1. Introduction
The growth of the elderly population has increased the prevalence of chronic diseases [1]. Iran has one of the fastest growth rates of the elderly population globally, and the share of the population over 65 years in Iran has increased from 4.2% in 1989 to 6.4% in 2019 [2]. Various studies have been conducted on the prevalence of chronic diseases among the elderly, and the issue of Chronic Disease Multimorbidity (CDM) has been discussed; Because this issue has increased the referral to the health system and has intensified the pressure on the resources of the health system [3]. 
One of the most important issues in the prevalence of chronic diseases is the role of socioeconomic factors, which have not been considered among the elderly in Iran. This study tried to calculate the prevalence of chronic diseases among the elderly in Tehran and investigate the prevalence among different socioeconomic groups.
2. Methods
The present study was a cross-sectional study of the elderly in Tehran in 2019; the required sample size was calculated using Cochran’s formula and the design effect of 1280 people. The data collection tool in this study was a researcher-made questionnaire including questions related to demographic characteristics, health status, and economic status. The face validity of this questionnaire was evaluated by 5 experts and 15 elderly people in Tehran. According to the purpose of the research and the importance of the socioeconomic status of the elderly, a combined variable was calculated for this issue, and socioeconomic classes were formed for the elderly. In order to classify chronic diseases, v. 11 of the International Classification of Diseases (ICD) and studies on diseases of old age have been used. In the present study, their self-expression is sufficient to identify chronic diseases among the elderly. Due to the limitations that self-declaration can have, in the questioning of the elderly, the criterion of the existence of the disease, the doctor’s diagnosis, and the use of medicine or health equipment by the elderly has been considered.
The sampling of the study was performed by considering the distribution of the elderly population in each region [4] from all 22 regions of Tehran. Sampling was done from the door of houses (29%), parks (48%), and nursing homes (23%) to ensure maximum diversity in the samples. The questionnaires were completed by trained interviewers using face-to-face interviews while obtaining informed consent from the respondents. The frequency and prevalence of the disease were used to evaluate the prevalence of chronic diseases among the elderly, and the chi-square homogeneity test was performed using SPSS v. 21 software at a significance level of less than 0.05. 
3. Results
The age range of the respondents in the study was 60 to 95 years (with a Mean±SD of 70.9±0.22). The highest frequency in the research sample in terms of demographic variables also included men (50.1%), married people (70.2%), and the elderly with primary-secondary education (37.4%). In terms of employment status, most elderly were retired (40.3%) or housewives (34.5%). Income of 2-3 million Tomans (100-150 USD per month) has been the most frequent income level among the elderly in Tehran. Among the sample, 6.3% of the elderly did not report any chronic disease, and 93.7% had at least one chronic disease. The prevalence rate of multiple chronic diseases (having two or more chronic diseases) among the elderly was 79.8%.
CDM prevalence is higher among women than men, and this rate has increased with age. The prevalence of CDM in the never-married elderly was the lowest and the highest among widows. The prevalence of chronic disease multimorbidity was lower in the elderly with higher education. The prevalence of CDM was significantly higher among housewives and retirees than the unemployed or employed elderly. The low-income elderly (less than one million tomans per month) (less than 50 USD per month) have the lowest (74.1%), and the middle-income low-income elderly (between one to two million tomans per month) (50-100 USD per month) have the highest (84.6%) CDM prevalence.
Among the sample elderly, hypertension (with a prevalence rate of 40.2%), musculoskeletal diseases (with a prevalence rate of 40.4), and hyperlipidemia (with a prevalence rate of 32.4%) were the most common chronic diseases of the elderly in Tehran. Cancer and eye diseases also had the lowest prevalence with 2.3% and 9%, respectively.
The homogeneity test shows that hypertension, cardiovascular diseases, diabetes, hyperlipidemia, neurological, and eye diseases were among the diseases that the probability of developing significantly differed among socioeconomic classes. Among the chronic diseases that had significant differences between different classes, the lowest prevalence belonged to the upper socioeconomic class. The highest prevalence of hypertension, hyperlipidemia and neurological diseases was related to the socioeconomic middle class. On the other hand, cardiovascular diseases, diabetes, and eye diseases have had a higher prevalence rate among the lower social class of the society (Table 1). 



4. Discussion
The high prevalence of CDM among the elderly in Iran (79.8%) indicates an unfavorable situation compared to developed countries. In addition, hypertension, musculoskeletal diseases, and hyperlipidemia have the highest prevalence and cancer, and eye diseases have the lowest prevalence, similar to those of the elderly in other parts of the world. The use of the social class combined variable showed that socioeconomic status plays an important role in the health of the elderly; However, the role of this variable in the development of various chronic diseases cannot be considered the same. The prevalence of CDM in the elderly in Iran should be considered under this variable. As a result, research and policy-making in chronic disease control in the elderly should be designed and implemented based on the existing inequalities in socioeconomic status.
Ethical Considerations
Compliance with ethical guidelines
In the present study, informed consent was obtained from the respondents and the research was approved by the ethics committee (Code: IR.USWR.REC.1398.068).
Funding
The research was funded by the Vice Chancellor for Research and Technology of the University of Social Welfare and Rehabilitation Sciences and under contract No. 2322 / T / 98, and there was no condition in the contract to remove or not publish findings that are not considered favorable by the research sponsor.
Authors' contributions
All research steps including conceptualization, methodology, investigation, writing and final approval performed by Seyed Hossein Mohaqeqi Kamal and Mehdi Basakha; Funding Acquisition, Seyed Hossein Mohaqeqi Kamal.
Conflicts of interest
The authors declared no conflict of interest.
Acknowledgments
The research team would like to thank the Vice Chancellor for Research and Technology of the University of Social Welfare and Rehabilitation Sciences, the data collection group and all the older people who participated in the research. 
 

1. Crimmins EM. Trends in the health of the elderly. Annual Review of Public Health. 2004; 25:79-98. [DOI:10.1146/annurev.publhealth.25.102802.124401] [PMID]
2. Mirzaie M, Darabi S. [Population aging in Iran and rising health care costs (Persian)]. Iranian Journal of Ageing. 2017; 12(2):156-69. [DOI:10.21859/sIran. J. Ageing-1202156]
3. Jacob L, Breuer J, Kostev K. Prevalence of chronic diseases among older patients in German general practices. German Medical Science. 2016; 14:Doc03. [PMID]
4. Basakha M, Yavari K, Sadeghi H, Naseri A. Health care cost disease as a threat to Iranian aging society. Journal of Research in Health Sciences. 2014; 14(2):152-6. [PMID]
5. Khosravi A, Taylor R, Naghavi M, Lopez AD. Differential mortality in Iran. Population Health Metrics. 2007; 5:7. [DOI:10.1186/1478-7954-5-7] [PMID] [PMCID]
6. Lehnert T, Heider D, Leicht H, Heinrich S, Corrieri S, Luppa M, et al., Health care utilization and costs of elderly persons with multiple chronic conditions. Medical Care Research and Review. 2011; 68(4):387-420. [DOI:10.1177/1077558711399580] [PMID]
7. Maghsoudi A, Abedi K, Omidvarijo F, Safayi F, Mohamadi Z, Riyahi S. [The study of prevalence of chronic diseases and its association with quality of life in the elderly of Ewaz (South of Fars province), 2014 (Persian)]. Navid No. 2016; 18(61):35-42. [DOI:10.22038/NNJ.2016.6610]
8. Alimohammadian M, Majidi A, Yaseri M, Ahmadi B, Islami F, Derakhshan M, et al. Multimorbidity as an important issue among women: Results of a gender difference investigation in a large population-based cross-sectional study in West Asia. BMJ Open. 2017; 7(5):e013548. [DOI:10.1136/bmjopen-2016-013548] [PMID] [PMCID]
9. Gijsen R, Hoeymans N, Schellevis FG, Ruwaard D, Satariano WA, van den Bos GAM. Causes and consequences of comorbidity: A review. Journal of Clinical Epidemiology. 2001; 54(7):661-74. [DOI:10.1016/S0895-4356(00)00363-2]
10. Fortin M, Dubois MF, Hudon C, Soubhi H, Almirall J. Multimorbidity and quality of life: A closer look. Health and Quality of Life Outcomes. 2007; 5:52. [DOI:10.1186/1477-7525-5-52] [PMID] [PMCID]
11. Wong MCS, Lao XQ, Ho KF, Goggins WB, Tse SLA. Incidence and mortality of lung cancer: Global trends and association with socioeconomic status. Scientific Reports. 2017; 7(1):14300. [PMID]
12. Levine RS, Foster JE, Fullilove RE, Fullilove MT, Briggs NC, Hull PC, et al. Black-white inequalities in mortality and life expectancy, 1933-1999: Implications for healthy people 2010. Public Health Reports. 2001; 116(5):474-83. [PMID]
13. Demakakos P, Biddulph J, Tsakos G, Marmot M, De Oliveira C. The role of subjective social status in socioeconomic inequalities in mortality: Evidence from the English longitudinal study of ageing (ELSA). Revue d’Épidémiologie et de Santé Publique. 2018; 66(5):S358. [DOI:10.1016/j.respe.2018.05.331]
14. Amini Rarani M, Rashidian A, Khosravi A, Arab M, Abbasian E, Khedmati Morasae E. Changes in socio-economic inequality in neonatal mortality in Iran between 1995-2000 and 2005-2010: An Oaxaca decomposition analysis. International Journal of Health Policy and Management. 2017; 6(4):219-28. [DOI:10.15171/ijhpm.2016.127] [PMID] [PMCID]
15. Haghparast-Bidgoli H, Rinaldi G, Shahnavazi H, Bouraghi H, Kiadaliri AA. Socio-demographic and economics factors associated with suicide mortality in Iran, 2001-2010: Application of a decomposition model. International Journal for Equity in Health. 2018; 17(1):77. [DOI:10.1186/s12939-018-0794-0] [PMID] [PMCID]
16. Dalstra JA, Kunst AE, Borrell C, Breeze E, Cambois E, Costa G, et al. Socioeconomic differences in the prevalence of common chronic diseases: An overview of eight European countries. International Journal of Epidemiology. 2005; 34(2):316-26. [DOI:10.1093/ije/dyh386] [PMID]
17. Kien VD, Van Minh H, Giang KB, Dao A, Weinehall L, Eriksson M, et al. Socioeconomic inequalities in self-reported chronic Non-communicable diseases in urban Hanoi, Vietnam. Global Public Health. 2017; 12(12):1522-37. [PMID]
18. Davari M, Maracy MR, Khorasani E. Socioeconomic status, cardiac risk factors, and cardiovascular disease: A novel approach to determination of this association. ARYA Atherosclerosis. 2019; 15(6):260-6. [PMID]
19. Nwani PO, Isah AO. Chronic diseases and multimorbidity among elderly patients admitted in the medical wards of a Nigerian tertiary hospital. Journal of Clinical Gerontology and Geriatrics. 2016; 7(3):83-6. [DOI:10.1016/j.jcgg.2015.10.001]
20. Ahmadi B, Alimohammadian M, Yaseri M, Majidi A, Boreiri M, Islami F, et al., Multimorbidity: Epidemiology and risk factors in the Golestan Cohort Study, Iran: A cross-sectional analysis. Medicine. 2016; 95(7):e2756. [DOI:10.1097/MD.0000000000002756] [PMID] [PMCID]
21. Zimmer Z, Liu X, Hermalin A, Chuang YL. Educational attainment and transitions in functional status among older Taiwanese. Demography. 1998; 35(3):361-75. [DOI:10.2307/3004043] [PMID]
22. Berkman CS, Gurland BJ. The relationship among income, other socioeconomic indicators, and functional level in older persons. Journal of Aging and Health. 1998; 10(1):81-98. [DOI:10.1177/089826439801000105] [PMID]
23. Wang F, Zhen Q, Li K, Wen X. Association of socioeconomic status and health-related behavior with elderly health in China. PloS One. 2018; 13(9):e0204237. [DOI:10.1371/journal.pone.0204237] [PMID] [PMCID]
24. Mohaqeqi Kamal SH, Basakha M, Ghaedamini Harouni G. Women’s well-being in Iran: Territorial analysis using a multidimensional approach. Social Indicators Research. 2018; 137(3):1061-72. [DOI:10.1007/s11205-017-1656-7]
25. Timmreck TC, Cole GE, James G, Butterworth DD. Health education and health promotion: A look at the jungle of supportive fields, philosophies and theoretical foundations. Health Education. 1987; 18(6):23-8. [PMID]
26. Keselman A, Smith CA. A classification of errors in lay comprehension of medical documents. Journal of Biomedical Informatics. 2012; 45(6):1151-63. [DOI:10.1016/j.jbi.2012.07.012] [PMID] [PMCID]
27. Khoja S, McGregor SE, Hilsden RJ. Validation of self-reported history of colorectal cancer screening. Canadian family physician Médecin. 2007; 53(7):1192-7. [PMID]
28. Kim MI, Johnson KB. Patient entry of information: Evaluation of user interfaces. Journal of Medical Internet Research. 2004; 6(2):e13. [DOI:10.2196/jmir.6.2.e13] [PMID] [PMCID]
29. Ming ME, Levy RM, Hoffstad OJ, Filip J, Gimotty PA, Margolis DJ. Validity of patient self-reported history of skin cancer. Archives of Dermatology. 2004; 140(6):730-5. [DOI:10.1001/archderm.140.6.730] [PMID]
30. Porter SC, Fleisher GR, Kohane IS, Mandl KD.The value of parental report for diagnosis and management of dehydration in the emergency department. Annals of Emergency Medicine. 2003; 41(2):196-205. [DOI:10.1067/mem.2003.5] [PMID]
31. Asadi-Lari M, Vaez-Mahdavi MR, Faghihzadeh S, Cherghian B, Esteghamati A, Farshad AA, et al. Response-oriented measuring inequalities in Tehran: Second round of Urban Health Equity Assessment and Response Tool (Urban HEART-2), concepts and framework. Medical Journal of The Islamic Republic of Iran. 2013; 27(4):236-48. [PMID]
32. Hosseini SR, Zabihi A, Savad Kouhi S, Bizhani A. [Prevalence of chronic diseases in elderly population in Amirkola (2006-2007) (Persian)]. Journal of Babol University of Medical Sciences. 2008; 10(2):68-75. https://www.sid.ir/en/Journal/ViewPaper.aspx?ID=121508
33. Tootoonchi P. [Chronic diseases and senile changes in the elderly population, Tehran, Iran (Persian)]. Payesh. 2004; 3(3):219-25. http://payeshjournal.ir/article-1-803-en.html
34. Marengoni A, Winblad B, Karp A, Fratiglioni L. Prevalence of chronic diseases and multimorbidity among the elderly population in Sweden. American Journal of Public Health. 2008; 98(7):1198-200. [DOI:10.2105/AJPH.2007.121137] [PMID] [PMCID]
35. Peiman H, Yaghoubi M, Seyed Mohammadi A, Delpishe A. [Prevalence of chronic diseases in the elderly in Ilam (Persian)]. Salmand: Iranian Journal of Ageing. 2012; 6(4):7-13. http://salmandj.uswr.ac.ir/article-1-470-en.html
36. Liu Z, Albanese E, Li S, Huang Y, Ferri CP, Yan F, et al. Chronic disease prevalence and care among the elderly in urban and rural Beijing, China-a 10/66 Dementia Research Group cross-sectional survey. BMC Public Health. 2009; 9:394. [DOI:10.1186/1471-2458-9-394] [PMID] [PMCID]