Perioperative Hemoglobin Decline After Deep Endometriosis Surgery: Association with Surgery Duration and Exploratory Analyses of Operative Extent

1. Introduction

Surgery for deep endometriosis may combine rectosigmoid and upper-bowel procedures in a single session. For the operating surgeon, the practical question is which surgeries are most likely to be followed by a hemoglobin decline of at least 3 g/dL. This information may inform preoperative preparation and the timing of postoperative testing [1,2,3].

Preoperative anemia and perioperative hemoglobin decline represent different clinical problems. A lower preoperative concentration leaves less reserve after any given decline, so anemia should be investigated and treated when indicated. Surgical features may identify a different group: patients whose surgery is likely to be followed by a larger decline and who may benefit from planned postoperative monitoring.

The extent of surgery varies widely among patients. Guidelines recommend specialist-center treatment and an individualized multidisciplinary plan [4,5,6]. Previous studies have linked bowel resection and extensive disease with longer operating times, greater measured blood loss, and postoperative morbidity [7,8,9]. However, perioperative hemoglobin decline has received less attention in this population.

Patient blood management (PBM) begins before surgery and continues through postoperative care. It combines identification and treatment of anemia, measures that limit avoidable blood loss, and appropriately timed blood counts [1,2]. However, transfusion is a treatment-dependent endpoint influenced by clinical thresholds, patient status, and local practice [10]. Many gynecologic prediction studies have nevertheless used transfusion as their endpoint [11,12], and published models vary substantially [13].

Our primary aim was to quantify the frequency of a hemoglobin decline of at least 3 g/dL and examine its association with surgery duration. Secondary objectives were to analyze decline as a continuous outcome, compare clinically recognizable categories of surgical type and extent, and test whether the duration finding persisted after preoperative hemoglobin was removed from the model.

2. Materials and Methods

2.1. Study Design and Setting

We conducted a single-center retrospective cohort study of patients who underwent surgery for deep endometriosis at the Clinical Department of Gynecologic Oncology, Prof. Tadeusz Koszarowski Opole Cancer Centre, an academic affiliate of the Institute of Medical Sciences, University of Opole. Data were drawn from the institutional deep-endometriosis registry, linked locally to laboratory records. The study period covered 1 January 2020 through 31 December 2025. Reporting followed the STROBE statement and the RECORD extension for routinely collected health data [14,15].

The Bioethics Committee of the University of Opole approved the study (decision No. UO/0029/KB/2026; 25 June 2026) and waived individual informed consent because of the retrospective design. Analyses were performed locally using pseudonymized data.

2.2. Participants

Surgeries recorded in the institutional deep-endometriosis registry during the study period were eligible if they had a same-day link to the operative record, a recorded start time and duration, and organ-specific procedure information. Availability of hemoglobin measurements was not an eligibility criterion. When a patient had more than one eligible surgery, the earliest was selected as the index surgery. Each patient therefore contributed one surgery to the analysis.

2.3. Hemoglobin Measurements and Outcomes

Preoperative hemoglobin (Hb) was the last measurement obtained from 7 days before surgery until the recorded start of surgery. The recorded postoperative nadir was the lowest available value after the end of surgery through postoperative day 3. No intraoperative hemoglobin measurements were available.

The primary outcome was an absolute hemoglobin decline of at least 3 g/dL, calculated as preoperative hemoglobin minus the postoperative nadir. This operational threshold was selected before the feasibility analysis but is not a validated minimum clinically important difference. Continuous decline was a key secondary outcome. Other secondary outcomes were a decline of at least 4 g/dL, a postoperative nadir below 9 g/dL and below 8 g/dL, and red blood cell (RBC) transfusion during the index hospitalization.

Transfusion was considered present if it was recorded in the dedicated transfusion table or with ICD-9 procedure code 99.04. Discordant records were not clinically adjudicated, and the timing of transfusion relative to the recorded postoperative nadir was unavailable.

2.4. Surgical Variables and Other Covariates

Surgery duration was modeled as a continuous variable and expressed per 60-minute increment. The primary model additionally included preoperative hemoglobin concentration, the number of organs involved in the procedure, and the presence of upper-bowel and bladder surgery. The organ count comprised upper bowel, bladder, rectum or sigmoid colon, uterus, vagina, and ureter. The upper bowel was defined as the small bowel, appendix, cecum, and colon proximal to the rectosigmoid junction; rectosigmoid procedures were counted separately. Age at surgery was also described, together with the length of hospital stay, counted as inclusive calendar days from admission through discharge. Conversion to laparotomy was reported descriptively because the registry did not contain a standardized field for initial operative access.

We used three exploratory groupings to characterize the type and extent of surgery. First, rectosigmoid surgery, reported as bowel technique in the tables and figures, was classified as no full-thickness resection or shaving only, discoid resection without segmental resection, or any segmental resection. Second, a separate registry field indicated whether more than one full-thickness bowel resection had been performed. Among completed records, surgeries were classified as no resection when no full-thickness resection was recorded, one resection when a resection was recorded without the multiple-resection flag, and multiple resections when the flag was present. Neither the exact number of resections nor their anatomical distribution was recorded, and a blank field was treated as missing rather than as zero, so this variable was analyzed in complete cases only. Third, the organ count was categorized as no more than one, two, or three or more. Two further variables were handled separately: anastomotic height above the anal verge was modeled as a continuous variable and reported per 5-cm increment among patients undergoing segmental resection, and body mass index (BMI) was included only in sensitivity analyses.

An additional exploratory analysis combined resection multiplicity and organ count into three operative-burden strata. Low burden was defined as no recorded full-thickness bowel resection together with involvement of no more than one organ. High burden was defined as an explicitly recorded multiple bowel resection or involvement of three or more organs; all other surgeries were intermediate. For these strata, a blank multiplicity field was handled as no recorded multiple resection; it could therefore prevent, but never create, assignment to the high-burden group. Definitions were set before the composite group outcomes were calculated; none used fitted probabilities or outcome-derived cut points. The display rule was outcome-dependent; it is specified in the Supplementary Methods. These strata were intended as a descriptive clinical summary rather than a validated prediction rule.

2.5. Statistical Analysis

Continuous variables were summarized as medians and interquartile ranges, and categorical variables as n/N (%). Exact 95% confidence intervals (CIs) were calculated for proportions.

Multivariable logistic regression was used to examine factors associated with a hemoglobin decline of at least 3 g/dL. The outcome threshold was fixed before preliminary feasibility work. Surgery duration was selected as the principal exposure after that work, and the primary model was fixed before the final analysis. It included surgery duration, preoperative hemoglobin, organ count, upper-bowel surgery, and bladder surgery. Results are reported as adjusted odds ratios (ORs) with 95% CIs. Continuous hemoglobin decline was examined with the same covariates using linear regression with HC3 robust standard errors.

Because preoperative hemoglobin is part of the calculated decline, its apparent association with that decline can arise partly from arithmetic rather than biology [16,17]. We therefore repeated the binary and continuous analyses without preoperative hemoglobin. We also compared how well preoperative hemoglobin alone, the surgical variables alone, and both together discriminated between patients in whom the primary outcome did and did not occur. We summarized discrimination with the area under the receiver operating characteristic (ROC) curve (AUC) and overall prediction error with the Brier score, and we assessed calibration as the agreement between predicted and observed proportions [18,19]. Model performance was internally validated by patient-level bootstrap resampling; full specifications are provided in the Supplementary Methods.

Primary analyses included patients with both hemoglobin measurements. We compared patients with and without a complete pair and repeated the duration analysis using inverse-probability weighting, multiple imputation of the two hemoglobin components, and prespecified missing-not-at-random scenarios [20,21,22,23]. In separate sensitivity analyses we excluded surgeries involving the bladder, excluded patients with a recorded transfusion, allowed a nonlinear duration effect, used separate organ indicators, and added BMI (Table S1). Age and year of surgery were examined separately in post hoc sensitivity models. We also examined the last platelet count, activated partial thromboplastin time (APTT), and international normalized ratio (INR) recorded from 7 days before surgery until the recorded start of surgery. Each marker was added separately and all three were then added together to assess information beyond the primary model. None of these analyses replaced the primary model. Full specifications are provided in the Supplementary Methods.

Analyses of surgical type and extent were exploratory. Continuous decline was the main outcome and the binary outcome was secondary. All models were adjusted for preoperative hemoglobin and were fitted both with and without surgery duration. For the post hoc operative-burden strata, we reported group-specific outcomes and an ordered trend across low, intermediate, and high burden. Adjustment for surgery duration was treated as an attenuation check because duration is itself one manifestation of operative burden. No outcome-based cut point, interaction, or confirmatory family was defined.

Analyses were performed in R version 4.6.1 (R Foundation for Statistical Computing, Vienna, Austria). Multiple imputation used mice version 3.19.0. Figures were produced with ggplot2 version 4.0.3, patchwork version 1.3.2, svglite version 2.2.2, ragg version 1.5.2, and scales version 1.4.0. Missing-data reporting followed TARMOS. The TRIPOD and PROBAST checklists guided reporting of discrimination and calibration, although this study did not develop or validate a clinical prediction model [23,24,25].

2.6. Relationship to Previous Publications From the Center

Previous publications from our center examined bladder endometriosis, a modified segmental bowel-resection technique, and colorectal dehiscence or rectovaginal fistula [26,27,28]. One prior bladder cohort reported descriptive and subgroup perioperative hemoglobin changes. None of the prior publications assessed the full cohort using the present 3-g/dL endpoint, postoperative-day- 0–3 window, and primary multivariable framework.

The reconstructed cohorts overlap: at least 402/424 patients appeared in one or more prior cohorts; the categories overlap, and the historical modified-resection count is a lower bound. The present analysis extends previous work from our center and is not based on an independent cohort. It does not re-evaluate the organ-specific complications reported in those studies.

3. Results

3.1. Hemoglobin Decline and Cohort Profile

The source cohort contained 443 eligible surgeries. After 19 repeat surgeries were excluded, 424 patients contributed one index surgery each (Figure 1). The median age was 38.3 years (IQR, 33.6–42.3), and the median length of hospital stay was 8 days (IQR, 8–9). Among 419 patients with a preoperative measurement, 41 (9.8%) had hemoglobin below 12 g/dL.

A rectosigmoid procedure was performed in 383/424 patients (90.3%), uterine surgery in 273 (64.4%), and bladder surgery in 82 (19.3%). A ureteral procedure was performed in 63 patients (14.9%) and an upper-bowel procedure in 57 (13.4%) (Table 1).

Preoperative hemoglobin was available for 419/424 patients, a postoperative nadir for 421/424, and both measurements for 416/424 (98.1%). Among the 416 patients with both measurements, the median hemoglobin decline was 2.6 g/dL (IQR, 2.0–3.3). The primary outcome occurred in 156/416 patients (37.5%; exact 95% CI, 32.8–42.3%), and 42/416 (10.1%) had a decline of at least 4 g/dL. Median hemoglobin decreased from 13.3 g/dL (IQR, 12.6–14.0) before surgery to 10.6 g/dL (IQR, 9.7–11.4) at the recorded postoperative nadir. A recorded nadir below 9 g/dL occurred in 42/421 patients (10.0%), and 8/421 (1.9%) had a recorded nadir below 8 g/dL.

The lowest recorded postoperative value occurred on postoperative day 1 in 179/421 patients (42.5%), day 2 in 123 (29.2%), and day 3 in 119 (28.3%). Because sampling times were not standardized, this distribution describes the lowest available value rather than measurements obtained on a common schedule. RBC transfusion was recorded in 42/424 patients (9.9%). Among the 42 transfused patients with both hemoglobin measurements, the primary outcome occurred in 29 (69.0%; Table 2).

3.2. Surgery Duration and Hemoglobin Decline

Longer surgery duration was associated with greater hemoglobin decline. In the primary model, each additional hour was associated with 61% higher odds of the primary outcome (OR, 1.61; 95% CI, 1.35–1.91). Removing preoperative hemoglobin did not materially weaken the association (OR, 1.50; 95% CI, 1.28–1.76).

Each additional hour also corresponded to a 0.20-g/dL greater continuous decline, both with preoperative hemoglobin in the model (95% CI, 0.13–0.26) and without it (95% CI, 0.12–0.27). The interquartile range of surgery duration was 170 to 286 minutes. Across that interval, the modeled probability of the primary outcome increased from approximately 25% to 45% (Figure 2).

Table 3 summarizes binary and continuous duration estimates; the complete coefficient sets are given in Table S4.

The preoperative-Hb model contains preoperative hemoglobin only. The surgical-variable model contains surgery duration, organ count, upper-bowel surgery, and bladder surgery. The combined model contains both sets of variables. The recorded-postoperative-nadir row is an algebraic re-expression of the continuous-decline row, because the nadir equals preoperative hemoglobin minus the decline in a model that already adjusts for preoperative hemoglobin; it is shown for completeness and is not an independent confirmation. The three logistic models in Panel B were internally validated with 1000 bootstrap resamples; internally validated values equal apparent performance minus the average bootstrap optimism. AUC differences used 2000 paired resamples. Hb, hemoglobin; OR, odds ratio; CI, confidence interval; AUC, area under the receiver operating characteristic curve.

Preoperative hemoglobin was strongly associated with crossing the fixed 3-g/dL threshold (OR per 1 g/dL, 2.32; 95% CI, 1.81–2.98). Because preoperative hemoglobin is part of the calculated decline, this coefficient reflects both the data and an arithmetic relationship. The association with surgery duration persisted in both models that excluded preoperative hemoglobin.

3.3. Clinically Recognizable Patterns of Surgical Extent

Hemoglobin decline increased with the extent of rectosigmoid surgery (Figure 3 and Table 4). The primary outcome occurred in 17/83 patients (20.5%) after no full-thickness resection or shaving only, 20/64 (31.2%) after discoid resection, and 119/269 (44.2%) after segmental resection (Table S2). With preoperative hemoglobin and duration in the model, the mean decline was 0.33 g/dL greater after discoid resection (95% CI, 0.05–0.61) than in the reference group. The corresponding difference after segmental resection was 0.40 g/dL (95% CI, 0.18–0.62).

A similar pattern was observed across the bowel-resection multiplicity categories. Among 379 patients with this field recorded, the outcome frequency was 9/62 (14.5%) with no full-thickness resection and 102/266 (38.3%) with one resection. It was 28/51 (54.9%) when multiple resections were performed during the same surgical session. In the same adjusted model, the mean decline was 0.40 g/dL greater after one resection (95% CI, 0.17–0.64). The corresponding difference after multiple resections was 0.70 g/dL (95% CI, 0.28–1.11). Corresponding ORs were 2.61 (95% CI, 1.13–6.04) and 3.46 (95% CI, 1.21–9.91). Because technique and multiplicity were separate registry fields, their denominators differed; details are provided in the Supplementary Methods.

A comparable gradient was observed for the number of organs involved. The outcome frequency was 28/108 (25.9%) with no more than one organ involved, 76/204 (37.3%) with two organs, and 52/104 (50.0%) with at least three (Table S2). After adjustment for preoperative hemoglobin and duration, the mean decline was 0.13 g/dL greater for two organs than for no more than one (95% CI, −0.08 to 0.34). The corresponding difference for at least three organs was 0.33 g/dL (95% CI, 0.05–0.61).

No clear association was observed between anastomotic height and hemoglobin decline. This variable was recorded in 240 of the 269 patients who underwent segmental resection. In this subgroup, the adjusted mean difference per 5-cm increase was −0.09 g/dL (95% CI, −0.33 to 0.15). Field availability is detailed in the Supplementary Methods.

3.4. Descriptive Operative-Burden Strata

The post hoc operative-burden strata summarized these overlapping surgical features. The definitions were specified before group outcomes were calculated, but the strata were retained for presentation only after minimum-size, minimum-event, and monotonic-outcome display conditions were met. They are descriptive and require external validation. Low-, intermediate-, and high-burden strata included 44/416 (10.6%), 256/416 (61.5%), and 116/416 (27.9%) patients, respectively. The outcome frequency was 11/44 (25.0%), 85/256 (33.2%), and 60/116 (51.7%), and the mean hemoglobin decline was 2.12, 2.64, and 3.06 g/dL, respectively (Table 5).

Low burden was defined as no recorded full-thickness bowel resection and no more than one organ involved. High burden was defined as an explicitly recorded multiple bowel resection or at least three organs involved; all remaining surgeries were intermediate. When a surgery met both the low-burden and the high-burden definition, the high-burden definition took precedence. A blank resection-multiplicity field was treated as no recorded multiple resection; it could therefore prevent, but never create, assignment to the high-burden group. The definitions were specified before group outcomes were calculated, but the strata were retained for presentation only after minimum-size, minimum-event, and monotonic-outcome display conditions were met. This display rule was outcome-dependent. The strata are descriptive and require external validation. Confidence intervals for proportions are exact (Clopper–Pearson). Hb, hemoglobin; IQR, interquartile range; CI, confidence interval.

After adjustment for preoperative hemoglobin and duration, mean decline was 0.30 g/dL greater per category increase (95% CI, 0.13–0.48). For the binary outcome, the corresponding estimate was imprecise (OR, 1.48; 95% CI, 0.96–2.28; Table S5).

3.5. Robustness of the Duration Finding

Combining preoperative hemoglobin with surgical variables yielded an internally validated AUC of 0.760. Without preoperative hemoglobin, the surgical-variable model had an AUC of 0.684. On the apparent-performance scale used for paired comparisons, adding preoperative hemoglobin to the surgical model increased AUC by 0.074 (95% CI, 0.031–0.125). Adding surgical variables to the preoperative-hemoglobin model increased AUC by 0.085 (95% CI, 0.054–0.131). Both preoperative hemoglobin and the surgical variables contributed to discrimination (Table 3). Calibration within this cohort is shown in Figure 4.

Alternative approaches to missing measurements produced comparable duration estimates. Inverse-probability weighting gave an OR of 1.61 (95% CI, 1.35–1.92). Multiple imputation gave an OR of 1.58 (95% CI, 1.33–1.88) and a continuous estimate of 0.19 g/dL per hour (95% CI, 0.13–0.25). Planned missing-not-at-random scenarios left the estimates essentially unchanged (Table S3).

Age was not clearly associated with either outcome (per 10 years: OR, 1.21; 95% CI, 0.82–1.79; continuous decline, 0.12 g/dL; 95% CI, −0.03 to 0.27). These intervals were wide and did not exclude a clinically relevant association. Including age did not materially change the duration estimate. Excluding surgeries involving the bladder yielded a consistent duration association (OR, 1.59; 95% CI, 1.31–1.94). There was no strong evidence of nonlinearity (p = 0.075). The per-hour estimate should be interpreted as an average association across the observed range. Year-of-surgery and other sensitivity analyses likewise preserved the duration association (Table S1).

Preoperative platelet count, APTT, and INR were available for 416, 415, and 413 patients with both hemoglobin measurements, respectively. None materially changed the duration estimate. Adding all three changed apparent AUC by 0.002 and provided no evidence of improved model fit (likelihood-ratio p = 0.822; Table S5).

4. Discussion

4.1. Main Findings

A hemoglobin decline of at least 3 g/dL occurred after 37.5% of surgeries for deep endometriosis. Surgery duration remained associated with both binary and continuous decline when preoperative hemoglobin was removed from the model. Exploratory gradients were observed across rectosigmoid techniques, bowel-resection multiplicity, and the number of organs involved. The post hoc operative-burden strata also showed a descriptive gradient, but their display was outcome-dependent and the duration-adjusted binary trend was imprecise (OR, 1.48; 95% CI, 0.96–2.28). These exploratory patterns require external validation.

4.2. Comparison with Previous Studies

Earlier studies of deep endometriosis have generally reported measured blood loss, surgery duration, or postoperative complications rather than a defined hemoglobin-change outcome. Direct comparison with the 37.5% primary outcome frequency is therefore limited. These findings are nevertheless consistent with reports that extensive disease and bowel resection are accompanied by longer operating times and greater blood loss [7,8]. Specialist-center cohorts also show that morbidity varies with operative complexity [9]. We report a frequently observed, explicitly defined laboratory outcome that complements that clinical picture.

4.3. Clinical Implications

Preoperative anemia and operative extent inform different steps in care. A lower preoperative hemoglobin increases the clinical impact of any given decline because the postoperative concentration will also be lower. Among patients with an available preoperative measurement, 9.8% had hemoglobin below 12 g/dL. These patients warrant evaluation for the cause of the anemia and treatment when indicated.

Surgery duration and operative extent may help identify patients likely to experience a greater decline. The directly recorded surgical features—not the post hoc composite strata—provide the most transparent basis for future work. Their potential use in prioritizing postoperative monitoring requires prospective evaluation before they can define a clinical rule.

The lowest recorded postoperative hemoglobin value was on days 2–3 in 57.5% of the 421 patients with a postoperative measurement. Because day-specific measurement completeness and within-patient day-1-versus-later comparisons were not available, this distribution may reflect both postoperative trajectory and surveillance intensity. It should not be interpreted as evidence that routine testing on days 2–3 is superior to testing on day 1.

Hemoglobin decline reflects a combination of bleeding, hemodilution from fluid administration, transfusion, and the timing of sampling [29,30]. Accordingly, the observed gradients indicate operative burden rather than providing direct estimates of blood loss.

4.4. Interpretation of the Analyses Excluding Preoperative Hemoglobin

A fixed decline threshold is more readily crossed when preoperative hemoglobin is higher, because the preoperative value is part of the calculation [16,17]. This mathematical relationship helps explain why preoperative hemoglobin improved model performance and why its odds ratio should not be interpreted as a purely biological effect. Importantly, longer duration remained associated with both the binary outcome and continuous decline when preoperative hemoglobin was removed. Adding the surgical variables to the model containing preoperative hemoglobin improved discrimination. However, the surgical-variable model did not clearly outperform preoperative hemoglobin alone (difference in AUC, 0.011; 95% CI, −0.060 to 0.092).

4.5. Added Value and Future Research

Previous publications from our center focused on organ-specific technique or postoperative complications. This analysis addresses a distinct PBM question across the full cohort. It quantifies a perioperative hemoglobin outcome and shows that the duration association persisted when preoperative hemoglobin was omitted. Exploratory measures of operative extent showed gradients that require external validation.

Future studies should prospectively record measured blood loss, fluid balance, the cause and treatment of preoperative anemia, and transfusion timing, and should apply a standardized postoperative sampling schedule. External validation should then test whether the operative-burden strata improve monitoring decisions and whether continuous decline or a clinically validated threshold is the better endpoint.

4.6. Strengths and Limitations

The study included consecutive eligible index surgeries, used clearly defined measurement windows, and captured both hemoglobin measurements in 98.1% of patients. After preliminary feasibility work, we fixed the primary model before the final analysis. The duration estimates were consistent across several approaches to the remaining missing measurements. All analyses were performed in a scripted R workflow.

The retrospective, single-center design precludes control of variation in perioperative care and limits the applicability of the estimates to other settings. Rectosigmoid procedures were performed in 90.3% of patients, and the median hospital stay was 8 days; both features may differ in other centers. Surgeon and team effects, ASA class, comorbidities, and anticoagulant exposure were not available for adjustment. Surgery duration should therefore be interpreted as a marker of operative complexity rather than a modifiable causal exposure. The cohort also overlaps with those of previous publications from our center and should be regarded as an extension rather than an independent validation cohort. Surgery duration was designated the principal exposure only after preliminary feasibility work; internal validation cannot account for that data-driven choice.

Quantitative measurements of blood loss and perioperative fluid volumes were not available, so bleeding could not be separated from hemodilution. Ferritin, transferrin saturation, and other measures of iron status were also unavailable; the cause of preoperative anemia could not be determined. Standardized fields for intravenous iron, tranexamic acid, and cell salvage were absent, precluding evaluation of these PBM measures. Transfusion status combined two discordant structured sources and was not clinically adjudicated; its timing relative to the recorded postoperative nadir was unavailable.

Eight patients lacked one or both hemoglobin measurements, and the missing-data analyses had little effect on the estimates. The missingness mechanism nevertheless cannot be established. Patients meeting the primary outcome had more postoperative measurements than those without it (mean, 3.07 vs. 2.64), which may have increased the chance of observing a lower value and created surveillance bias in this nadir-based outcome. Day-specific measurement availability was not reconstructed, and the timing of the lowest recorded value was described for the cohort as a whole rather than according to surgical extent. The observed distribution therefore cannot establish whether hemoglobin declined after day 1. Moreover, the lowest recorded value was on day 3 in 28.3% of patients, so a later true nadir may have been missed.

The 3-g/dL threshold is not a validated minimum clinically important difference. Resection multiplicity was unavailable for 37 patients with both hemoglobin measurements, and the surgical-extent analyses involved several exploratory comparisons without adjustment for multiple testing. Presentation of the post hoc operative-burden strata depended on minimum group sizes and monotonic outcome gradients. A blank multiplicity field was treated as no recorded multiple resection for those strata and could prevent, but not create, high-burden assignment. These choices may have led to under-assignment of surgeries to the high-burden group and limit the evidential weight of the gradient; the strata require external validation. Finally, the narrow observed ranges of APTT and INR limit the hemostasis analysis; its findings do not apply to patients with clinically important coagulopathy.

5. Conclusions

Longer surgery duration was associated with greater perioperative hemoglobin decline after surgery for deep endometriosis. Exploratory analyses also showed gradients across bowel-resection multiplicity and the number of organs involved, but these findings do not define a monitoring or prediction rule. These routinely available features warrant prospective evaluation as possible monitoring indicators, while lower preoperative hemoglobin indicates less reserve after a similar decline. The optimal timing of postoperative hemoglobin measurement cannot be determined from these retrospective data and should be evaluated prospectively with a standardized sampling schedule.