Implications of key findings
Findings from this study suggest that the progression of chronic inflammatory disease may exert a significant negative impact on QoL in both emotional and social domains. This study establishes a conceptual foundation that demonstrates the feasibility and importance of conducting more detailed and sophisticated research on the formative and summative sociomedical dynamics of chronic inflammation. Likewise, it can help to corroborate previous research on the fundamental social causes of inflammation itself, especially findings from other studies using NSHAP data [
62].
While preserving a robust and representative sample of people in the NSHAP, stratifying my analyses by CRP levels below and above 6 mg/L also limited the extent to which outlying values attributable to measurement error could skew findings. Using evidence-based stratification techniques optimized internal validity without dismissing the potential contributions of very high CRP values to the overall study results. This facilitated consideration of why the much smaller samples of people with biomarker levels greater than 6 mg/L showed no significant associations between chronic inflammation and QoL, whereas their peers had markedly different results. As noted previously, among people with fewer than 6 mg/L of the CRP marker, chronic inflammation was significantly associated with both emotional and relational QoL.
Other sociological studies of QoL reflect that different QoL outcomes may be more or less salient to different people. They also suggest that depending on the particular elements of QoL they capture, these same measures may be more or less sensitive to influence from specific biological and social forces. When measured using CRP, chronic inflammation appears to have the potential to influence people's overall happiness, as well as their satisfaction in intimate relationships, to a small but consistent degree. Other QoL outcomes may show different patterns of association with CRP.
This same literature has important implications for the remainder of my analyses of possible relationships between CRP and QoL. Any associations documented in older populations are likely to be more pronounced among younger groups, as older individuals may expect a certain amount of decline in health and functionality as a normal part of the experience of growing older. The social role of "older adult" in contemporary American society thus includes changes in physical health and functionality, despite growing pressure to minimize these changes as much as possible. Demonstrating evidence of impairment in the course of performing their prescribed social role thus affords older adults a certain amount of legitimacy and credibility, even if they experience very large declines in health and/or functionality.
Likewise, presenting evidence of diminished health and/or functionality can lead older adults to begin receiving assistance, either informally from family members or formally from home health professionals. Extant sociological literature suggests that while people often experience negative feelings related to loss of independence when entering into a caregiving arrangement, receiving care from other people can also exert significant positive impacts on QoL. Researchers in a variety of disciplines have documented how living with any type of persistent illness can produce feelings of isolation and loneliness, both of which negatively affect QoL. These feelings can become particularly pronounced for people who experience a level of functional disability substantial enough to make living alone difficult, but not severe enough to warrant seeking outside assistance. Taken together, these trends may explain why a significant association between CRP levels and QoL outcomes did not emerge for NSHAP participants with CRP levels in excess of 6 mg/L.
As noted in the results section, visual analysis of NSHAP data revealed nuances in the relationships between specific levels of CRP and the odds of reporting high QoL across both outcome measures. While overall correlations between CRP and QoL were consistently negative and clear trendlines emerged, the curves of best fit for these data did not entirely resemble straight lines. This may owe in part to measurement considerations, especially the fact that while higher levels of CRP do predict chronic inflammation in an overall population, specific individuals may have levels significantly higher than average without experiencing significant pathology.
The higher average levels of CRP found among female-bodied NSHAP participants may speak to this phenomenon. As noted in the literature review, females tend to have higher average levels of inflammatory biomarkers regardless of disease status when compared to their male peers. However, newer research suggests that higher levels of CRP in females may owe strongly to underlying social inequalities that impact females disproportionately [
61].
Differences in average levels of CRP among subgroups within the full NSHAP sample may also stem from sociomedical phenomena related to the lived experience of chronicity at particular levels of inflammatory pathology. Across different levels of disease pathology, people's expectations for their health and resultant QoL may differ. Extant literature shows strong associations between cognitive dissonance—the mental anguish people experience when reality differs unfavorably from expectations—and reduced QoL among people who develop chronic conditions, including inflammatory diseases.
The fact that social structure factors did not confound observed relationships between CRP levels and either QoL outcome indicates that these forces may exert other types of influences. Fundamental causation [
77] of chronic inflammation by structural disadvantage is one of the most likely ways in which these influences may operate. Indeed, previous research using the NSHAP [
62] supports conceptualizing chronic inflammation as a socially conditioned outcome. Likewise, a variety of other studies have shown strong associations between structural disadvantage and the onset of different chronic inflammatory conditions [
59],[
78],[
79]. Future research should thus analyze inflammatory biomarkers themselves as potential mediators of relationships between social disadvantage and QoL.
Strengths of this study include background and perspectives from multiple disciplines, parsimonious model specifications, and use of data on older adults. The interdisciplinary approach used in this research helps to bridge gaps in prior work on inflammation, health, social life, and QoL. As such inquiry into chronic inflammation's potential effects on QoL remains relatively new, parsimonious models were used to guard against reporting of significant associations in cases where none were present. The choice of NSHAP as a data source, and specifically the advanced ages of its participants, allowed for insight into potential cumulation of disadvantage in QoL. This advantage persisted despite the limited number of waves currently available in the NSHAP.
Limitations of this study include lack of longitudinal data, challenges with data capture in the NSHAP, and narrow generalizability. Two specific factors limiting generalizability are the advanced ages of participants and relatively high average levels of QoL among these individuals. Only two waves of NSHAP data are currently available, so the potential for using panel designs to study these phenomena remains limited. NSHAP researchers also experienced problems with certain measurements in the dataset, including the CRP assay. Although this study accounts for potentially problematic CRP values via stratification, it is nonetheless possible that some values in the focal range were also erroneous. Using data on older adults confers drawbacks in addition to the advantages noted above. Specifically, the relatively small range of ages captured by the NSHAP restricts the generalizability of these findings to late midlife and elderly adulthood. Similar processes may occur in children, adolescents, and younger adults without manifesting in the same ways.
Irrespective of the specific strengths and limitations of a particular study, it should also be noted that no research can unilaterally confirm or refute the potential influence of chronic inflammation on QoL. Rather, the statistical significance and reliability of results from this study lend support to the idea of inflammatory biomarkers as meaningful correlates of QoL. These findings likewise recommend further investigation of the topic in both community and clinical settings.
In addition to their consistent statistical significance, these findings may indeed possess some clinical significance. Specifically, the potential for a 1 mg/L difference in CRP levels to produce 8 to 10 percent lower odds of reporting high levels of QoL has important implications for people's everyday lives. A person with severe inflammatory disease is likely to experience substantially elevated levels of CRP within a certain range [
62]. Stratifying regression analyses using a cutoff value of 6 mg/L of CRP allowed for examination of variability across the vast majority of the analytic sample. For each computed model, the size of the population group with CRP serolevels between 0 and 6 mg/L exceeded that of its counterpart with levels in excess of 6 mg/L by approximately a factor of 8.