In June 2023, it was decided to perform an intermediate analysis to assess the benefit or harm of the intervention.

The aim of this clinical trial is to associate paracentesis with positive metabolic derangement components for surgical determination in preterm infants with necrotizing enterocolitis. The main questions it aims to answer are:

• are the combination of metabolic derangement components and paracentesis good indicators for determining early surgery in preterm patients with necrotizing enterocolitis?
• With the use of the previously mentioned indicators, can mortality in these patients be reduced? Participants will undergo determination of the 7 parameters of the metabolic deterioration components and those with three or more positive points will undergo paracentesis and if this is also positive, it will be determined that the patient requires surgical management.

The researchers will compare with a control group to see the differences in surgical variables (length of necrotic bowel, length of viable bowel, need for intestinal diversion and surgical reintervention) and mortality.

Controlled, randomized, double-blind clinical trial approved by the hospital's Ethics and Research Committee, subject to the Declaration of Helsinki. This study follows the CONSORT trial reporting guidelines. Patients younger than 37 weeks of gestation, with NEC stage IIa or greater according to Bell criteria were included.

Two groups of patients were formed (Control and Intervention), randomization was based on a randomization sequence generated using the Graphard 2021 program.

Control group: determination of surgery in a conventional manner according to Bell's criteria (absolute indication: pneumoperitoneum), with radiographic surveillance every 6 hours.

Intervention Group: CMD measurement every 3rd day, it was considered positive when 3 or more points were present. These patients underwent paracentesis and if it was positive (faecal aspiration, serous fluid with Gram + staining, or serohematic fluid) the decision for surgery was made.

Both groups received the same pre- and post-surgical medical management by neonatology.

As a safety criterion, it was defined that when a surgeon considered that a neonate had an indication for surgery, in spite of not meeting the criteria previously indicated for each group, the surgical intervention would be performed.

The diagnosis, implementation of the randomization to each group, and blood sampling were performed by the neonatology staff, the surgical research team being blinded; paracentesis was performed by a pediatric surgeon in the patient's crib with asepsis measures, antisepsis with chlorhexidine, sedation and analgesia (fentanyl 2 to 4 mcg /kg/dose), 1% lidocaine infiltration at the puncture site (iliac fossa right), a 22 G punzocath and a 10 ml syringe were used .

Follow-up was performed until the patient was discharged or died. Mortality was the dependent variable. The time from the patient's admission to the study until surgery was considered; during exploratory laparotomy, the length of intestinal necrosis, length of viable intestine, need for intestinal bypass, surgical intervention performed, and surgical reintervention were determined. In addition, fasting days and parenteral nutrition (TPN) were quantified.

Sample size. Based on the calculation for the difference of two proportions, a size of 31 patients per group was obtained, with a mortality difference of 30%, one-tailed, alpha of 5%, and statistical power of 80%. In June 2022, it was decided to carry out an intermediate analysis to assess the benefit or harm of the intervention, determined by the p value ˂ 0.05 in the dependent variable.

Statistical analysis. Descriptive statistics with measures of central tendency and dispersion were used. For qualitative variables frequencies and percentages, for quantitative variables mean and standard deviation. The Shapiro-Wilk test was used to test normal distribution.

For inferential statistics, group comparisons were performed using the χ2 test, Fisher's exact test, Student's t test or Mann Whitney U test, according to the type and distribution of the variable. The relative risk (RR) was obtained as a measure of association, with 95% confidence interval (CI). In addition, a bivariate analysis was performed between mortality and secondary variables. Statistical significance was established with a p-value <0.05.

Inclusion Criteria:

• Patients equal to or less than 36.6 weeks gestation, with a diagnosis of NEC stage II A or greater according to Bell criteria.

Exclusion Criteria:

• Patients with a history of gastroschisis, suspected Hirschsprung's disease. Patients who do not give authorization for participation.

Elimination criteria:

Patients who during follow-up died due to some other condition. Patients who during the study declined to participate. Patients who in the surgical findings had data of Spontaneous Intestinal Perforation.

The diagnosis, application of randomization to each group and blood sampling were performed by the neonatology staff, both the patients and the surgical team that provided care were blinded.