We conducted a thorough search in the PubMed, Embase, and Web of Science databases for all relevant articles until March 1st, 2023. The key words included “arteriovenous fistula”, “vein” and “diameter”. After removing duplicated articles, additional items were sought by manual review the reference of all articles. We initially screened the articles mainly by the title and abstract, then we read the full text of all potentially eligible articles carefully. The inclusion criteria were listed as follows: (1) articles were written in English; (2) study design was randomized controlled trial (RCTs) or observational study; (3) the AVF was constructed for hemodialysis in renal failure patients; (4) the AVF was constructed by autogenous veins;(5) the patients with different vein diameter were compared on outcome indicators such as functional maturation rate or postoperative patency rates; (6) the threshold for patients grouping was 2 mm on vein diameter. The exclusion criteria included: (1) no independent record of functional maturation in patients with different vein diameter; (2) difficult to make a distinction between AVF and AVG. The whole review process was accomplished by two authors independently and the controversies were discussed strictly to achieve consensus finally. The study was designed and reported in accordance with Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) Statements (8) and guidelines and recommendations of Meta-analysis of Observational Studies in Epidemiology (MOOSE) (9) respectively. Patients or the public were not involved in the design, or conduct, or reporting, or dissemination plans of our research.

A predefined data extraction sheet was used to record all study characteristics from all included articles: title, first author, year of publication, country, study design and number of patients in different groups. Several baseline characteristics and procedure outcomes including age, gender, previous history, follow-up time, surgical procedure, vein diameter, tourniquet using, functional maturation rate, primary patency rate, cumulative patency rate and so on were also recorded for further comparison. The data was recorded by two authors with repeated confirmation. If the relevant indicators above were not clearly illustrated or not mentioned at all in the article, we contacted the first and corresponding authors to ensure the integrity and accuracy of the data. According to the KDOQI clinical practice guideline for vascular access: 2019 update, the functional maturation was defined as a fistula became suitable for providing prescribed dialysis consistently with 2 needles. The duration of time from fistula placement to thrombosis or any intervention to facilitate, maintain, or re-establish patency (e.g., angioplasty) was defined as the primary patency, while the duration of time from fistula placement to access abandonment was defined as cumulative patency (same as secondary patency).

We used the Newcastle–Ottawa scale (NOS) (10) for cohort study to evaluate the methodological quality of the included articles. The total NOS score ranged from 0 to 9, which was divided into three parts—participants selection, comparability and outcome. Two authors independently assessed the articles and mean scores were calculated. Cohort studies were graded as follows: 0–3 = low, 4–6 = moderate, and 7–9 = high (11). In addition, the certainty of evidence was assessed in accordance with Grading of Recommendations Assessment, Development, and Evaluation (GRADE) system (12) on GRADEpro website (https://www.gradepro.org/).

Categorical variables were presented by dividing the number of events by the number of cases. The bilateral χ² test or Fisher's exact test was used for comparison. The continuous variables were shown as the mean ± standard deviation (SD) or median and range (quartile) and the t-test or Mann–Whitney U-test was used. Pooled odds ratio (OR) and 95% confidence intervals (CIs) were showed in forest plot. The heterogeneity was calculated by I², which represented the proportion of the difference caused by non-sampling error in the total heterogeneity (13). I²< 50% indicated low heterogeneity and the fixed effect model was the best choice while I²≥ 50% indicated high heterogeneity and the random effect model was more appropriate (14). To clarify the potential source of heterogeneity, we performed sensitivity analysis by leave one-out approach to find studies design bias and confounding factors. The publication bias was evaluated by Egger's test and funnel plot. Subgroup analysis was conducted by gender, tourniquet using and vein diameter distribution based on fix effect model (15). A P-value < 0.05 was considered statistically significant. All statistical analysis was conducted in R studio (version 4.2.0; https://www.r-project.org) with “meta” and “metafor” R packages.

The article screening process is presented in Figure 1. Two thousand two hundred forty-six articles were searched firstly (644 from PubMed, 762 from Embase and 840 from Web of science) and 1,320 articles progressed to further review after removing duplicated records. 384 articles were preliminarily included after title and abstract screening. Two hundred ninety-six articles were excluded for lacking comparison on functional maturation rates between different vein diameters, 67 for involving synthetic graft, 8 for unexpected diameter threshold and 5 for non-English articles. Finally, 8 articles were included in this systematic review and meta-analysis (16–23). The basic characteristic of all included articles was showed in Table 1. All articles were cohort studies. Three of them were prospective study (16, 17, 19) while others were designed retrospectively. Among a total of 1,075 patients (675 males and 400 females), 227 and 809 patients had available ultrasound data and possessed <2 mm and ≥2 mm vein respectively, while others did not provide available ultrasound data. Among these, most of cases were constructed with radio-cephalic AVF except 11 cases with brachial-cephalic AVF and 5 cases with brachial-basilic AVF. Based on NOS evaluation, 6 of 8 included articles were assessed as high quality and 4 of them reached the maximum score of 9 (18, 20–23) (Supplementary Table S1).

To explore the influencing factors of AVF maturation, we compared some baseline indicators firstly (Table 2). Apart from gender and coronary artery disease, there was no significant difference in age, diabetes, hypertension or radial artery diameter between maturation and non-maturation groups. The functional maturation rate of AVF was higher in males and lower in coronary artery disease patients (P < 0.05). The functional maturation rates for patients with different vein diameter were demonstrated in Table 3. The forest plot indicated that the functional maturation rate was lower in patients with <2 mm vein according to fixed effect model [OR = 0.19, 95% CI (0.12, 0.30), P < 0.01] (Figure 2). There was no significant difference in one-year primary [OR = 0.63, 95% CI (0.12, 3.25), P = 0.58] or one-year cumulative patency rates [OR = 0.40, 95% CI (0.13, 1.19), P = 0.10] (Figure 3).

As for the subgroup analysis, 5 articles evaluated the vein diameter with tourniquet by ultrasound and 7 articles enrolled more males. Considering the difference in the vein diameter distribution in each study, we defined the proportion of patients with vein diameter <2 mm less than 30% or more than 70% as unbalanced distribution. There was no significant difference in functional maturation rates in between articles with different tourniquet using, gender ratio and vein diameter distribution (Supplementary Table S2 and Figure S1). The funnel plot (Supplementary Figure S2) showed there was no publication bias among 8 included studies, which was further confirmed by Egger's test (P = 0.22). According to GRADE system, the certainty of evidence was evaluated moderate to high certainty (Supplementary Figure S3). Although the included articles all belonged to observational study, the quality of evidence was upgraded due to large effect and assessing plausible confounding factors.