The characteristics and specific process of study inclusion and exclusion are detailed in Fig. 1 Initially, a total of 223 relevant studies were identified from the aforementioned four databases. Following automated duplication removal of 37 studies and manual elimination of 38 studies, 6 studies (comprising meta-analyses, reviews, guidelines, and conference abstracts) were excluded. Subsequently, the full texts of the remaining 142 studies were assessed for credibility, leading to the exclusion of 115 articles based on predetermined criteria. These criteria included study subjects not being CRC patients, absence of pertinent data on SII and prognostic indicators, studies involving cancers metastatic to the intestine from other sites, insignificant findings, illogical cohort study designs, and unavailability of full-text data. Ultimately, 27 articles were deemed suitable and included.

Identification of studies via databases and registers

Among the included studies, 19 studies were conducted in China [12, 18, 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37], 4 in Japan [18, 38,39,40], 3 in Italy [28, 41, 42], and 1 in the USA [43]. Of these, 18 studies included patients with primary CRC [12, 18, 21, 22, 26,27,28, 30,31,32,33,34,35,36,37,38,39, 44], while 9 studies focused on recurrent CRC [23,24,25, 29, 40,41,42,43, 45]. Among the studies, 22 analyzed the relationship between SII and OS [12, 18, 22,23,24,25,26,27,28,29,30, 32,33,34, 36, 37, 39,40,41,42,43,44,45], 11 analyzed the relationship between SII and PFS [12, 23, 25, 34,35,36,37, 41,42,43, 45], 8 analyzed the relationship between SII and DFS [21, 22, 26, 27, 30, 33, 42, 44], and 3 analyzed the relationship between SII and RFS [18, 29, 38]. The sample sizes of the 27 studies ranged from 41 to 1383 individuals, with a median age range of 45–68 years. Among the 27 studies, 26 were retrospective cohort studies [12, 18, 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40, 42,43,44,45], and 1 was a prospective cohort study [41]. The main characteristics of the 27 studies included in our study are presented in Table 1. The NOS scores of the 27 studies ranged from 6 to 9, with scores exceeding 7 indicative of high quality. Reasons for lower quality included inadequate and imprecise surgical records, non-independent or non-blinded outcome assessment, inadequate follow-up duration, incomplete follow-up, and absence of analysis concerning lost follow-up subjects.

The prognostic analysis of SII and OS was conducted on 8347 patients across 22 studies [12, 18, 22,23,24,25,26,27,28,29,30,