Department of Reproductive Health and Research, World Health Organization, Geneva, Switzerland
SirsWe read with interest the article by Dickersin on systematic reviews and meta-analysis of observational studies.1 She notes obstacles for conducting systematic reviews of observational studies as compared to those of intervention studies (randomized trials) of medical and non-medical forms of care. She also suggests steps to improve their methodological quality.
The World Health Organization (WHO) has been fully engaged in systematic reviews in the area of reproductive health for several years. It has now become necessary to expand this activity beyond the issues of effectiveness of interventions. For example, considerable information describing the incidence and prevalence of eclampsia, haemorrhage, abortion and other morbidities, and mortality is continuously generated. However, these efforts are not efficiently co-ordinated; data are of uneven quality, not synthetized, and overall results are seldom taken into consideration for service planning. We think that global effort similar to that made for randomized clinical trials (e.g. The Cochrane Collaboration) is needed to map the magnitude and distribution of reproductive morbidity and mortality, especially in developing countries.
With this concept in mind, the Department of Reproductive Health and Research of WHO is conducting a systematic review of maternal and perinatal morbidity and mortality covering published and unpublished studies from 1997 to 2002. The main objective of the review is to provide a comprehensive tabulation of available data on the prevalence/incidence of maternal morbidity and mortality globally. Through this systematic review we also aim to estimate case-fatality rates for maternal conditions, and calculate the proportion of maternal deaths that could be averted (population-attributable fraction) by eliminating or reducing the prevalence of selected morbidities.
As of October 2002, the protocol for the systematic review, including a generic form for data-extraction and a critical appraisal strategy have been developed and externally peer-reviewed, following the same procedures for any other research protocol supported by WHO. Internal and external methodological consultations with experts on analytical strategies have also taken place. Data abstraction has been completed for 1997 and over 1000 data entries were recorded. We expect that during 2003 over 5000 entries will be included in the WHO database.
Although efforts to develop methodologies for searching, critically appraising, and analysing data from observational studies exist, these focus largely on effects of health care interventions that are difficult or impossible to evaluate through randomized controlled trials (Cochrane methods groups). Conducting a systematic review of prevalence/incidence studies poses new challenges: developing a search strategy that has a satisfactory level of sensitivity and specificity is difficult; relevant studies are dispersed widely in many bibliographic databases, Internet sites, and grey literature. Other methodological difficulties include dealing with population- and institution-based data, and the lack of a standardized set of definitions of maternal conditions and diagnosis methods which affect results.24 Heterogeneity due to study design could be larger than real measurable changes over time or by regions, and may make pooling of results impossible.
The WHO systematic review will also assist in identifying a standard set of definitions for maternal morbidities, test a critical appraisal instrument for future reviews, and set a global database to guide epidemiological research. Eventually routine vital registration systems should be adopted in all developing countries; previous experiences in many countries in Latin America demonstrate that this is feasible. These efforts should result in better and sounder grounds for capturing, quantifying, and tackling the burden of reproductive ill-health.
To our knowledge, this is the first time that such global effort based on systematic review principles is being undertaken. Tackling this challenge requires active international collaboration within the epidemiological community. Support from researchers in the field, university departments, governments, and non-governmental institutions could help ensure comprehensiveness. We invite all researchers to participate in this effort by providing WHO with any results that are not easily available by standard searching techniques so that they could be included in this systematic review. Appropriate credit to the sources will always be given. In her article Dr Dickersin also encourages the development of registers of observational studies, including a description of the variables examined. We agree that a register of large multicentre observational studies on maternal and perinatal mortality and morbidity should be started and offer the Department of Reproductive Health and Research at WHO as a secretariat for such a register.
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2 Ronsmans C, Campbell O, Collumbien M. Effect of supplementation with vitamin A or beta carotene on mortality related to pregnancy. Slight modifications in definitions could alter interpretation of results. BMJ 1999;319:120203.
3 North RA, Taylor RS, Schellenberg JC. Evaluation of a definition of pre-eclampsia. Br J Obstet Gynaecol 1999;106:76773.[ISI][Medline]
4 Graafmans WC, Richardus JH, Macfarlane A, Rebagliato M, Blondel B, Verloove-Vanhorick SP. Comparability of published perinatal mortality rates in Western Europe: the quantitative impact of differences in gestational age and birthweight criteria. Br J Obstet Gynaecol 2001; 108:123745.[CrossRef][ISI]