Overall 1218 patients were treated for acute appendicitis during the study period. According to the aforementioned criteria, 450 patients were classified as CA. Univariate analysis revealed equal sex distribution in both groups. Patients with CA were significantly older and had a higher BMI and ASA Score. They had a median symptom duration that was longer than in the UA group and were more likely to undergo preoperative CT scan. Procedures for CA were more likely to be converted to open and lasted significantly longer. These patients had a higher chance to undergo revision surgery, and postoperative length of stay was prolonged. For details, see Table 1.

According to the surgical reports of the complicated cases, the appendix was classified as gangrenous in 52.9%, a peri-appendicular abscess was reported in 32.2%, the appendix appeared perforated in 65.6%, and peritonitis was present in 51.8%.

Any one criterion was fulfilled in 38.9% of CA while there was an overlap of two criteria in 28.0%, of three criteria in 24.7%, and of all four criteria in 8.4% respectively. Postoperative antibiotic continuation was significantly more common in CA patients at 72.2% compared to 9.9% in the UA group. For details, see Table 2.

We created a binary logistic regression model to explore predictors for continued antibiotic therapy after appendectomy in our cohort. The overall model was significant with a χ2 of 741.592, p < 0.001. The model accounted for approximately 45.8% (Cox and Snell R2) to 63.8% (Nagelkerke’s R2) of the variance and was able to correctly predict postoperative continued antibiotics in 87.4%. Hosmer and Lemeshow goodness-of-fit test indicated a decent fit as it was not significant p = 0.738. In the multivariate analysis ASA Score of 3, elevated WCC, high CRP, presence of gangrenous or perforated appendicitis, peritonitis, and microbiologic testing were independent predictors of prolonged antibiotics, where age and intraoperative finding of an abscess were not. The strongest predictors were visible perforation followed by high increased C-reactive protein (CRP), and the decision to send a microbiologic specimen. For details, see Table 3.

Microbiological specimens were sent for culture in nearly 40% of UA and 75% of CA. Of the 301 cultures from UA cases, a third grew pathologic bacteria, while nearly four out of five cultures in the CA patients were positive. 65% of positive cultures were poly-microbial in the UA group compared to 89% poly-microbial positive cultures in the CA group. A total of 1224 bacteria were cultured with 147 different strains. The most common isolates were E. coli (n = 228, 18.7%) Bacteroides fragilis (n = 124, 10.1%), Bacteroides thetaiotaomicron (n = 94, 7.7%), Bacteroides ovatus (n = 60, 4.9%), Bilophila wadsworthia (n = 60, 4.9%), Pseudomonas aeruginosa (n = 57, 4.7%), Streptococcus anginosus (n = 44, 4.6%), Enterococcus avium (n = 36, 2.9%), Enterococcus faecalis (n = 28, 2.3%) and Parabacteroides distasonis (n = 28, 2.3%). There was no statistically significant difference between UA and CA although there was a trend toward more anaerobic bacteria in the CA group (50.1% vs 44.2%, p = 0.084).

Overall, there were 58 documented infectious complications in the form of surgical-site infections (4.8%), nine in the UA group—four superficial (0.5%) and five intra-abdominal SSI (0.6%) as well as 49 in the CA group—five superficial (1%) and 44 intra-abdominal SSI (9.8%).

In the CA group, of the four defining criteria, only gangrenous appendicitis (β 0.918, odds 2.505 (CI 95% 1.298–4.835), p = 0.006) and perforated appendicitis (β 1.006, odds 2.735 (CI 95% 1.180–6.338), p = 0.019) were independent risk factors for SSI, while the presence of peritonitis and abscess was not.

Of the nine patients with an SSI in the UA group six received antibiotics, compared to 48 of the 49 patients in the CA group (p < 0.001). In addition to antibiotics, no intervention was necessary in three UA patients (33.3%) compared to 16 CA patients (32.7%). Wound opening/superficial abscess drainage was performed in three UA patients (33.3%) compared to four CA patients (8.2%). CT-guided drains were placed in one UA patient (11.1%) vs 11 CA patients (22.4%). Revision surgery was performed in two UA patients (2.2%) vs 18 of CA patients (36.7%).

We compared the results of microbiological specimen collected from the initial surgery and during the SSI complication. Forty-three patients (74.1%) had material cultured during their appendectomy, whereas 15 (25.9%) did not. Thirty-seven patients (63.8%) had new microbiological testing done during the SSI.

Eighteen patients (31.0%) had either no material sent for culture or cultures yielded no growth. Twenty-two patients (37.9%) did not have microbiologic testing for their SSI. In the cases where bacteria were cultured during both the appendectomy and the SSI, all SSI bacteria were present on index swabs in five patients (8.6%), one different bacterium was cultured in six patients (10.3%), and more than one different/additional bacteria were grown in seven patients (12.1%). Therefore, 13 out of 18 patients (72.2%) that had both positive cultures during the appendectomy and the SSI had a different bacterial spectrum at the SSI that was not fully represented by the index swab.

In a next step, we examined whether antibiotics prescribed during SSI matched bacterial antibiotic resistances. Four patients (6.9%) did not receive any antibiotics. Seven patients (12.0%) either had no swab or showed no growth. The antimicrobial selection matched the bacteria encountered during the appendectomy and the SSI in twelve cases (20.7%). In 18 cases (31.0%), it matched only the index appendectomy but not the SSI, (nine patients had no swab during the SSI and six grew no bacteria). The remaining three grew bacteria that were not covered by the chosen antibiotic regimen. In eight cases (13.8%), the empiric antimicrobial during the SSI matched the bacteria cultured from the SSI but not the index bacteria (three cases grew bacteria, that were not properly covered, no initial cultures were done in five). In nine patients (15.5%), even though there were microbiologic results available from the appendectomy and the SSI, the antibiotics did not fully cover either.

With the exception of four patients who had superficial wound infections opened in the emergency room, all patients had antibiotic treatment.

Overall, the outcome was favorable in 57 out of 58 patients with the remaining patient requiring further intervention due to an anastomotic leak after initial appendectomy followed by a leak of the appendix base treated with ileocecal resection.