Studies describing patients with SVT as the patient population and DVT and/or PE as the outcome and reporting on predictive factors were selected for this review. The PICOTS of this study is described in box 1. The inclusion criteria were studies (1) including patients with SVT based on clinical and/or ultrasonography diagnosis, (2) selected in primary and secondary care settings and (3) reporting on the outcome DVT and/or PE. Both cross-sectional—that is, assessing predictors for the outcome DVT and/or PE at baseline (ie, concomitant DVT/PE) and prognostic—that is, assessing the outcome DVT and/or PE at a follow-up time point after diagnosis of isolated SVT—studies were included. The exclusion criteria were (1) studies only describing therapeutic predictors, (2) study designs other than original research studies such as reviews, editorials and commentaries and (3) studies not written in the English language. PubMed/MEDLINE and Embase were systematically searched until 3 March 2023. Conference abstracts were omitted from the search. To identify predictive factor studies specifically, the Haynes broad filter for prognostic factor studies and its update were added to a general SVT search.14 Together with a medical librarian trained in systematic review, the final search string was designed and is presented in online supplemental table 1. After removal of the duplicates, the studies meeting all the inclusion criteria and none of the exclusion criteria were independently selected based on title and abstract by two investigators (FS-AvR and G-JG). Cases of doubt were discussed until consensus was reached, if needed, a third investigator (SvD) was consulted for consensus. If deemed eligible, the study underwent full-text screening before final inclusion.

P: patients with superficial venous thrombosis (SVT)

I and C: all potential predictive factors

O: deep venous thrombosis and/or pulmonary embolism

T (timing): predictive factors measured at diagnosis of SVT. Outcome assessed at diagnosis (cross-sectional) or at follow-up (prognostic)

S (setting): both hospital and primary care