ANA are the most commonly measured biomarkers for autoimmunity and the easiest to assess at the population level. Estimating the prevalence and types of ANA in the U.S. is critical to understanding their etiology and changes over time. This study provides the first nationally-representative estimates of the prevalence of ANA in sociodemographic groups. Our investigation also included a determination of the patterns of ANA by standard immunofluorescent methods, as well as the identification of specific autoantibodies by a reliable immunoprecipitation assay in ANA-positive sera. Our finding of an overall ANA prevalence of 13.8% at a 1:80 serum dilution-level is similar to some small studies in selected healthy populations (3
), however, ANA prevalence in other reports at the same dilution level ranged from 1.1% to 20% (3
). These differences likely relate to the different populations under study and variations in ANA assessments in different laboratories.
Our findings of higher ANA prevalences in females and older individuals are similar to several earlier reports (7
). The reason for the female predominance in autoimmune diseases is not completely understood, nevertheless, the finding of a similar pattern of female dominance in ANA production suggests that hormonal or other factors in females play a role in this process (32
). While some reports suggest that aging is associated with autoimmunity and the prevalence of ANA is higher in the elderly (5
), this trend was not apparent in other studies (3
). Our finding of non-linear variations in ANA prevalence among different age groups could be the result of the differential exposure to factors related to development of ANA in certain age groups, non-linear intrinsic variations in the aging of the immune or endocrine systems, or sampling bias.
The variations in the female:male ratios of ANA in different age groups in our study are similar to patterns seen in systemic autoimmune diseases, which are strongly associated with ANA production (10
). One report suggested a lack of sex effects on ANA prevalence in subjects under 20 years of age (31
), but the prevalence of many autoimmune diseases increases in females during the child-bearing years. For example, a female:male ratio of ~9:1 is seen in SLE patients with onset between 20 to 40 years, but this ratio is only ~2:1 in children with SLE (0–9 years old) and in elderly-onset SLE (≥ 60 years old) (37
). The female:male ratio in young to middle-aged adults who develop RA is ~4:1, but it is only ~1:1 in elderly-onset RA (39
), and similar patterns are reported in scleroderma (40
ANA prevalence in non-Hispanic blacks was modestly increased compared to non-Hispanic whites. This is consistent with the higher incidence of SLE (37
) and increased prevalence of certain lupus autoantibodies, such as anti-U1RNP, -Sm, and –Ku autoantibodies, in non-Hispanic blacks (41
The lower prevalence of ANA in overweight and obese subjects, particularly in females, in the present study may be unexpected given the ability of adipose tissue to produce pro-inflammatory cytokines (22
) and estrogens (42
). Nonetheless, the effect of obesity on the immune system is complex, sometimes resulting in immunosuppression (43
), and an inverse association of ANA frequency with obesity has been previously reported in women (44
). Also, in a study of chronic obstructive pulmonary disease, ANA were not associated with smoking but their frequency was significantly higher in those with a low BMI (< 22 kg/m2
) compared to subjects with a normal or high BMI (45
). Additional investigations are needed to understand the cause of lower prevalence of autoantibodies in individuals with high BMI.
Despite some studies that suggest smoking as a risk factor for SLE, RA, and other autoimmune diseases (19
), we did not find any evidence suggesting an association or dose effect of current or past smoking with ANA.
Although ANA pattern distributions among healthy individuals vary among studies, a nuclear pattern is usually the most commonly identified, followed by cytoplasmic and nucleolar patterns (5
), as was observed in the present study. The most commonly identified autoantibodies in those who were positive for ANA were anti-Ro autoantibodies (3.9% among those with ANA and 0.53% among persons in the U.S.) and anti-Su autoantibodies (2.4% among those with ANA and 0.33% among persons in the U.S). The prevalence estimate of anti-Ro autoantibodies in this study is similar to that in an investigation of 5,000 blood donors (0.44%) (5
), but lower than that seen in a Japanese report (2.66% in 2181 subjects) (10
). Several investigations using reliable methods reported anti-Ro autoantibodies in 0.12 to 2% of blood donors or pregnant women (5
), while other small studies using enzyme-linked immunosorbent assays reported an even higher percentage of anti-Ro positives among healthy individuals (48
). It is difficult to compare these data, however, due to variations in the sensitivities and specificities among the assays used (2
). Because not all anti-Ro and anti-Su autoantibodies show strong immunofluorescence (2
), some sera containing these autoantibodies may not have met the threshold for immunoprecipitation testing in our study. Thus, the actual prevalence of these autoantibodies in the general population is likely higher than our estimate. The prevalence of autoantibodies associated with multiple systemic autoimmune diseases did not increase with age, consistent with a study of anti-Ro autoantibodies in female blood donors (5
In contrast to autoantibodies that are associated with multiple systemic autoimmune diseases, such as anti-Ro and -Su autoantibodies (2
), autoantibodies that are highly specific for certain diseases or disease phenotypes (e.g., anti-Sm, -topoisomerase I, -RNA polymerase I/III, and -Jo-1 autoantibodies) (1
) were rarely if ever seen in this study, supporting their disease specificity and rarity.
Our study has limitations. First, the institutionalized U.S. population (e.g., nursing home residents) was not sampled by NHANES and this may have led to an underestimate of ANA prevalence, especially in the elderly. Also, small sample sizes of certain subgroups may have limited our power to detect differences in ANA prevalence for some factors. Furthermore, not all types of autoantibodies were assessed by our testing and there are other potential causes of autoantibody production in addition to autoimmune diseases, including certain infections, cancers and drugs (2
). Due to limitations inherent in the NHANES data collection methodology, including cross-sectional sampling, we could not identify which ANA might be persistent versus transient, and we were unable to assess associations with specific autoimmune or other diseases. Finally, the prevalence of specific autoantibodies with less intense immunofluorescence may be underestimated since only ANA positive samples by immunofluorescence were tested by immunoprecipitation.
These findings demonstrate a high prevalence of ANA in the U.S., especially in females and older individuals. With the aging of the population, the number of individuals with ANA will likely increase beyond our estimate of 32 million persons. These first population-based estimates of ANA by indirect immunofluorescence, including their cellular staining patterns and specific autoantibody reactivities, resolve the uncertainties related to other published estimates from selected populations. These findings should be kept in mind by physicians when assessing ANA results and will serve as a useful baseline for future investigations of changes in ANA prevalence over time and the factors associated with their development.