This study showed a high prevalence of anemia in the population aged 65 years and older, regardless of sex. The prevalence of anemia was found to increase significantly with age, and the trend of prevalence also showed considerable increase within years. Comorbid conditions including being underweight and presence of DM, rheumatoid arthritis, cancer, and CRF were shown to be significant risk factors.
There are few reports on anemia in the older population in Korea. Kim et al  showed that the incidence of anemia in population aged over 60 years in Korea was 7.2%, and most of them were chronically anemic. Han et al  analyzed healthy individuals who underwent routine medical checkup in a single center and showed that mild anemia, defined as having a hemoglobin level between 10.0 g/dL and 12.9 g/dL in men and 10.0 g/dL and 11.9 g/dL in women, is a risk factor for cancer and cardiovascular death in the elderly population. Jeong et al  also showed a high incidence of anemia in the population aged 80 years and more, and most of them were anemic from unknown reasons, which was closely related to malignancy as analyzed by routine medical checkup of laboratory data. These studies are small community-based cross-sectional studies within a selected population and are limited in reflecting the general Korean population. This study analyzed data from the KNHANES, which was elaborately designed and conducted as a government-led nation-wide survey with carefully sampled participants. Therefore, the results of this study potentially represent the current status of older population in Korea.
However, results from this study are limited in their ability to be generalized, because populations with laboratory data displayed significantly varying characteristics compared to populations without laboratory data. The decision to conduct laboratory tests was left entirely to the participants and the decision was made by the family unit in KNHANES; hence, the group with missing laboratory data was not randomly distributed. This seems to contribute to the observed differences.
The occurrence of anemia in older people is not fully understood. Hemoglobin levels in the older population are reportedly lower than the reference values for other population groups. Some reports concluded that this decrease in hemoglobin levels might be one of the consequences of the normal aging process; hence, the criteria for anemia should be reassigned in this population [13, 24-27]. However, many reports also showed that anemia in older individuals is related to the presence of underlying health conditions and is therefore associated with high mortality and morbidity. Furthermore, most people with anemia have been shown to have nutritional deficiency, but the etiology is unknown in one-third of the anemia cases [17, 28]. This unexplained category of anemia includes aging-related clonal hematopoiesis (ARCH), idiopathic cytopenia of undetermined significance (ICUS), and pre-myelodysplastic syndrome (MDS); these conditions are associated with a low but potential risk of hematologic malignancy [19, 20, 29]. Therefore, older people with anemia are recommended to undergo evaluations for etiology analysis . However, there is no guideline or consensus for the range and frequency of the evaluation and management in this population.
The reported prevalence of anemia differs across various studies, and these differences might be attributed to the diversity of the subjects and cohorts of previous studies [4, 17, 23, 24, 27]. Studies conducted in nursing home- and hospital-based populations have shown significantly higher prevalence of anemia and morbidity [3, 10]. Most of these studies seem to show similar findings with regard to the prevalence of anemia in older adults—that is, the prevalence of anemia tends to increase with age, and there is no sex-based difference, with some studies even showing a higher prevalence of anemia among males. In particular, a study in the US showed that in people aged > 85 years, the prevalence of anemia was higher among males .
In this study, the prevalence of anemia in the population aged ≥ 65 years was 14.0%, which is higher than that reported in previous studies, for example, 10.6% in the US  and 8.33% in Bang et al’s study . Since study settings and participants enrolled are different, it is not reasonable to compare these studies directly. However, this finding is quite striking because our study was based on populations who were relatively healthy and lived in a secure environment. Moreover, it is expected that the prevalence of anemia would be much higher in those who live in a less secure environment, are admitted in hospitals or social facilities, and experience malnutrition due to financial reasons. Studies of hospitalized patients and people with diseases have shown that anemia is associated with a high risk of complications and poor outcomes [3, 5, 6, 13].
This study showed that older adults with anemia tend to be underweight, which might be related to malnutrition. However, this finding is inconsistent with those of previous studies, showing that being a beneficiary of social allowance or having a low household income is not a risk factor for anemia. Malnutrition-related anemia in the elderly has been reported to be 34 -62% of the anemia of known etiology [17, 23, 30, 31]. Most cases of malnutrition-related anemia involve iron, folate, and vitamin B12 deficiency . Among them, iron deficiency is the most common and is usually accompanied by comorbidities, i.e., gastrointestinal tract bleeding, chronic inflammation, among others. Malnutrition also can be induced by chronic diseases, i.e., chronic renal failure, or psychological disorders, i.e., depression and anorexia . Moreover, malnutrition also correlates with the economic status or household financial burden [28, 31]. This nationwide survey did not include any other specific data for malnutrition apart from body weight. Further studies are thus required to clarify the relationship between anemia and malnutrition.
In addition, as shown in Table1, participants without laboratory data tends to be in worse circumstances; they tend to be older, to have economic problems, and live alone. Prevalence of comorbidities were also higher in those without laboratory data. Laboratory tests were distributed according to the statistically designed manner considering weight and stratifications. However, participants could choose whether to do the laboratory tests or not in the end. Worse circumstances might have hindered from time-consuming, troublesome laboratory tests. It is difficult to find out where these differences came from, but it seems to be obvious that our prevalence of anemia and its burden might be underestimated. If the whole individuals had laboratory data and included in our study, the result would have been quite different.
This study is limited in its ability of the KHNANES data being unable to show a causal relationship between variables. To determine the cause-and-effect relationship between these variables, large well-designed prospective cohort studies are necessary. Another limitation is that the questionnaire items of the KHNANES are being revised every 3 years for economic reasons, making it impossible for researchers to investigate the possible etiologies of anemia in the study population. As the etiologies of anemia seem to be more complicated in older adults than in the younger population, anemia may impose a heavy medical burden on older people in countries such as South Korea.