Monitoring Global Progress in Core Diabetes Control Metrics: Protocol for a Systematic Review of Prevalence (2015–2023)

Background

Diabetes is a major chronic disease that continues to be a leading public health concern. For 2021, the estimated global prevalence of people living with diabetes was 529 million; with Type 2 diabetes (T2D) accounting for 96% of all cases (GBD 2021 Diabetes Collaborators, 2023). Furthermore, over 1.3 billion people are projected to be living with diabetes by 2050, with low- and middle-income countries (LMICs) being disproportionately impacted by the burden of the disease (GBD 2021 Diabetes Collaborators, 2023). Uncontrolled diabetes is associated with an increased risk of adverse health outcomes, including acute and chronic complications, hospitalisation, and death (Elafros et al., 2023; Feleke et al., 2024; Khunti et al., 2023; Lazzarini et al., 2023; Tomic et al., 2022; Yaow et al., 2023). In addition to the long-term health impacts, the economic burden associated with diabetes is high. For 2022, the estimated cost of diagnosed diabetes was $412.9 billion in the United States (US), with 25% of healthcare expenditure incurred by those living with diabetes and 15% directly attributable to diabetes (Parker et al., 2024).

People with diabetes are at risk of microvascular and macrovascular complications, with the latter including cardiovascular diseases (Chait et al., 2023; Karimi et al., 2024; Khunti et al., 2023; Tomic et al., 2022). These deleterious outcomes can be prevented or mitigated by a combination of healthy lifestyle choices and the effective monitoring and management of glycated haemoglobin (HbA1c), blood pressure, low-density lipoprotein cholesterol (LDL-C) and non-high-density lipoprotein cholesterol levels (non-HDL-C). The attainment of recommended diabetes control goals is likely to extend the life expectancy of people living with diabetes; however, the achievement of these targets globally remains suboptimal (Kianmehr et al., 2022). For 2015–2018, only 22% of adults with diabetes living in the US achieved comprehensive HbA1c (<8% or <7%), blood pressure (<140/90 mm Hg or <130/80 mm Hg) and non-HDL-C control (<130 mg/dL) (American Diabetes Association Professional Practice Committee, 2024). The attainment of optimal LDL-C control in people with diabetes is as crucial as achieving recommended glycaemic and blood pressure goals (Poli et al., 2023). Statin therapy is the recommended evidence-based approach for LDL-C management in those aged 40–75 years living with diabetes to reduce the risk of developing atherosclerotic cardiovascular disease; yet, the implementation and translation of this recommendation into clinical practice has been suboptimal and inequitable (Decicco et al., 2023; Eastwood et al., 2021; Marcus et al., 2022; Thompson-Paul et al., 2023; Yun et al., 2022; Zhang et al., 2024). Of people with diabetes who participated in the 2015–2018 cohort of the US National Health and Nutrition Examination Survey (NHANES), only 53% were on a statin, suggesting underuse of this therapy (Leino et al., 2020).

In 2013, the World Health Organization (WHO) adopted the Non-communicable Diseases (NCD) Global Monitoring Framework, which was followed by an international commitment to monitor the progress of NCD prevention and control at a 2014 High-level Meeting of the United Nations (UN) General Assembly (WHO, 2014; WHO, 2015). To address the growing global epidemic of diabetes, the WHO launched the Global Diabetes Compact in April 2021 (Hunt et al., 2021), with the goal of reducing the burden of diabetes and improving health outcomes for people living with diabetes. Metrics were considered across four domains: factors at a structural, system, or policy level; processes of care; behaviours and biomarkers; and health events and outcomes; and three risk tiers (diagnosed diabetes, high risk, or whole population). Metrics were reviewed and prioritised according to their health importance, modifiability, data availability, and global inequality; published reports of national-level prevalence of each metric were assembled to evaluate global variation and set appropriate goals. This process led to five core national metrics and target levels for UN member states by 2030: (1) 80% of people living with diabetes are clinically diagnosed; (2) 80% of those diagnosed achieve glycaemic control; (3) 80% of those diagnosed achieve blood pressure control; (4) 60% of those aged ≥40 years receive statin therapy; and (5) 100% of those with Type 1 diabetes (T1D) have access to insulin, blood glucose meters and test strips (Gregg et al., 2023).

A substantial amount of the epidemiological information on diabetes control is derived from data in high-income countries. The performance of healthcare systems in LMICs is difficult to evaluate, as nationally representative data is often unavailable (Rahim et al., 2023). A recent study of 55 LMICs found that the combined pharmacological treatment coverage for diabetes was 14.3%, with further differences across regions and subgroups. This study concluded that controlling glucose levels and risk factors for cardiovascular disease, such as hypertension and high cholesterol are global priorities for diabetes management (Flood et al., 2021). Previous reviews of diabetes risk factor control prevalence suggest that strategies to improve risk factor management for people living with diabetes are needed. A review of studies from sub-Saharan Africa reported that the pooled prevalence of patients with T2D categorised as achieving good glycaemic control was 30% (Fina Lubaki et al., 2022). Another review from Africa reported pooled prevalence estimates of 27%, 38% and 42% of optimal glycaemic, blood pressure and LDL-C control, respectively, in patients with T2D (Kibirige et al., 2022). Although these reviews are informative, and highlight the challenge of diabetes control internationally, they are region-specific, spanning 2000–2022, and are restricted to adult patients with T2D. Epidemiological information on patients living with T1D is often limited (Ogle et al., 2023). It is necessary to determine the current status of diabetes control target achievement on a global level, and to identify attainment gaps, to inform diabetes surveillance, prevention, and treatment.

Aims and objectives

The aim of this systematic review is to determine the status of core diabetes control metrics, and to describe regional and country-level variation in attainment gaps for these metrics. The research questions guiding this systematic review are:

Protocol registration

The review protocol follows the JBI guidance for systematic reviews of prevalence and incidence (Munn et al., 2020), and will be reported in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines. The review protocol was structured using the PRISMA-Protocols checklist (PRISMA-P; Moher et al., 2015). The search strategy will be reported using the PRISMA literature search extension (PRISMA-S; Rethlefsen et al., 2021), and the completed review will be reported according to the PRISMA 2020 Statement (Page et al., 2021). The review protocol is registered on the International Prospective Register of Systematic Reviews (PROSPERO; CRD42024505286). All materials relating to this review will be available on the Open Science Framework (OSF; https://doi.org/10.17605/OSF.IO/DZYJK).

Eligibility criteria

The eligibility criteria are informed by the CoCoPop (i.e., Condition, Context, Population) framework (Munn et al., 2015; Munn et al., 2020), as presented in Table 1. We will include population-based observational studies conducted in all geographic settings with adult populations (aged ≥18 years) with diagnosed T1D or T2D. Studies will only be included if reporting a sample size of ≥150 people with diagnosed diabetes, and if the population samples are representative at the national, subnational (i.e., covering ≥1 subnational regions, >3 urban communities or >5 rural communities) or community level (i.e., up to 3 urban communities or up to 5 rural communities). The study population inclusion criteria were adapted from NCD Risk Factor Collaboration methodology (NCD-RisC, 2023). Only studies published between 2015 and 2023 with a data collection period that began in 2015 will be included. These dates were chosen in order to capture the status of diabetes control since the international commitment to monitoring progress in NCD prevention and control using the WHO NCD Global Monitoring Framework (WHO, 2014; WHO, 2015). There will be no language restrictions.

Outcomes

The primary outcomes are levels of:

Information sources

A search strategy was developed in consultation with an information specialist (KW) for the selected databases (restricted from January 2015 to January 2024): Ovid MEDLINE, Embase, Scopus, Cochrane Library, and WHO Global Index Medicus. The search strategy for Embase is presented in Table 2. The full search strategy for each database is available on OSF (https://doi.org/10.17605/OSF.IO/DZYJK; see Supplementary File 1).

Data management and study selection

Study records will be imported into EndNote and uploaded to Covidence to manage screening and data extraction. Two reviewers will independently assess retrieved records against eligibility criteria. Titles and abstracts will be screened for relevance, followed by full-text review to examine if inclusion criteria are met. Disagreement among reviewers will be resolved by consulting a third reviewer to reach consensus. The reference lists of included publications will be checked for other potentially eligible studies. A PRISMA flow diagram will be generated to map out the number of records identified, included and excluded, and the reasons for exclusion in the review.

Data extraction and synthesis

Two reviewers will use a piloted, standardised data extraction form, developed in Covidence, to independently extract study population, methodological, and outcome data (see Table 3 for draft template). Disagreement among reviewers will be resolved by a third reviewer.

Data will be synthesised using descriptive statistics, such as proportions and counts for categorical variables, and means, standard deviations and ranges for continuous data. Tables and graphs will be used to present descriptive data on study characteristics, participant characteristics, prevalence estimates, and quality assessment ratings. Depending on data availability and structure, potential differences in attainment of diabetes control targets will be explored between regions and countries. Where possible, analyses will be stratified for diabetes type, gender, or other population characteristics. The potential for equity gaps in diabetes control will be explored in the data, and summarised narratively.

If possible, a meta-analysis will be conducted, using a random-effects model. This will require consideration of the various methodological challenges that exist in relation to conducting and reporting systematic reviews of prevalence (Borges Migliavaca et al., 2020; Hoffmann et al., 2020; Mueller et al., 2018). Heterogeneity is a significant issue when undertaking a meta-analysis of prevalence data, which is often addressed using the I ² statistic. Accordingly, the I ² statistic will be presented; however, its limitations will require careful interpretation, and reporting will be supplemented by prediction interval estimation and sensitivity analyses per recommendations (Migliavaca et al., 2022). If a meta-analysis is not feasible, then reporting will follow the Synthesis Without Meta-analysis guideline (SWiM; Campbell et al., 2020).

Quality assessment

Two reviewers will independently conduct a quality assessment of included studies, using the JBI Critical Appraisal tool for studies reporting prevalence data (Munn et al., 2015; Munn et al., 2020). The critical appraisal checklist consists of nine questions (addressing study design, conduct and data analysis), and includes three possible ratings for assessing methodological quality (i.e., yes, no, unclear). Disagreement will be resolved by consulting a third reviewer.