In recent decades, diabetes mellitus has become a major health threat worldwide. The global prevalence of diabetes in those aged 20 to 79 years has reached pandemic proportions in 2019 and has increased from 9.3% in 2019 to 10.5% in 2021 [1]. In South Korea, the prevalence of diabetes increased from 10.3% in 2011 to 13.9% in 2020 [2]. However, approximately one-third of people with diabetes aged ≥ 30 years are unaware of diabetes, and diabetes awareness has decreased from 72.4% in 2007–2009 to 67.8% in 2016–2018. Approximately one million people are estimated to live without knowing that they have diabetes [3]. In addition, the prevalence of prediabetes, an early stage in the hyperglycemic continuum, was 26.9% in 2018 [4].

Prevention of diabetic complications is a key issue in maintaining the quality of life of people with diabetes. Diabetic complications result from an abnormal metabolic environment caused by chronic hyperglycemia [5]. The duration of diabetes and degree of hyperglycemia affect the risk of diabetic complications [6–8]. Early detection and management of prediabetes and diabetes can delay the development and progression of diabetes and diabetic complications. However, diabetes is often asymptomatic in its early stages and remains undiagnosed for many years, and diabetic complications can often occur before the clinical diagnosis of type 2 diabetes [9,10]. Thus, the early identification of prediabetes and diabetes is the first step in diabetes care.

In 2021, the Korean Diabetes Association (KDA) recommended annual prediabetes and diabetes screening for adults aged ≥ 40 years and adults aged ≥ 30 years with diabetes risk factors such as overweight or obesity [11]. However, the age thresholds for screening individuals were determined based on the guidelines of other countries, and the evidence for screening for Koreans was limited [12]. In addition, in South Korea, the incidence of diabetes among adults aged 20 to 39 years has increased [13]. Therefore, this study aimed to determine the age thresholds for diabetes screening to provide evidence for screening guidelines.

In the KNHANES, there were 24,822 adults aged ≥ 20 years without diagnosed diabetes, representing 35,151,173 Korean adults (Supplementary Table 1). In the NHIS-NSC, there were 417,768 adults aged ≥ 20 years without diagnosed diabetes.

When classified by age group at 5-year intervals, the NNS to detect diabetes decreased from 63 to 34 between the ages of 30–34 years and 35–39 years in the KNHANES. In the NHIS-NSC, the NNS decreased from 71 to 42 in the same age range, and the greatest declines were noted between the ages of 25–29 year and 30–34 years (from 143 to 71) (Fig. 1 and Supplementary Table 2). In detecting prediabetes and diabetes, the NNS was approximately 10 in those aged 20 to 24 years, which continued to decrease with age (Supplementary Table 3).

When universal screening was applied by age, in the KNHANES, the NNS slightly increased from 15 (rate of positive test = 6.8%) in adults aged ≥ 40 years (age threshold of the KDA recommendation for universal screening) to 20 (rate of positive test = 4.9%) in adults aged ≥ 20 years (Table 1). In the NHIS-NSC, the NNS increased from 26 to 33 (rate of positive test, 3.8% to 3.0%) in the same age groups (data not shown). When universal screening was applied to adults aged ≥ 35 years, the NNS was similar to that of adults aged ≥ 40 years (Table 1).

Half of the adults aged 20 to 34 years were overweight or obesity, and approximately 20% had abdominal obesity (Supplementary Table 1). In adults aged 20 to 34 years who were overweight or obesity, the rate of missed screening positive was the lowest at 10.6% compared to the other risk factors (obesity, 20.0%; abdominal obesity, 17.6%; family history of diabetes, 55.6%; hypertension, 67.5%; and dyslipidemia, 43.8%). However, NNS was the highest at 48 (Table 2). When considering the risk factors of obesity and/or abdominal obesity, the rate of missed screening positive increased from 10.6% to 17.6%; however, NNS decreased from 48 to 34 compared to overweight or obesity (Table 2). Supplementary Table 4 shows the diabetes screening effectiveness according to age groups from 20–24 years and up to 35–39 years for each diabetes risk factor.

Compared to the KDA guidelines, NNS was slightly increased when applying the newly suggested screening criteria (adults aged ≥ 35 years and adults aged ≥ 20 years with diabetes risk factors). However, the rate of missed screening positive in adults aged ≥ 20 years decreased substantially from 4.0% to 0.2% in the KNHANES (Table 1).

This study provides estimates of the NNS to detect diabetes in adults in South Korea. When NNS is estimated by 5-year age categories, the NNS noticeably begins to decrease after 35 years and continues to decline until 75 years and older. When universal screening was applied to adults aged ≥ 35 years, the NNS was similar to that of adults aged ≥ 40 years.

In 2021, the KDA recommended annual universal screening for the detection of prediabetes and diabetes in adults aged ≥ 40 years and adults aged ≥ 30 years with diabetes risk factors [11]. It was established by considering to the prevalence of diabetes by age in Korea and previous studies in other countries [12]. However, in the United States, the optimal age cut-point for opportunistic universal screening based on the NNS was suggested as 35 years old [20]. Recently, the U.S. Preventive Services Task Force recommended screening in adults aged 35 to 70 years who are overweight or obesity, which decreases the age from 40 to 35 years to consider the increase in diabetes incidence in young adults [21]. The American Diabetes Association also recommends universal screening for adults aged ≥ 35 years and adults of any age with overweight or obesity (BMI ≥ 25 or ≥ 23 kg/m² in Asian Americans) who have one or more risk factors, such as a history of CVD, hypertension, and dyslipidemia [22]. The Diabetes Canada Clinical Practice Guidelines Expert Committee suggested that the age to begin screening is 40 years, based on the Diabetes Screening in Canada Study [23,24]. Adults at high risk on the Canadian Diabetes Risk were also included for prediabetes and diabetes screening [24].

In South Korea, the incidence of diabetes has increased from 1.3 to 1.7 per 1,000 individuals in adults aged 20 to 39 years, with increasing obesity from 2006 to 2015 [13]. People diagnosed at 20 to 39 years of age had worse status of insulin resistance and β-cell function with severe obesity than those diagnosed at older ages [25]. In addition, the proportion of diabetes awareness was 76.4% in adults aged 65 years or older but decreased to 65.8% in adults aged 30 years or older [26]. The onset of diabetes at a younger age is associated with prolonged disease exposure and increased risk of diabetic complications [27]. Thus, active diabetes screening is required in young adults. However, screening individuals unlikely to benefit can result in unnecessary test burdens and costs for the healthcare system. In this study, the evidence for screening all adults aged 20 to 34 years is unclear. Compared to screening people aged 35 to 39 years, the NNS increased markedly from 34 to 63 in the KNHANES and from 42 to 71 in the NHIS-NSC for ages 30 to 34 years. In addition, when universal screening was changed from ≥ 35 to ≥ 20 years, the NNS increased from 16 to 20. However, excluding young adults from diabetes screening would have resulted in almost 100,000 people potentially being missed. Thus, we need to screen adults aged 20 to 34 years with diabetes risk factors. When adults were overweight or obesity based on the KDA guideline, the rate of missed screening positive was the lowest, but NNS was the highest among the diabetes risk factors. In addition, approximately 50% of adults aged 20 to 34 years must be screened. However, although the rate of missed screening positive increased, adults with obesity or abdominal obesity had a relatively low NNS and screening eligibility rate. Thus, diabetes screening may be more beneficial for adults aged 20 to 34 years with obesity and/or abdominal obesity than for those who are overweight or obesity.

Prediabetes may be an important stage for early intervention to prevent the development of diabetes and its complications. A lifestyle intervention based on a healthy diet or physical activity prevented the progression of prediabetes to diabetes [28]. The Diabetes Prevention Program trial showed that weight loss through lifestyle intervention reduced the risk of developing diabetes by 44% [29]. In addition, lifestyle interventions in prediabetes reduced the risk of diabetic complication and diabetes-related mortality [30,31]. In this study, the NNS to detect prediabetes or diabetes remained stable under 10 among adults of all ages. Thus, the early detection of prediabetes through the screening may be an effective strategy to delay the development of diabetes and its complications.

The benefits of diabetes screening in older adults remain controversial. In the United States, the proportion of deaths or regression to normoglycemia from prediabetes was higher than progression to diabetes in older adults aged > 70 years with prediabetes [32]. The U.S. Preventive Services Task Force does not recommend diabetes screening for adults aged > 70 years [21]. In South Korea, almost 20% of adults aged ≥ 65 years with prediabetes progress to diabetes. In addition, in adults aged ≥ 75 years diagnosed with prediabetes or diabetes, the risk of diabetic complications did not increase [33]. However, in the current study, we included all adults aged ≥ 20 years, including older adults, to evaluate the effectiveness of screening for diabetes. Thus, further studies on the upper age limit for diabetes screening are required.

In the current study, compared to when fasting plasma glucose used alone as the positive test definition, the rates of positive test result were higher when both fasting plasma glucose and HbA1c were used simultaneously. In general, fasting plasma glucose, 2-hour plasma glucose during a 75g oral glucose tolerance test (OGTT), or HbA1c are all needed for diagnosing prediabetes and diabetes. Among the tests, OGTT is the diagnostic standard for diabetes. However, it is difficult to use for screening purposes because of time-consuming and inconvenient to the patient. Although fasting plasma glucose and HbA1c test is a simple and time-saving test that can be performed instead of OGTT, the measurement of HbA1c is a more reliable than fasting plasma glucose. And, HbA1c is better associated with diabetic complications than fasting plasma glucose [34]. In addition, the combination of fasting plasma glucose and HbA1c may be a more sensitive and specific screening tool for early identification of high-risk individuals with early diabetes [35]. Thus, it is necessary to consider HbA1c as well as fasting plasma glucose for diabetes screening.

The current study had several limitations. First, diabetes is diagnosed based on the two or more abnormal test results using fasting plasma glucose, 2-hour plasma glucose during a 75 g OGTT, or HbA1c value [11]. However, our study considered only one positive screening test. Second, the NNS is easy to calculate and intuitively shows the effectiveness of screening for clinicians and patients. However, we were unable to consider the risk and cost of screening.

In conclusion, universal screening for adults aged ≥ 35 years and selective screening for adults aged 20 to 34 years, considering diabetes risk factors such as obesity and/or abdominal obesity, may be appropriate for detecting prediabetes and diabetes in South Korea.