Objective
The purpose of this study was to determine whether there is an association between self-reported and biologic measures of stress in low-income, reproductive-age women.
Study Design
Between 1999 and 2005, randomly selected reproductive-age women from the 1998 welfare rolls in Chicago, IL, were interviewed yearly to assess psychosocial, socioeconomic, and health characteristics. The association of 2 stress-sensitive biomarkers (Epstein-Barr virus antibody titer (EBV) and C-reactive protein level) with self-reported stress was assessed.
Results
Of the 206 women who were interviewed, 205 women (99%) agreed to provide a blood sample. There was no difference in mean EBV or C-reactive protein levels based on age, race, parity, employment, marital status, or education. Women who reported a higher degree of perceived stress or reported experiences of discrimination had significantly higher levels of EBV ( P < .05).
Conclusion
Measures of self-reported psychosocial stress are associated with elevated levels EBV antibody in a low-income population of reproductive-age women.
Psychosocial stress has been proposed as a potential cause for adverse pregnancy outcomes, such as preterm delivery. Three biologically plausible physiologic pathways that link stress and preterm birth have been proposed. First, stress is known to activate the hypothalamic-pituitary-adrenal axis stress response, which results in a hormone-mediated cytokine release that stimulates the myometrium. Second, stress, through increased glucocorticoid production, inhibits immune function and increases susceptibility to infection-mediated labor. Third, chronic stress has been reported to up-regulate the inflammatory response that leads to a chronic proinflammatory state.
The epidemiologic evidence that links stress and poor pregnancy outcomes has been intriguing, although somewhat inconsistent. Although several studies have shown a link between maternal self-reported stress and low birthweight or preterm birth, other studies have failed to demonstrate an association. Similarly, it also has been hypothesized that maternal stress may play a role in explaining the racial disparities that are seen in preterm birth rates in the United States. Yet, the available data have not consistently established disparities in stress as the underlying cause for disparities in preterm birth.
Several factors may explain the inconsistent associations that have been reported between maternal stress and preterm birth. Much of the research that has been conducted to date relies heavily on self-report measures of stress. In addition, numerous measures of maternal stress have been used, with little consistency in the operational definitions across studies. Perhaps most importantly, existing research largely has focused on acute stress with little emphasis on chronic stress. This is important because chronic stress is a biologically plausible mechanism that links stress with preterm birth.
Epstein-Barr virus (EBV) antibody and C-reactive protein (CRP) are 2 examples of biomarkers that have been associated positively with chronic stress. CRP is an acute phase protein that is produced in the liver in response to proinflammatory cytokine stimulation. Previous research has found that perceived stress is an independent predictor of CRP, which suggests that inflammation may be an important pathway through which psychosocial factors increase the risk of poor health outcomes. Elevated CRP has also been associated with self-reported stress, fewer economic resources, chronic fear, and work demands. EBV is a ubiquitous herpes virus that infects 80-90% of adults in industrialized countries. Once infected, the virus remains in a latent state; most of those adults who are infected remain asymptomatic for the rest of their lives. Adequate cell-mediated immune function is required to maintain EBV in its latent state. Immunosuppression allows the EBV to reactivate and release viral antigens, which causes the body to initiate an immunoglobulin G (IgG) antibody response. Increased levels of EBV IgG antibodies therefore provide an indirect measure of an important aspect of cell-mediated immune function and a measure of immune dysfunction that is associated with chronic stress. Elevated EBV antibody titers have been associated with a wide range of chronic stressors.
The association of EBV and CRP with self-reported chronic stress has not been investigated in reproductive-aged women. Correspondingly, the objective of this study was to assess the association between self-reported stress and 2 biologic measures that are sensitive to chronic stress in a cohort of low-income, reproductive-aged women.
Materials and Methods
The Illinois Family Study (IFS) is a longitudinal, cohort study that was designed to assess the effects of welfare reform on families in Illinois who were receiving Temporary Assistance for Needy Families (TANF) in 1998. The complete methods for this study have been described elsewhere. In brief, the sample population consisted of 1899 randomly selected TANF grantees who, between 1999 and 2005, completed a yearly interview that included demographic information and questions that were related to psychosocial stress. The survey was administered in each participant’s home by a nonmedical, community-based interviewer.
The IFS sample represents a stratified random sampling design that is based on data from 2 geographic regions of the state of Illinois: Cook County and the remainder of the state. Nine Illinois counties were selected for the IFS study, which represented 80% of the TANF caseload in June 1998. The total TANF caseload at this time was 122,720. The 9 counties that were chosen as the sampling frame represented 92,893 TANF grantees.
Approximately 950 TANF grantees were selected randomly per stratum (for a total of 1899 eligible TANF grantees). Within each stratum, a systematic sample with a random start was selected from the grantee populations. To achieve greater precision in the sample results, the grantees were sorted by various demographic and service variables (including race/ethnicity, marital status, age, and duration of TANF receipt) before sampling from each stratum to achieve “implicit” stratification of the sample.
The sample for the current analysis included a complete sample of the IFS participants from Cook County in 2005. At the time of their yearly interview in 2005, IFS study participants answered several additional survey questions regarding self-reported stress and provided a blood specimen. Given the community setting of the IFS study, blood specimen collection was conducted in each participant’s home. Specifically, IFS interviewers were trained to collect a dried blood spot specimen from a finger stick. The blood spots were collected on standardized filter paper that commonly is used for neonatal screening (no. 903; Schleicher and Schuell, Keene, NH), were allowed to dry for 4 hours, and then were returned to and stored in a central laboratory at –30°C until batched analysis.
Blood spots were analyzed for CRP and EBV antibodies with the use of highly sensitive standardized enzyme immunoassay protocols that were developed for use with dried blood spot samples. After discs of whole blood were punched from the dried filter paper cards and eluted overnight, the eluate was added in duplicate to microtiter wells that had been precoated with capture antibody. The quantity of CRP in each sample was determined on the basis of a comparison with calibrators that had been standardized against the World Health Organization International Reference Preparation for CRP (X0923; Dako Corp, Carpinteria, CA). The lower limit of detection was 0.028 mg/L; samples above the upper limit of detection (10 mg/L) were diluted and measured again, and the result was multiplied by the dilution factor. Low, mid, and high control samples were included with each assay; the percent coefficient of variation among assays was 10.4, 9.4, and 7.4, respectively. The EBV antibody protocol was based on an adaptation of a commercially available enzyme immunoassay kit for measuring EBV viral capsid antigen IgG antibodies in serum (# P00160 6A; DiaSorin, Vercelli, Italy). The assay directly quantifies IgG antibodies against the p18 polypeptide, which is a marker protein that contains the immunodominant epitopes of the viral capsid antigen complex. Discs of whole blood were eluted overnight, and the eluate was added in duplicate to microtiter wells that had been precoated with antigen. Sample values were determined on the basis of a comparison with calibrator material that was provided with the kit and reported in enzyme-linked immunosorbent assay (ELISA) units. Low, mid, and high control samples were included with each assay; the percent coefficient of variation between assays was 10.0, 7.4, and 10.7, respectively. In the present analysis, no samples were below the assay range; samples that were above the highest calibrator were diluted and assayed again, and the result was multiplied by the dilution factor. IgG antibodies were used to measure reactivation of latent EBV (characterized by an IgG response), rather than primary EBV exposure (characterized by high IgM response, with little IgG). Reactivation of EBV is thought to be the result of deficits in cell-mediated immunity that may be related to chronic stress. Previous validation studies indicate a high degree of correlation between results that are obtained from paired blood spot and serum samples for both CRP and EBV antibodies.
Psychosocial factors were grouped into 4 categories, based on a previously reported conceptual model of chronic stress. This model posits 4 domains that are particularly relevant to understanding the actual internal stress experienced by an individual: (1) external stressors (life events, hardships in the home environment, hardships obtaining food, hardships obtaining medical care, having a child with chronic illness in the home, and home crowdedness); (2) enhancers of stress (depression, mental health, and drug or alcohol use); (3) buffers against stress (social support, community group involvement, and coping skills); and (4) perceptions of stress (perceived economic hardship, self-rated health, perceived neighborhood safety, and discrimination). The questions and validated scales from the IFS questionnaire are shown in Table 1 .
