Prevalence and incidence of gout in Korea data from the national

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DOI 10.1007/s00296-017-3768-4 EPIDEMIOLOGY OF RMD

Prevalence and incidence of gout in Korea: data from the national health claims database 2007–2015

Ji‑Won Kim¹ · Sang Gyu Kwak² · Hwajeong Lee¹ · Seong‑Kyu Kim¹ · Jung‑Yoon Choe¹ · Sung‑Hoon Park¹

Received: 20 February 2017 / Accepted: 29 June 2017

2025 were 16.59 (15.85–17.34) per 1000 persons and 3.81 (3.14–4.47) per 1000 persons. The prevalence and incidence of gout increased in Korea between 2007 and 2015.

Men and the older population had a higher prevalence and incidence of gout compared to women and the younger population. However, the incidence of gout in the younger population has increased rapidly in recent years.

Keywords Gout · Prevalence · Incidence · Korea

Introduction

Gout is a common inflammatory arthritis resulting from monosodium urate (MSU) crystal deposition around joints.

Persistent elevation of serum uric acid levels leads to deposition of MSU crystals in peripheral joints, typically involving metatarsophalangeal joints. Early episodes of acute gouty arthritis occur abruptly and cause severe pain, but resolve spontaneously within several days or weeks. It may appear as chronic arthritis with formation of tophi and joint damage, which contribute to disability and decreased qual- ity of life. Moreover, gout may accompany other comor- bidities such as hypertension, diabetes mellitus, obesity, metabolic syndrome, and cardiovascular diseases, which increase the burden on patients and the health care system [1–3].

Epidemiologic studies of gout have been performed in many countries worldwide. The prevalence of gout varies among ethnic groups and countries. The highest prevalence is reported in Taiwan aborigines [4, 5] and the Maori people in New Zealand [6]. Gout is prevalent in developed countries, including North America and Europe. In the USA, the prevalence of gout was 3.9% in the National Health and Nutrition Examination Survey (NHANES) Abstract The purpose of the present study was to inves-

tigate the prevalence and incidence of gout in Korea and predict the future prevalence and incidence of gout. Data were collected from the national health claims database.

Patients who had at least one claim for gout between 2007 and 2015 were included in the study. The prevalence of gout from 2007 to 2015 and the incidence of gout from 2009 to 2015 were determined. We estimated sex- and age-specific prevalence and incidence of gout during the period. The prevalence and incidence of gout were predicted using time series analysis. The prevalence of gout (95% CI) increased from 3.49 (3.48–3.51) per 1000 persons in 2007 to 7.58 (7.55–7.60) per 1000 persons in 2015. The incidence of gout (95% CI) was 1.52 (1.51–1.53) in 2009 and rose to 1.94 (1.93–1.95) per 1000 persons in 2015. The prevalence and incidence of gout were higher in men than in women. The older population had a higher prevalence and incidence than the younger population. The increase in prevalence was higher in the older population than the younger population, whereas the increase in incidence was higher in the younger population than the older population.

The predicted prevalence and incidence of gout (95% CI) in

INTERNATIONAL

Electronic supplementary material The online version of this article (doi:10.1007/s00296-017-3768-4) contains supplementary material, which is available to authorized users.

* Sung-Hoon Park [email protected]

1 Division of Rheumatology, Department of Internal

Medicine, Catholic University of Daegu School of Medicine, 33, Duryugongwon-ro 17-gil, Nam-gu, Daegu 42472, Republic of Korea

2 Department of Medical Statistics, Catholic University of Daegu School of Medicine, 33, Duryugongwon-ro 17-gil, Nam-gu, Daegu 42472, Republic of Korea

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2007–2008, which was determined by interviewing participants for history of physician-diagnosed gout [7]. In the UK, by assessing the Clinical Practice Research Datalink (CPRD), the prevalence of gout was 2.49% in 2012, with a 63.9% increase during the period from 1997 to 2012 [8]. A study from China, conducted by randomly surveying people in the city of Qingdao, showed the prevalence of gout to be 0.36% in 2002 [9], increasing to 0.53% in 2004 [10].

Despite the ethnic variations among studies, the prevalence of gout has been increasing in recent decades.

There are relatively few studies regarding the incidence of gout compared to those of prevalence, and the results are quite different among the studies. Incidence of gout increased in the USA between 1977–1978 and 1995–1996 [11]. However, a recent study from the UK reported that the incidence of gout stabilized between 2005 and 2012 [8], and another recent study from Taiwan showed a decreasing trend of incidence from 2005 to 2010 [12].

The epidemiologic studies of gout are limited in Korea.

The prevalence of gout among the hyperuricemic population was reported by surveying data from a single health screen- ing center in 2002 [13]. After that, the prevalence of gout in the Korean population was studied using the national health claims database from 2001 to 2008 [14]. The prevalence of gout was 0.171% in 2001, and increased 2.317-fold to 0.397%

in 2008. No further studies have been performed since then, and the incidence of gout has not yet been studied in Korea.

The aim of the study was to investigate the prevalence and incidence of gout in the Korean population between 2007 and 2015. Furthermore, we analyzed the trend of prevalence and incidence of gout and predicted their values for next 10 years.

Methods Data source

The national health claims database managed by the Health Insurance Research and Assessment (HIRA) service of South Korea was employed in this study. The Korean National Health Insurance system, which was developed in 1989, covers 97% of the population; the remaining 3%

of the population is supported by the Medical Aid program [15]. HIRA reviews all the health claims in South Korea, irrespective of whether the patients are covered by the National Health Insurance system or the Medical Aid program. The claims database contains detailed information including patient demographics, diagnoses, medical procedures, and prescribed medications. As the database pro- vides a nationwide source of information, it is well suited for epidemiologic studies from Korea [15–17].

Study population and identification of gout

The beneficiaries that were studied included all patients who were registered to have gout as a primary or secondary diagnosis in an outpatient visit or hospital admission during 1 January 2007 to 31 December 2015. The data included patients who attended a primary care hospital, secondary care hospital, or tertiary care hospital. We collected data from patients of all ages and both sexes. They had at least one claim for the diagnosis of gout, and the first visited date for gout was obtained. Gout was identified using the diagnostic code M10 from the Korean Standard Classification of Diseases (KCD), which is similar to the International Classification of Diseases, 10^th Revision (ICD-10). This study was approved by the institutional review board committee of Daegu Catholic University Medical Center (CR-16-113-L).

Estimation of prevalence and incidence

The annual prevalence of gout from 2007 to 2015 was calculated using the number of cases during each year divided by the total population of that year. Data of the total South Korean population were available from the Korean Statistical Information Service (http://www.kosis.

kr), estimated from the Population and Housing Census 2010. Data for annual prevalence were represented as cases per 1000 persons.

Annual incidence of gout was calculated using the number of yearly incident cases divided by the total population of the year. The total population was derived from the same data as was used for prevalence. Incident cases were defined as newly diagnosed gout during the year and they were determined through an algorithm which was statistically made. Individuals discovered to have gout prior to each year were excluded in incident cases of the year. Because our study had claims data from 2007, we established a 2-year washout period for estimating the annual incidence in 2009. Annual incidence from 2009 to 2015 was obtained to assess the trend in gout incidence.

Data for annual incidence were represented as cases per 1000 persons.

Prevalence and incidence of gout in men and women were investigated. Age-specific prevalence and incidence were obtained from the database. The prevalence and incidence of gout in people before they entered the sixth decade (20–49 years) were estimated separately for an analysis in the younger aged population. The trends of prevalence and incidence in the younger aged population were compared to those in patients with older age (50 years and over).

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Statistical analysis

The trends of prevalence and incidence of gout were compared between different age groups by proportion compari- son analysis. Predicted values for annual prevalence and incidence were estimated by the autoregressive integrated moving average (ARIMA; p, d, q) model of time series analysis. Data of estimated (2007–2015) and predicted (2016–2025) prevalence and incidence are represented with a 95% confidence interval (CI). All statistical analyses were performed using SAS Enterprise Guide 6.1 M1 (SAS Insti- tute Inc., Cary, NC, USA) and IBM SPSS statistics 19.0 (IBM Corp., Armonk, NY, USA). A p value <0.05 was considered statistically significant.

Results

Overall and sex‑specific prevalence and incidence of gout between 2007 and 2015

The prevalence and incidence of gout overall, in men, and in women are shown in Table 1. Among 48,597,652 total persons, 169,796 cases were observed in 2007, resulting in a prevalence of 3.49 (95% CI 3.48–3.51) per 1000 persons. The prevalence increased to 7.58 (95%

CI 7.55–7.60) per 1000 persons in 2015 (383,471 cases among 50,617,045 persons), which is a twofold increase compared to the prevalence in 2007. The prevalence of

gout (95% CI) in men increased from 6.23 (6.21–6.25) per 1000 persons in 2007 to 13.57 (13.54–13.60) per 1000 persons in 2015. The prevalence (95% CI) in women also rose from 0.73 (0.73–0.73) per 1000 persons in 2007 to 1.58 (1.58–1.58) per 1000 persons in 2015. Overall gout prevalence (95% CI) in adult aged ≥20 years was 4.61 (4.58–4.63) per 1000 persons in 2007 and increased to 9.45 (9.41–9.47) per 1000 persons in 2015.

There were 74,939 incident cases of gout in the total population of 49,182,038 in 2009, which increased to 98,244 incident cases among a total population of 50,617,045 in 2015. The incidence of gout (95% CI) during 2009-2011 remained stable, ranging from 1.52 (1.51–1.53) to 1.55 (1.54–1.56) per 1000 persons.

However, the incidence (95% CI) increased to 1.69 (1.67–1.70) per 1000 persons in 2012, followed by 1.82 (1.81–1.83) per 1000 persons in 2013 and 2014, and to 1.94 (1.93–1.95) per 1000 persons in 2015. Simi- lar patterns of incidence appeared in both men and women. The incidence of gout (95% CI) in men was 2.59 (2.57–2.60) per 1000 persons in 2009, which became 3.21 (3.20–3.23) per 1000 persons in 2015. The incidence (95% CI) in women was 0.45 (0.45–0.46) per 1000 persons in 2009, growing to 0.67 (0.66–0.68) per 1000 persons in 2015 (Fig. 1). In the adult aged ≥20 years, gout incidence (95% CI) remained stable from 1.96 (1.94–1.97) to 1.99 (1.98–2.01) per 1000 persons in 2009–2011, which increased to 2.40 (2.39–2.42) per 1000 persons in 2015.

Table 1 Prevalence and incidence of gout between 2007 and 2015

95% CI 95% confidence interval

Year Overall Men Women

No. Rate 95% CI No. Rate 95% CI No. Rate 95% CI

Prevalence (per 1000 persons)

2007 169,796 3.49 3.48–3.51 152,071 6.23 6.21–6.25 17,725 0.73 0.73–0.73 2008 189,231 3.87 3.85–3.88 170,029 6.92 6.90–6.94 19,202 0.79 0.79–0.79 2009 208,247 4.23 4.22–4.25 187,582 7.61 7.58–7.63 20,665 0.84 0.84–0.84 2010 233,397 4.72 4.70–4.74 210,445 8.50 8.47–8.53 22,952 0.93 0.93–0.93 2011 253,975 5.10 5.08–5.12 229,211 9.19 9.16–9.22 24,764 1.00 1.00–1.00 2012 285,838 5.72 5.70–5.74 257,113 10.27 10.24–10.30 28,725 1.15 1.15–1.15 2013 321,419 6.40 6.38–6.42 288,401 11.48 11.45–11.50 33,018 1.32 1.32–1.32 2014 346,685 6.88 6.85–6.90 310,115 12.30 12.27–12.33 36,570 1.45 1.45–1.45 2015 383,471 7.58 7.55–7.60 343,387 13.57 13.54–13.60 40,084 1.58 1.58–1.58 Incidence (per 1000 persons)

2009 74,939 1.52 1.51–1.53 63,857 2.59 2.57–2.60 11,082 0.45 0.45–0.46 2010 76,527 1.55 1.54–1.56 64,736 2.61 2.60–2.63 11,791 0.48 0.47–0.48 2011 76,299 1.53 1.52–1.54 64,288 2.58 2.56–2.59 12,011 0.48 0.48–0.49 2012 84,288 1.69 1.67–1.70 70,155 2.80 2.79–2.82 14,133 0.57 0.56–0.57 2013 91,247 1.82 1.81–1.83 75,579 3.01 2.99–3.02 15,668 0.62 0.62–0.63 2014 91,837 1.82 1.81–1.83 75,362 2.99 2.97–3.00 16,475 0.65 0.65–0.66 2015 98,244 1.94 1.93–1.95 81,241 3.21 3.20–3.23 17,003 0.67 0.66–0.68

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Age‑specific prevalence and incidence of gout between 2007 and 2015

Prevalence and incidence of gout in age groups 0–9, 10–19, 20–29, 30–39, 40–49, 50–59, 60–69, 70–79, and 80 years and over are shown in Online Resource 1. Prevalence and incidence of gout increased as participant age increased.

The highest prevalence of gout was observed in people age 70-79 years from 2008 to 2015. The incidence of gout also peaked in people age 70–79 years.

The prevalence of gout increased in most age groups from 2007 to 2015. However, there were different trends in incidence of gout according to age groups from 2009 to 2015. People aged 20–29, 30–39, and 40–49 years old had an increasing trend of incidence during this period, while those aged 50–59 and 60–69 years showed a decreasing or stable trend of incidence. Those aged 70–79 and 80 years and over had an increased incidence compared to 2009, but showed a relatively stable incidence from 2013 to 2015 (Fig. 2).

Prevalence and incidence of gout in the younger age group (20–49 years) versus the older age group (≥50 years)

Prevalence and incidence of gout in the younger population (20–49 years) were compared to those in the older population (50 years and over) (Table 2). The older population had a higher prevalence and incidence of gout than the younger population. The prevalence (95% CI) in the

younger population was 3.18 (3.15–3.20) per 1000 persons in 2009, then steadily increased to 5.73 (5.70–5.76) per 1000 persons in 2015. The prevalence of gout (95%

CI) in the older population was 9.60 (9.53–9.63) per 1000 persons in 2009 then, similarly, steadily increased to 14.33 (14.25–14.36) per 1000 persons in 2015. The increase in the prevalence of gout was significantly higher in the older population than the younger population (Δ 2.55 per 1000 persons versus Δ 4.74 per 1000 persons, p < 0.001). The incidence of gout (95% CI) in the younger population was 1.32 (1.30–1.33) per 1000 persons in 2009, rising to 1.81 (1.79–1.82) per 1000 persons in 2015, while the incidence (95% CI) in the older population was quite stable from 3.13 (3.10–3.16) per 1000 persons in 2009 to 3.18 (3.15–3.21) per 1000 persons in 2015. The incidence of gout grew significantly faster in the younger population compared to the older population (Δ 0.49 per 1000 persons versus Δ 0.05 per 1000 persons, p < 0.001). Data are also shown in the figure (Online Resource 2).

Predicted values for prevalence and incidence of gout from 2016 to 2025

The predicted values for the prevalence and incidence of gout in the population overall, in men, and in women are shown in Fig. 3. Because the prevalence of gout increased from 2007 to 2015, it was predicted to continue to increase.

The overall prevalence of gout (95% CI) is predicted to be 8.28 (8.05–8.52) per 1000 persons in 2016, growing to

Fig. 1 a Prevalence and b incidence of gout overall, in men, and in

women between 2007 and 2015 Fig. 2 a Age-specific prevalence and b incidence of gout between 2007 and 2015

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16.59 (15.85–17.34) per 1000 persons in 2025. Accord- ingly, the incidence of gout is also predicted to increase in the next 10 years. The overall incidence of gout (95% CI) is predicted to be 2.06 (1.85–2.27) per 1000 persons in 2016 and 3.81 (3.14–4.47) per 1000 persons in 2025. Detailed data are given in Online Resource 3.

Discussion

Our study investigated the prevalence and trends in the incidence of gout in Korea. The annual prevalence from 2007 to 2015 and the annual incidence from 2009 to 2015 were obtained from the national claims database. We investigated the incidence of gout for the first time in Korea (and, for our knowledge, the second in Asia) and expanded the data of gout prevalence to 2015. Prevalence and incidence of gout increased during the study period. We stratified the data according to sex and age. Men had a higher prevalence and incidence of gout than the women. The older population (sixth decade and over) had a higher prevalence and incidence of gout than the younger population (before sixth decade). However, the incidence of gout is growing faster

in the younger population compared to the older population. From the current data, we predicted the prevalence and incidence of gout for the next 10 years.

The present study corroborates an increasing trend in the prevalence of gout in Korea that was presented in a previous study. In a previous study using the national health claims database, the prevalence of gout was 0.17% in 2001, which increased to 0.26% in 2004, followed by 0.35% in 2007 and 0.40% in 2008 [14]. There were some differences between that study and the present study. First, we included patients who had gout as a primary or secondary diagnosis, while the previous study enrolled only patients who had gout as a primary diagnosis. Second, we used the total South Korean population each year for estimating the annual prevalence, while the previous study used the total population of South Korea in 2005 for estimating the annual prevalence from 2001 to 2008.

The prevalence of gout in Korea is quite low compared to other countries. The prevalence of gout in Korea increased to 0.76% in 2015, while the USA reported a prevalence of 3.9% in 2007–2008 and the UK reported a prevalence of 2.49% in 2012 [7, 8]. The prevalence of gout in Italy was 0.91% in 2009, and in France it was 0.9% in

Table 2 Prevalence and incidence of gout in the younger age group (age 20–49) and the older age group (age 50 and over)

95% CI 95% confidence interval

a Increased prevalence (per 1000 persons) from 2009 to 2015 is higher in the older population than the younger population (p < 0.001)

b Increased incidence (per 1000 persons) from 2009 to 2015 is higher in the younger population than the older population (p < 0.001)

Year 20–49 years 50 years and over

No. Rate 95% CI No. Rate 95% CI

Prevalence (per 1000 persons)

2007 64,075 2.65 2.63–2.67 104,742 8.38 8.32–8.42

2008 69,980 2.91 2.89–2.93 118,272 9.02 8.96–9.06

2009 75,705 3.18 3.15–3.20 131,508 9.60 9.53–9.63

2010 83,248 3.52 3.50–3.55 149,022 10.40 10.33–10.44

2011 89,294 3.79 3.77–3.82 163,439 10.89 10.83–10.93

2012 99,255 4.25 4.22–4.28 185,188 11.83 11.76–11.86

2013 110,787 4.77 4.75–4.80 209,068 12.83 12.76–12.87

2014 118,727 5.15 5.12–5.18 226,417 13.40 13.33–13.44

2015 131,326 5.73 5.70–5.76 250,488 14.33 14.25–14.36

Δ^a 2.55 4.74

Incidence (per 1000 persons)

2009 31,335 1.32 1.30–1.33 42,900 3.13 3.10–3.16

2010 32,151 1.36 1.35–1.38 43,591 3.04 3.01–3.07

2011 32,329 1.37 1.36–1.39 43,091 2.87 2.84–2.90

2012 34,861 1.49 1.48–1.51 48,442 3.09 3.06–3.12

2013 37,960 1.64 1.62–1.65 52,177 3.20 3.17–3.23

2014 38,322 1.66 1.64–1.68 52,436 3.10 3.07–3.13

2015 41,391 1.81 1.79–1.82 55,656 3.18 3.15–3.21

Δ^a 0.49 0.05

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2013 [18, 19]. Various ranges of prevalence have been doc- umented in Asian countries including China, Singapore, and Taiwan [10, 12, 20]. In Taiwan, the prevalence of gout was higher in Aborigines than in Han [5]. The difference in prevalence among ethnic groups suggests that genetic factors contribute to gout.

Environmental factors as well as genetic factors are known to be important contributors to gout. The familial aggregation of gout was confirmed by demonstrating the prevalence and risk of gout in individuals with affected family members in the study of Kuo et al. [21]. Family members share not only similar genetic backgrounds, but also similar lifestyle and environmental influences. Both factors influence the familial clustering of gout. They found that environmental factors had a relatively large contribu- tion to gout, whereas genetic factors had a smaller, but still substantial, role.

Dietary factors, increasing age, and comorbid conditions were found to be the environmental influences of gout. Increased meat and seafood intake were associated with increased risk of gout [22]. Consumption of alcohol, sugar-sweetened soft drinks, and fructose also increased the risk of gout [23, 24]. Obesity, weight gain, hypertension, and diuretic use were independent risk factors for gout

from the Health Professionals Follow-up Study [25]. Pro- spective data from the Framingham Heart Study revealed that a higher uric acid level, increasing age, obesity, alcohol consumption, hypertension, and diuretic use were associated with a risk of incident gout [26]. The present study demonstrated that the older population had a higher prevalence and incidence of gout than the younger population.

Increased longevity and an enlarged population of elderly people in Korea had a direct effect and an indirect effect on the prevalence and incidence of gout. Age-related comor- bidities and treatments also contribute to the increased prevalence of gout in the older population [27]. In addi- tion, a rapidly increasing incidence of gout was recently observed in Korea in the younger population. Dietary and lifestyle have changed among the younger population in recent decades, and these change may be contributing to the increased incidence of gout. The Korea National Health and Nutrition Examination Survey (KNHANES) 2014 found that consumption of meat and alcohol has steadily increased in South Korea since the 1990s. In particular, men in their fourth decade had poor management of their health and exhibited the highest rate of high-risk alcohol intake, a lower rate of physical activity, the highest rate of excessive caloric intake, and the highest rate of obesity compared to other age groups. These changes in dietary and lifestyle patterns increased the risk of gout in Korea, especially in the younger population. Epidemiologic studies from other countries identified that there is a different prevalence of gout across different geographical areas [10, 18]. Those findings were also attributed to different dietary patterns in the different areas.

Gout is more prevalent and has a higher incidence in men than in women. The reason for this male predomi- nance is not clear. One explanation is that estrogen exerts a uricosuric effect in premenopausal women, thereby low- ering the prevalence of gout [28]. Postmenopausal women had a relative risk of 1.26 for incident gout compared to premenopausal women [29]. Moreover, different dietary and lifestyle patterns of men may contribute to the higher prevalence and incidence of gout in men.

Data on the incidence of gout are relatively scarce throughout the world. The early Johns Hopkins Precur- sors Study followed medical students for a median of 29 years and concluded that the incidence of gout was 1.73 per 1000 person-years [30]. The recent Athero- sclerosis Risk in Communities Study was a US population based-cohort study that enrolled middle-aged adults in 1987 with annual follow-up visits until 2012;

they reported the incidence of gout to be 8.4 per 10,000 person-years [31]. The incidence of gout in the UK increased in the 1990s, but remained stable in the 2000s [8, 32–34]. The incidence of gout was also investigated in Italy (0.95 per 1000 person-years in 2009), Taiwan

Fig. 3 a Predicted prevalence and b incidence of gout for the next 10 years

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(2.74 per 1000 person-years in 2010), and Western Swe- den (1.9 per 1000 person-years in 2012) [12, 18, 35]. Our study is the first to report the incidence of gout in Korea.

The overall incidence of gout was relatively stable from 2009 to 2011 (annual incidence of 1.52–1.55 per 1000 persons), but it rose to 1.69 per 1000 persons in 2012 and 1.94 per 1000 persons in 2015. These patterns were consistently observed in the incidence of gout in men and women. The trend in the incidence of gout was different among different age groups. The incidence in the younger population increased from 2009 to 2015, whereas the incidence in the older population was relatively stable throughout the period. The main strength of our study is that we performed the study using the national health claims database, which includes all claims that were filed in South Korea.

There are several limitations of the present study. First, as the study was based on the claims database, we could not assess the clinical characteristics of the study participants such as the presence of tophi, the results from syno- vial fluid analysis, and uric acid levels. To confirm the diagnosis of gout, additional information from the study participants and medical records are sometimes needed.

Second, as we were provided data starting 1 January 2007, patients who were first diagnosed with gout prior to 2007 might be included in the incident cases of gout.

However, we established the washout period for estimating the annual incidence of gout from 2009 to 2015 and there were no difficulties in investigating the trend in the incidence of gout.

Our study has some implications to the internationally diverse readers. Although Korea has relatively low gout prevalence, incident gout increases rapidly in younger population. With continuously increasing elderly population, the prevalence of gout is also steadily increasing. This is not only a problem for just one country. Gout patients are at increased risk of hypertension, metabolic syndrome, and cardiovascular diseases. The disease gives burden to individual patients and national society.

In conclusion, the prevalence and incidence of gout increased between 2007 and 2015. Men and the older population had a higher prevalence and incidence of gout compared to women and the younger population. Nevertheless, we observed a recent trend of rapidly increasing incidence of gout in the younger population. The prevalence and incidence of gout are predicted to increase steadily for the next 10 years. The communities and the country should be con- cerned with these increasing numbers of gout patients and focus on optimal management for them.

Acknowledgements This work was supported by a grant of the Research Institute of Medical Science, Catholic University of Daegu (2016).

Compliance with ethical standards

Conflict of interest Ji-Won Kim, Sang Gyu Kwak, Hwajeong Lee, Seong-Kyu Kim, Jung-Yoon Choe, and Sung-Hoon Park declare that they have no conflict of interest.

Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. For this type of study, formal consent is not required.

Human and animal rights This article does not contain any studies with animals performed by any of the authors.

Funding No external funding was obtained for performing this study.

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