Search results yielded a total of 2799 study, of them 2774 were excluded via title and abstrct screening for being either duplicates or non-revlevant to the topic of the review. 25 studies were included in the full text-screening, 12 exluded for being dupplicates and 1 study was exluded because it was not a RCT , leaving 12 studies to be included in the final meta-anlysis. Figure 1 demenostrates the PRISMA flow chart and the studies selection process. patients numbers were diversed among the studies, with a total of 1456 patients radmoized to recive either PC (753 patients) or DPC (703 patients), Table 1 shows charachertistics of included studies. Gender distribution was considered in all studeis,with a majority of 829 (57%) patients as males, while 627 (43%) of them were females. Regarding difference in age groups, two studies include pediatics patients only [17, 23], and four studies include adults only[7, 15, 24, 25], while the rest of the studies included both pediatrics and adults patients[1, 5, 16, 26–28].
Clincal diagnoses, appendicities was diagnosed in seven studies [15–17, 23, 24, 26, 27], appendicities plus other clinical conditions was the diagnosis in three studies, ,. peritonitis was in only one study , and in other study the diagnosis was ulcerative colitis . Surgeries were done correspondingly with clinical diagnosis, appendectomy was done in eight studies[5, 15–17, 23, 24, 26, 27],two studies performed laparotomy ,, and one study did reverse closer of ileostomy . Regarding surgical incisions, right lower quadrant muscle splitting (McBurney’s point) was done in four studies [5, 23, 24, 27], three studies performed midline incision [5, 25, 28],other three studies did Grid Iron incision [15, 16, 26], and two studies used para-median incision [5, 26]. On the other hand, the type of incision used wasn’t mentioned in 3 studies [1, 7, 17]. Regadiding patients follow-up period, it wasn’t mentioned in two studies [16, 28], other than that the duraiton varied among the studies ranging from 5 days up to 5 weeks post-operativelly.
Risk of bias assessment of the included stuides revealed that most of the studies had a high risk of bias, Fig. 2, illustrates risk of biass assement rsults of the included studies. The most common source of bias was allocation concelment. Chatwiriyacharoen et al study showed the highest rsik of bias comared to other studies , in some studies the risk of bias wasn’t clear to be assessed[26, 28].
Regarding surgical prophylaxis, in six studies the patients were given the antibiotics pre and post operatively [15–17, 23, 24, 26], whereas three studies reported giving antibiotics preoperatively only [7, 25, 27]. In one study the antibiotics were given postoperatively only  ,and in two studies there was no mention of antibiotics administration [1, 28].Types of antibiotics administrated were not specified in most of the studies, however five studies mentioned the types, which are demonstrated in Table 2.The duration of antibiotics administration after the operation was specified only in three studies, with Chatwiriyacharoen et al stating a duration of 5–10 days postoperative , while in Duttaroy et al study antibiotics where administered for 7 days postoperative , and Patients in third study where given antibiotics till discharge from the hospital .
Interventions done on the DPC patients varied across the studies, both in the context of type and frequency. In four studies [5, 15, 23, 24], the wound was left open and dressed with saline-soaked gauze, whereas Chatwiriyacharoen et al and Chiang et al used betadine soaked-gauze for wound dressing[17, 27]. In one study , the surgeons inserted the sutures to the wound but the edges were left open, and dry absorbent dressing was used. Other three studies[7, 16, 28] did not specify the type of dressing nor the specific intervention. In yet another two studies[1, 25], the wounds were left open and packed with saline soaked gauze without dressing. Regarding the frequency of dressing, the daily change of dressings was the method used in six studies [15–17, 23, 27, 28]. In one study, the dressings were changed twice daily, where as in another study , the wounds were dressed only once for the 48 hrs. after surgery. There is no data available regarding the frequency in the remaining four studies.
Regarding Differences in the definition of SSI among the studies, two studies defined SSI as the presence of gross purulent discharge with or without positive bacterial culture [23, 27]. Another three studies said that any purulent discharge from the wound requiring drainage or repeated dressings is considered as SSI [17, 25, 26], but one of them also added having surrounding cellulitis as an alternative criterion for the definition of SSI., in other four studies they have used the CDC criteria of diagnosing SSI’s in their trials, the CDC criteria can be concluded by saying “the presence foul-smelling purulent discharge from the incision with or without laboratory confirmation and with or without local inflammatory or systemic signs (fever, tachycardia)” [1, 5, 7, 24]. The remaining three studies didn’t mention a definition of SSI [15, 16, 28].
The Time of the first evaluation of DPC had some variation between the studies. In one study they started to evaluate the wound immediately the first day after the surgery . Another study started on day 2 postoperatively. three studies started to evaluate the wound on the 3rd postoperative day[1, 25, 28]. two studies started their wound evaluation on day 3 to the 5th and day 3 to the 7th-day, respectively [16, 24]. In another study, they started on day 4, and in another, they started on day 5 respectively [23, 27]. three studies didn’t mention when they have started the first wound evaluation [15, 17, 26].
The most commonly isolated bacterial pathogens are E. coli, pseudomonas aeruginosa, Bacteroides, klebsiella, staphylococci and streptococci. In 4 studies [5, 24, 26, 27], E. coli represented approximately 50% of the isolates, and it accounted for 72.8% of the isolates in another study . additionally, Pettigrew et al reported that Bacteroides fragilis accounts for 46% of the isolates from SSI. Hospital charges and treatment costs were mentioned by two studies, Cohn et al study reported no significant difference between the two groups, among DPC patients they were $22,258 [range $10,001–$47,927] and $26,352 [range $5,127–$45,822] among PC patients, while Siribumrunwong et al study stated that the costs of treatment were significantly higher among DPC with a median of with a median difference of -2083 Baht ( approximately 60 US$)[1, 24].
Surgical site infections
12 studies compared surgical site infections between primary closure group and delayed primary closure group and were included in the analysis [1, 5, 27, 28, 7, 15–17, 23–26]. Using a fixed-effects model the results favored the DPC group with a risk ratio of 0.56 [95% CI:0.44, 0.72], as the risk of SSIs between PC and DPC groups, and the difference between the groups was statistically significant (p value < 0.001) Fig. 3. However, the heterogeneity among the studies was high (Heterogeneity: Chi² = 47.66, df = 11 (P < 0.00001); I² = 77%), to identify the potential sources of heterogeneity different factors were explored including the age groups of the included patients, usage of surgical prophylaxis, and studies that has high weight, though none of these factors were recognized as a source of heterogeneity. Because of the high heterogeneity, random-effects model was used, yielding a risk ratio of 0.59 [95% CI:0.32, 1.08] favoring DPC group, however the difference between the group was not significant (p value = 0.09) Fig. 4.
Subgroup analysis was conducted for appendectomy surgeries, in which 7 studies were included, the risk ratios favoring delayed primary closure group were 0.70 [95% CI:0.51, 0.95], and 0.75 [95% CI:0.32, 1.78] using fixed and random effects models, respectively. This difference was significant when using a fixed-effects model (p value = 0.02), however it wasn’t when random-effects was used (p value = 0.52) Figs. 5, 6.
Length of hospital stay
Regarding the length of hospital stay (LOS), 7 studies have compared it between primary closure group and delayed primary closure were included in the analysis[1, 16, 23–25, 27, 28]. pooling of data using a fixed-effects model revealed that the length of hospital stay was slightly lower in the primary closure group with a mean difference of 0.25 [95% CI:0.02, 0.48] days from delayed primary closure group, additionally, this difference between the two groups was statistically significant (p value = 0.03) Fig. 7. pooling the data from the 7 studies demonstrated high heterogeneity among the studies (Heterogeneity: Chi² = 184.95, df = 6 (P < 0.00001); I² = 97%), the source of heterogeneity was not identified, so a random-effects model was used in which the length of hospital stay was also lower in primary closure group compared to delayed primary closure group with a mean difference of 0.81 [95% CI:-0.76, 2.38] days, however this time the difference was not statistically significant ( p value = 0.31) Fig. 8.