*3.1. Baseline Characteristics*

No significant di fferences in age, gender, and smoke habit distribution were observed between CD patients and the control group.

At the time of surgery for CD, 16 patients were smokers, 15 former smokers, and 19 had never smoked. In about 54% of the patients, the disease was localized in the ileum, in 34.6% in the colon and only 6 (11.5%) patients had a disease involving both segments. Many of the patients (41%) presented extra-intestinal diseases (skin and arthritis), 45.10% perianal disease, 55.5% had already undergone surgery, and in 30.6% of the cases, familial IBD was observed.

#### *3.2. Oxidative Damage and Antioxidant Capacity*

All of the oxidative damage parameters measured were significantly higher in the serum of CD patients when compared to the controls (Table 2). In particular, the di fference between the AOPP levels in the serum of CD patients was very high when compared to healthy volunteers with a median value of 11.25 (5.02–15.15) vs. 1.36 (0.75–2.70) μmol/g of proteins, respectively (Table 2 and Figure 1A).

Based on the analysis of ROC curves, we assessed the diagnostic utility of the oxidative damage biomarkers as predictors of CD. The area under the ROC curve (AUC) was 0.6938 for TBARS, 0.7412 for CO, 0.7195 for AGEs, and 0.5765 for FRAP. The AOPP determination resulted in a much higher AUC value of 0.9306 for the prediction of Crohn's disease (Figure 1B).

The antioxidant capacity, measured as FRAP values, in the CD patients and controls was similar (Table 2).

Through univariate analysis, we observed that CD patients treated with azathioprine (*n* = 24) presented FRAP levels significantly higher (*p* < 0.05) than those untreated (*n* = 18). Moreover, serum AOPP levels were significantly reduced in patients treated with mesalazine (*p* < 0.01), but the number of untreated patients was very low (6 vs. 41).


**Table 2.** Mean values of oxidative markers in serum samples from Crohn's patients or healthy volunteers.

Means ± SD or median (interquartile range 25–75); *p*-values by t test or by Mann–Whitney test. AU: Arbitrary Units.

**Figure 1.** (**A**) Scatter dot plot of the advanced oxidation protein products (AOPP) levels in the serum of Crohn' patients and healthy volunteers. \*\*\*\* *p* < 0.0001 by Mann–Whitney test. (**B**) ROC curve for AOPP.

Positive correlations existed among almost all of the different markers of oxidative stress, both in the CD patients and in the controls (Tables S1 and S2). Figure 2 shows the correlation between circulating AOPP and TBARS levels in CD patients. No correlation between CDAI and oxidative-stress parameters was observed.

**Figure 2.** Correlation between AOPP and TBARS levels in the serum of Crohn's patients.
