Meta-analysis
Literature search and study selection. We performed a PubMed,
EMBASE, and SCOPUS search to identify eligible articles. A forward
search of the retrieved articles was performed and ‘google scholar’ was
also assessed to screen for non-indexed publications. The last search
was performed on December 12th, 2019. The search terms
included: “ANCA vasculitis” OR “ANCA-associated vasculitis” OR
“antineutrophil cytoplasmic antibody vasculitis” OR “AAV” AND
complement. Publications were screened first by title, second by
abstract, and finally by full-text, based on our eligibility criteria.
Inclusion and exclusion criteria. We included cross-sectional or
longitudinal studies, which compared plasma levels of complement factors
in patients with AAV and healthy controls. We excluded studies that have
measured complement levels in the urine, other body fluids or biopsy
specimens. The exclusion criteria also included review articles, case
reports, and animal experiments.
Data extraction and outcome. Data extraction was carried out as
recommended by the Cochrane handbook, and included authors, year of
publication, study design, participants, demographic characteristics,
and measurement of serum complements.
Quality assessment. This meta-analysis was conducted and reported
according to the PRISMA (Preferred Reporting Items for Systematic
Reviews and Meta-Analysis) statement (Supplementary table S1). Risk of
bias of individual studies at the outcome level was assessed by using
the Newcastle-Ottawa scale. The scoring was performed independently by
two researches (Lee JM and Shin JI) (Supplementary table S2).
Statistical analysis and evaluation of heterogeneity and
publication bias. In the meta-analysis, the standard mean difference
(SMD) method and corresponding 95% confidence intervals (CIs) were used
to compare complement levels (mean±SD). Random effect models were used
because of heterogeneity of the included studies. We assessed the
heterogeneity of the studies by the Cochran Q test, and P -value
of <0.1 was considered significant. The inconsistency across
the studies was also measured by I2 metric, as a
measure of the percentage of total variation across the studies because
of the heterogeneity. I2 values of <25,
25-75 and >75% considered to represent low, moderate, and
high levels of heterogeneity, respectively. Publication bias of each
article was estimated by inspecting funnel plot. All analyses were
conducted using RevMan 5.3.