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.