Risk of bias
The assessment of study quality by QUADAS-2 is presented in Figure 2 and Supplementary Figures S2-5. Overall methodological quality was mixed, with 5 studies presenting a low risk of bias across all domains32-36 and 64 presenting a high risk of bias or applicability in at least one domain.
In patient selection, 22 studies presented a low risk of bias, with 73 and 30 presenting an unclear or high risk respectively. Non-consecutive or non-random selection, two-gate selection for cases and controls, and having a highly selected group of participants (infertility cohort, surgery for a narrow indication etc.) were the main reasons for a high risk of bias.
Symptom based index tests presented an unclear or high risk of bias due to a lack of definition of a positive test and of blinding. Just 9 studies presented a low risk in symptom-based tests across all groups. Index tests applicable to clinical history or investigations performed better, with 66 studies presenting a low risk. Reasons for an unclear or high risk of bias were a lack of pre-specified criteria for a positive test; no blinding to results of the reference standard; and inter-observer variability regarding imaging. Studies in the partial verification group assessed a proportionally higher number of index tests nulligravidity and BMI ≥30kg/m2, which, less subjective to interpretation presented a lower risk of bias.
The risk of bias regarding the reference standard performed best in the complete and partial verification groups where 74 studies were at low risk of bias. Those with an unclear or high risk lacked information on how likely the surgery was to correctly classify the target condition or operators not blinded to the result of index test(s). In the database/self-reporting group, 5 studies assessed for probable surgical confirmation by means of additional codes at the time of recording and therefore presented a lower risk, all other studies were high risk.
In flow and timing, the complete verification group presented the lowest risk. An unclear or high risk of bias was attributable to a long (>12 months) or unclear time interval, and a high or unclear withdrawal of participants from analysis. All other studies presented a high risk as not all participants received the same reference standard.