Objectives To critically appraise the quality of the studies underpinning the GBD 2017 estimates for MDD with respect to i) the GBD 2017 inclusion criteria and ii) population coverage.
Methods Of 431 studies underpinning the GBD 2017 estimates, 400 were retrieved. Country-level samples used in multi-country studies were disaggregated to give 467 country-level studies. Each study was critically appraised with respect to the four GBD 2017 inclusion criteria: representativeness, study method and sample, diagnostic criteria and publication from 1980 onwards. Population coverage was calculated by country, by region and in total.
Results Only 262/467 (56.10%) of studies reported specifically on MDD and more than a third did not use DSM or ICD criteria: 94/467 (20.13%) did not specify any diagnostic criteria and 68/467 (14.56%) relied on self-reported depression for diagnosis. Overall, 107/195 (54.87%) of countries were covered by at least one prevalence study. Population coverage varied by region from only 6.28/100,000 in the SEARO region compared with 7617.09/100,000 in the PAHO region. Regional estimates of coverage were distorted by overrepresentation of some countries, for example respondents from the USA accounted for 90.61% of PAHO region respondents despite the USA only making up approximately a third of the total PAHO population and similarly, Vietnamese respondents accounted for 73.60% of WPRO region respondents although Vietnam only makes up 5% of the total WPRO population. Less than half of studies (221/467, 47.32%) were nationally representative. The majority of studies (400/467; 85.63%) provided sufficient information to assess the quality of the study; 10 did not report on the age group and 57 did not report response rates. Only 62/467 (13.28%) of studies were from 2011-2017.
Conclusions Studies that do not use diagnostic criteria or report specifically on MDD were included in the GBD 2017 estimates for MDD. Self-reported estimates of depression are known to inflate estimates and the lack of specific data of on MDD may have led to the inclusion of cases that would not meet diagnostic criteria for MDD. The estimates were also based on incomplete country and population coverage and there was a lack of nationally representative studies. Given these critical flaws in the data, underpinning the GBD 2017 estimates they are not uniformly reliable and, as such, may have only limited value in international policymaking. At country level policymakers should interpret the estimates with caution, as reliance on poor quality data will lead to the misallocation of resources in prioritising disease conditions and health services.
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