Are there socioeconomic differences in responses to a commonly used self report measure of chronic illness?

Background: Single questions on self-reported morbidity are commonly used in social or health surveys. It has been suggested that these may underestimate socioeconomic gradients in health because more disadvantaged social groups may have higher thresholds for defining illness. Method Face-to-face interviews by research nurses with community-based respondents in the West of Scotland, using a specially designed suite of prompts following up on responses to the UK General Household Survey (GHS) long-standing illness question. Participants were 858 respondents born in the early 1930s and 852 respondents born in the early 1950s (mean age at interview 59 and 40, respectively) classified by occupational social class and area deprivation.
Results: Adjusted for age and sex, the Relative Index of Inequality (RII) for reporting any condition in response to the GHS question was 2.14 (95% CIs 1.49-3.08) for social class and 2.01 (1.41-2.87) for Depcat. Among those not reporting any conditions to the GHS question, the RII for reporting conditions to any further prompts was 1.54 (0.87-2.70) for social class and 0.86 (0.50-1.46) for Depcat. The RIIs for reporting any condition after the initial question and all prompts were 2.16 (1.40-3.33) for social class and 1.50 (0.98-2.29) for Depcat. Across a range of conditions defined as more serious, and conditions classified by different ICD categories, socioeconomic status (SES) gradients after the initial question and all prompts were similar to, or less steep than, those produced by the GHS question alone.
Conclusions: These data do not support the hypothesis that poorer social groups are more stoical and more likely to need detailed prompting in order to elicit morbidity. Nor do they support the hypothesis that SES gradients in morbidity are underestimated by using the GHS question rather than more detailed questioning. This suggests that responses to this type of question can be used in epidemiology and health needs assessment without major socioeconomic bias.