Strengths and limitations
We undertook a thorough search of the current literature, undergoing analysis by two independent reviewers with strict quality assessment. Attempts were made to mitigate inter-study heterogeneity by division of studies into groups. All index tests are relevant to primary care and immediately available without novel techniques or additional training.
There were, however, limitations. Due to difficulties in data extraction from some non-English journals, 15 studies were excluded from the analysis. Some studies, such as Chapron et al 2005, which was seminal in providing a clinical prediction model for moderate/severe endometriosis, were not able to be included due to the inability for construct 2x2 tables.37 We did not contact authors to obtain individual data that was not available in the published text.
Overall, there was significant methodological variance and population heterogeneity in age; presentation; and stage of disease. Variation in selection of cases and controls may not reflect a clinically representative population. Prevalence of disease was higher than seen in the general female population, which may reflect a high degree of surgical accuracy, but also indicates the selective nature of study populations.
There is the possibility of inappropriate assignment of cases and controls, occurring in both directions due to uneven application of the reference standard, although we attempted to account for this by assigning groups. We included studies that diagnosed endometriosis by visual inspection and there is debate regarding this in the absence of histological confirmation.38-40
There was variation in the definition of positive symptom index tests. This is common across many reviews and although there is guidance on symptom reporting, it was not clearly followed in all studies.41-43 Assessment of symptoms varied, with most studies using a self-administered questionnaire. Although the use of standardised validated tools would better allow for comparison across studies, the nuance and detail acquired through clinical history taking is likely to better grasp the nature and significance of a symptom and its implications.
It is likely that imaging and surgical techniques have developed over time. A trend towards recent studies may mitigate this.
Considering the balances of strengths and weaknesses, however, we believe that our data synthesis presents an objective summary of the current evidence.