Participants and procedure

One hundred and eighteen participants were recruited between November 2022 and 2023 through targeted online media advertisements on Instagram and Facebook. These advertisements were aimed at individuals who worried about their health or identified as having health anxiety, using messages such as “Are you suffering from health anxiety?” and “We are seeking research participants who experience persistent and frequent worries about their health…”. Interested individuals were directed to the Black Dog Institute website, which outlined the eligibility criteria and provided a link to a Qualtrics (2022) questionnaire. The questionnaire assessed eligibility, and eligible participants received an electronic participant information statement and consent form. Eligible participants were aged over 18, Australian residents, fluent in English, and self-identified as experiencing persistent and frequent concerns and worries about their health. The study received approval from the University of New South Wales Human Research Ethics Committee (HC220649), and all participants provided electronic informed consent to participate.

After providing consent, participants completed a 45-minute online questionnaire and were invited to participate in an online interview via Zoom Videoconferencing (2016), ranging between 15–60 minutes in duration. The purpose of the interview was to assess diagnostic status for IAD, SSD and comorbid disorders including generalized anxiety disorder, obsessive and compulsive disorder, panic disorder, agoraphobia, and major depressive disorder. Participants were reimbursed at a rate of $35 per hour through an electronic gift-card upon the completion of both the survey and interview.

Measures

Statistical analyses

Sample demographic characteristics and associations between DSM-5 IAD and SSD

Demographic results for the entire sample can be found in S1 Table, whereas demographic information for IAD and SSD are presented in Table 1. The majority of the sample identified as female (n = 99, 83.9%), had a mean age of 41 (range = 18–72 years, SD = 12.4), and were of Australian ethnicity (n = 74, 69.2%) and born in Australia (n = 86, 72.9%). The sample primarily spoke English at home (n = 105, 89%) and resided in major cities in Australia (n = 89, 75.4%). Approximately half of the sample were in a de facto or married relationship (n = 58, 49.2%), and more than a half were employed part-time or full-time (n = 76, 64.4%) and had completed an undergraduate degree (n = 47, 39.8%) or postgraduate degree (n = 32, 27.1%). Most of the sample (n = 105, 89%) had sought treatment for their mental health in the past, with just over a half (n = 67, 56.8%) currently receiving mental health treatment. Of those currently receiving treatment, 60.8% (n = 45) were undergoing therapy with a psychologist, 60.8% (n = 45) were taking medication, and 45.9% (n = 34) were receiving mental health support from their general practitioner.

Prevalence of DSM-5 IAD and SSD

Inter-rater reliability was assessed for IAD, SSD, and comorbid disorders. Kappa estimates between the two raters were 0.96 for IAD, 0.88 for SSD. Discrepancies for IAD and SSD arose from disagreements regarding the severity of somatic symptoms. Inter-rater agreement for other comorbid diagnoses was 100% except for obsessive compulsive disorder (kappa = 0.73).

Approximately one in four participants met criteria for IAD (n = 28, 23.7%), while just under 40% met criteria for SSD (n = 47, 39.8%). Comorbid IAD and SSD were present in 11 participants (9.3%), and 32 participants (27.1%) entered the study with self-identified health anxiety but did not meet criteria for either IAD (current or modified) or SSD, which was mostly due to participants not meeting the 6-month duration criteria for the disorders.

Among participants meeting IAD criteria, the majority self-identified as fluctuating between seeking and avoiding medical care (n = 20, 71.4%), followed by 7 (25%) indicating they were care-seeking, and 1 (3.6%) indicating they were care-avoidant, as illustrated in Table 2. Please note that we do not report the care-seeking and care-avoidance results for SSD, as SSD in the DSM-5 does not contain these subtypes.

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Table 2. Nature and course of health anxiety of individuals with IAD and SSD.

https://doi.org/10.1371/journal.pone.0342493.t002

Participants meeting criteria for SSD differed from those with IAD in terms of significantly lower levels of fulltime employment, higher levels of casual work, more time off work, retirement and disability pension, greater past use of medication for mental health treatment, and recipients of greater levels of current psychological therapy sessions.

Illness onset, nature and course of health anxiety

Illness nature and course for the overall sample are depicted in S2 Table and in Table 2 for IAD and SSD. Overall, the average age of health anxiety onset was 24 years (range 4–65; median 39). The majority of the sample (n = 99, 83.9%) reported fears related to multiple illnesses, while 16.1% (n = 19) reported fearing the same illness. The most frequently reported feared illnesses included cancer, multiple sclerosis, and heart disease. More than a half of the participants (n = 65, 55.1%) reported experiencing greater than 7 distinct episodes of health anxiety lasting two or more weeks in their lifetime, and 56.8% (n = 67) reported feeling anxious about their health for more than four years in their lifetime. Comparisons between participants meeting criteria for IAD and SSD showed no differences in terms of illness onset and course.

Symptom severity, quality of life, and health care service utilization

As outlined in Table 3, the IAD and SSD groups did not show significant differences on health anxiety severity (d = 0.01), depression severity (d = 0.45), generalized anxiety (d = 0.19), general quality of life (d = 0.27) and quality of life related to mental health (d = 0.12). Participants that met SSD criteria reported significantly higher levels of somatic symptoms (d = 0.56) and lower quality of life related to physical health compared to participants that met IAD criteria (d = 1.1). Participants with SSD reported significantly more health professional appointments in the past three months (d = 0.75), including more general practitioner appointments (d = 0.55) and psychologist appointments (d = 0.59) compared to participants meeting IAD criteria. However, there were no significant differences between groups in the number of psychiatrist appointments (d = 0.26), medical specialist appointments (d = 0.46), and other health practitioner appointments (i.e., physiotherapy, massage therapist, and chiropractors) (d = 0.47). Please note, due to an experimenter error three participants in the total sample did not complete the health service utilization section of the survey, with two being in the SSD group, bringing the total sample to 115 and SSD sample to 45 for this section. The symptom severity, quality of life, and health care service utilization results for the overall sample can be found in S3 Table.

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Table 3. Symptom severity, quality of life, and service utilization in individuals with IAD and SSD.

https://doi.org/10.1371/journal.pone.0342493.t003

Mental and physical comorbidities

Mental and physical health comorbidities are presented in Table 4. Fifteen participants (53.6%) with IAD and 74 participants with SSD (62.7%) met criteria for other DSM-5 diagnoses that were assessed. There were no group differences between IAD and SSD groups in terms of the proportion of those experiencing comorbid mental disorders. Participants meeting SSD criteria experienced muscular-skeletal disorders, back problems, and other conditions (i.e., bowel disease, eye conditions, and lung conditions) significantly more than participants meeting IAD criteria, but the groups did not differ on any of the other chronic health conditions that were assessed. Mental and physical comorbidities for the entire samples are reported in S4 Table.

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Table 4. Health anxiety comorbidities with other mental health disorders and physical health conditions.

https://doi.org/10.1371/journal.pone.0342493.t004

Discussion

The present study is the first to examine the features and characteristics of IAD and SSD in individuals drawn from the community who self-identified as experiencing health anxiety. We compared the two DSM-5 disorders on demographic and clinical characteristics, mental health comorbidities and health service use. Overall, we found limited differences between IAD and SSD in people with health anxiety, with differences primarily being related to severity. These results suggest that individuals with health anxiety who meet SSD criteria appear to be more severe in their presentation.

Similarities and differences between IAD and SSD

Consistent with our previous research in a treatment-seeking sample who participated in a clinical trial for health anxiety treatment [6], we found no significant differences between IAD and SSD on most demographic characteristics and the nature and course of health anxiety, including number of health anxiety episodes, lifetime duration, subtypes and illness fears. However, we found that SSD was associated with a greater number of chronic health conditions compared to IAD. In contrast to our previous work, SSD was also associated with lower rates of full-time employment and greater medication use and therapy with psychologists than IAD. Given that individuals with SSD are debilitated by moderate-to-severe somatic symptoms, we speculate that the debilitating nature of these physical symptoms may prevent them from engaging in greater hours of work. However, it is unknown why SSD is associated with more medication use and therapy with psychologists. Although we did not find statistically significant differences between IAD and SSD on mental health comorbidities, numerically SSD had greater mental health comorbidities than IAD. Thus, it is possible that individuals with SSD need more psychological support and medication to manage these comorbidities.

In our sample, participants with health anxiety who met criteria for SSD had greater somatic symptom severity and poorer quality of life related to their physical health compared to participants with IAD, with medium to large effect sizes. These findings align with the broader literature on SSD indicating an association between SSD and reduced physical quality of life [18–20]. Past research in health-anxious samples has also found SSD to be significantly more comorbid with generalized anxiety disorder, major depressive disorder, panic disorder and agoraphobia, and associated with higher levels of health anxiety severity, depression severity, and functional impairment compared to IAD, with medium to large effect sizes [6, 7]. In contrast to this previous research, in the current sample of people from the community with health anxiety, we found non-significant small to medium differences between IAD and SSD on these variables. The reasons underlying these discrepancies are unclear but may stem from sampling differences, as our study recruited health-anxious participants from the community, while past research recruited participants signing up to a treatment trial. It is possible that there may have been differences in severity or other factors which may have influenced the findings. Notably, however, we observed a numerical difference in depression severity, with greater depression severity in SSD that was approaching (but did not reach) statistical significance, indicating that our sample may have been underpowered to detect these differences between IAD and SSD.

Investigating healthcare utilization in individuals with health anxiety is important for developing targeted interventions that reduce unnecessary healthcare visits and promote symptom self-management when appropriate. Consistent with previous research [6], our results showed that participants who experienced health anxiety and met SSD criteria reported a greater total number of appointments with health professionals, as well as more visits to general practitioners and psychologists in the past three months compared to participants with IAD, with medium to large effect sizes. In addition, Newby et al. [6] found that SSD was associated with more frequent visits to psychiatrists and other health professionals, with a medium effect size. These findings may suggest that higher somatic symptom severity in SSD may prompt individuals to seek more medical care than those with no or milder somatic symptoms, such as in IAD. Although speculative, interventions aimed at reducing or managing somatic symptoms could help reduce unnecessary healthcare visits, thereby lessening the burden on both the individuals and the healthcare system.

Illness anxiety nature, course, and subtypes

Very few studies have examined the nature of the newly identified IAD diagnosis in DSM-5 [21]. This study replicates our previous research in treatment-trial samples [6], and shows that health-anxious individuals drawn from the community experience IAD in a long and episodic course, fear multiple illnesses, and begin worrying about their health in early adulthood. Similarly, we found that the IAD care-avoidant subtype was rare, the care-seeking subtype was more common, and, interestingly, the majority of our sample fluctuated between seeking and avoiding medical care. These findings warrant further investigation into the features and characteristics of these subtypes, including why some individuals with IAD seek while others avoid medical care, as well as what prompts an individual to fluctuate between the two (i.e., a specific period in their lives, an unexpected somatic symptom, or their health anxiety severity). Such findings would call into question the necessity of categorizing individuals with IAD into two different subtypes, as they might be better suited as behavioral symptoms of IAD.

Validity of somatic symptom severity criteria in IAD

There has been criticism regarding the clinical utility of the somatic symptom criterion (Criterion B) in DSM-5 IAD, particularly as clinical observations indicate that individuals with IAD can present with moderate-to-severe somatic symptoms [9]. This study is the first to test the validity of Criterion B by directly comparing individuals who meet current IAD criteria with those who meet modified IAD criteria (with moderate-to-severe somatic symptoms) across various demographic, clinical and behavioral characteristics. While some differences emerged (i.e., IAD modified group experienced greater casual employment history, past psychiatric care, poorer physical quality of life, more healthcare appointments than IAD current), the groups did not differ on many other demographic characteristics and clinical characteristics (i.e., illness anxiety course, symptom severity for generalized anxiety, health anxiety and depression, mental health comorbidities and chronic health conditions). This suggests that the presence or severity of somatic symptoms may be less important for clinicians when diagnosing an individual with IAD. More research is needed to examine the course and treatment response of IAD with or without somatic symptoms in various samples (i.e., treatment-seeking, primary and secondary care), as well as the role of differential DSM diagnostic criteria, such as Criterion F, in distinguishing IAD from related disorders including SSD.

Limitations

The present study has several limitations that need to be addressed. First, although participants were drawn from the community rather than as a treatment-seeking sample, the majority (90%) had sought treatment in the past for their general mental health concerns. Therefore, it is unclear whether our results would generalize to self-identified health-anxious individuals in the community who had not previously sought mental health treatment. We also did not ask our participants whether they sought treatment specifically for their health anxiety or for other mental health concerns in the past. In addition, our recruitment strategy required participants to self-identify with health anxiety, which may have introduced sampling bias and further limited the generalizability of our findings to the broader clinical population. The study was also only powered to detect large differences and underpowered to detect small-to-moderate between-group differences, meaning that we may not have detected differences of this size in our study. In addition, multiple comparison corrections were not applied to the analyses, as they were not specified prior to the analysis. Further, there were high levels of comorbidity between IAD and SSD, illustrating limitations of the current DSM-5. However, comorbidity between IAD and SSD was not reported at an individual level, as this was not the primary focus of the current study. High levels of comorbidity between SSD and IAD modified were also observed and should be considered when interpreting between-group comparisons. Finally, the cross-sectional nature of our study precludes conclusions about how IAD and SSD evolve and change over time. A longitudinal study would offer valuable insights into the progression and trajectories of these disorders.

Clinical implications

The findings from this study have important implications for diagnosis, assessment and treatment of health anxiety. First, our results indicate that IAD is less common than SSD, suggesting that clinicians are more likely to encounter health-anxious individuals with moderate-to-severe somatic symptoms. Second, given the minimal differences observed between IAD and SSD, it may be appropriate for clinicians to consider a broader diagnosis for health-anxious individuals that is not constrained by somatic symptom severity. This would better reflect real-world clinical presentations, in which health anxiety can present across somatic symptom severity levels.

From a treatment perspective, cognitive–behavioral therapy (CBT) remains a well-supported intervention for health-anxious individuals [22], with past research showing that both IAD and SSD respond similarly to CBT [23]. Therefore, regardless of diagnostic label or somatic symptom severity, CBT appears to be an appropriate treatment option when health anxiety is present. However, for individuals with SSD who experience persistent and debilitating somatic symptoms in the absence of significant health anxiety (criterion B2), other interventions which target somatic symptom burden (i.e., drawn from the research into treatments for functional somatic symptoms) may be more appropriate [24, 25].

Finally, the findings from this study highlight the limitations of the categorical diagnostic model (i.e., DSM-5), which presumes discrete boundaries between disorders and fails to adequately capture the continuum of health anxiety presentations [26]. However, dimensional models, such as the Research Domain Criteria (RDoC) framework, offer a potential alternative by focusing on underlying neurobiological (e.g., brain functioning, genetics, emotional responses) and behavioral (e.g., family environment, social and cultural context) processes [27]. In the context of health anxiety, this approach would better capture the heterogeneity of health anxiety presentations and provide a more nuanced understanding of symptoms, such as excessive checking behaviors and online health searches, and persistent rumination about physical sensations and their meanings.