in early postoperative wound infection. The infection rate was 9.8% with mannitol and 12.2% without it, corresponding to an absolute difference of approximately 2.4 percentage points and a relative risk of approximately 0.80. These findings are in line with the previous study.4 The numerical direction favored mannitol, but the observed difference was not statistically significant. More importantly, the small number of outcome events limits the precision of the estimate. The appropriate interpretation is therefore that this study did not demonstrate a statistically significant incremental benefit, rather than that mannitol has been proven ineffective or that the two strategies are formally equivalent. A previous study also reported comparable results.5 The comparison is clinically informative because both groups received saline rectal preparation. The intervention therefore represents the additional contribution of oral mannitol rather than a comparison between bowel preparation and no preparation at all. Within the limitations of the data set, the results do not demonstrate a clear wound-infection advantage from adding the osmotic agent. These findings are similar with the previous studies.6 The subgroup findings also failed to identify a consistent patient category in which mannitol clearly improved outcomes. Some numerical differences were observed—for example, among women, patients weighing 60 kg or less, and patients undergoing longer procedures—but the subgroup event numbers were small and none of the reported comparisons was statistically significant. The apparent increase in infection with age is also noteworthy. In the no-mannitol group, the highest observed infection frequency occurred among patients older than 50 years, while infection frequencies in the mannitol group were more evenly distributed. However, the age-specific comparisons were not statistically significant, and the study was not powered to establish age-treatment interactions. These observations suggest that the clinical value of mannitol should not be inferred from the ability to achieve mechanical evacuation alone. If an additional preparation component is to be routinely used, its value should ultimately be demonstrated through patient-important outcomes. The broader literature has similarly produced heterogeneous findings regarding mechanical bowel preparation and mannitol.6-8 The original dissertation identified conflicting results from previous mannitol studies and noted variation in bowel-preparation practice. The present findings add to that uncertainty rather than resolving it. Modern colorectal practice also increasingly considers mechanical bowel preparation together with oral antimicrobial prophylaxis. The present cohort was not designed to evaluate contemporary bowel preparation plus oral antibiotic protocols, so its findings should not be extrapolated to those strategies. Clinical interpretation Three conclusions can be drawn cautiously from the available data. First, oral mannitol was not associated with a statistically significant reduction in early wound infection. Second, the observed absolute difference was small. Third, no subgroup demonstrated a statistically significant and consistent advantage attributable to mannitol. Taken together, these findings support consideration of a simplified preparation strategy in selected patients, but they do not establish equivalence or non-inferiority of saline preparation alone. Strengths The underlying study was randomized with equal allocation to the two preparation strategies. The secondary analysis also reports effect magnitude and examines consistency across the patient and operative characteristics that were available in the original dataset. Limitations The principal limitation is the modest sample size and low number of wound-infection events. Only 18 infections occurred among 164 patients, limiting statistical precision and preventing reliable adjustment for numerous covariates. The study was conducted at a single institution. Wound infection was assessed one week after surgery, so later infections could have been missed. The available dataset also lacks several variables important to contemporary colorectal SSI research, including detailed operative procedure, oral antibiotic prophylaxis, microbiological findings, anastomotic leak, length of stay and 30-day readmission. The subgroup analyses were exploratory and should not be interpreted as evidence of treatment effects within individual demographic or operative categories. Finally, this manuscript represents a secondary analysis of a previously published randomized cohort. The original publication must be cited and the secondary nature of the analysis disclosed to the target journal. The present analysis should therefore be considered hypothesis-generating rather than an independent new clinical trial.

In this randomized cohort, adding oral mannitol to saline rectal preparation did not demonstrate a statistically significant reduction in early postoperative wound infection. Although the observed infection rate was numerically lower with mannitol, the absolute difference was small and the number of events was limited. No consistent subgroup-specific advantage was demonstrated. These findings support further evaluation of simplified bowel-preparation protocols but do not establish equivalence or non-inferiority of mannitol-free preparation.

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