Hidradenitis Suppurativa Epidemiology

Prevalence and Incidence

Prevalence and incidence are the most common metrics used to assess the burden of disease in a population. Prevalence assesses the proportion of a population with a disease over a point in time. In contrast, incidence is the rate of new, or newly diagnosed, cases of a disease over a period of time (typically 1 year). Taken together, incidence and prevalence can show the rate at which a disease is being diagnosed and the duration of time that patients carry the diagnosis. The ability to determine these statistics is affected by data source, coding, and other biases.

Estimates of the overall prevalence of HS range from 0.03% to 4% with a likely prevalence of around 1% ( Table 2.1 ). Several types of datasets such as those from patient-facing surveys, cohort, and population-based studies have noted differences in prevalence that may be attributed to study design or regional study population differences. Typically, the threshold of 47 cases per 100,000 people or below is defined “rare.” While HS has been classified as a rare disease in the past, increasing data suggest that it should no longer be considered rare.

Large cohort studies using claims databases or billing codes may underestimate prevalence, as they only capture patients who seek medical care, while survey studies may report higher rates of prevalence, as patients with the disease may be more incentivized to respond. Several methods of analysis exist. Claims database studies use large databases derived from either the provider side or the insurer side. A population-based observational and case-control study using the U.K. Clinical Practice Research Datalink found an overall point prevalence of HS diagnosis to be 0.77%; however, when probable cases (individuals confirmed by primary care physician to have a history of between one and four skin boils in flexural regions) were also included, the prevalence rose to 1.19%. Some studies control for incorrect coding by performing a full or partial chart review to estimate the positive predictive value of each coded diagnosis. For example, a 2017 US population analysis used a chart-verified billing code model to estimate HS prevalence at 0.10% in the US population (98 per 100,000 persons).

Several smaller cohort studies have used dermatologist-rendered diagnosis instead of billing code diagnosis resulting in prevalence estimates ranging from 0.03 to 0.2% (see Table 2.1 ). Survey studies report a higher estimate of prevalence on average, with the notable caveat that survey validation may influence the resultant reporting. Studies performed in Denmark and Australia used previously validated HS surveys to report a general prevalence of 0.67–2.1%. Other survey studies, using unvalidated questionnaires, have reported prevalence ranging from 0.41% to 4%.

Breakdown of prevalence by disease severity is rarely reported, as it is difficult to ascertain through billing codes and patient surveys. However, one US analysis of predominantly white patients in Olmsted County, Minnesota, reported a distribution of Hurley stage as Hurley I 59.7%, Hurley II 38.1%, and Hurley III 2.2%. Smaller studies of HS patient cohorts have observed a higher burden of moderate to severe HS with over half of patients in Hurley stages II and III, and high-severity patients may be overrepresented in samples drawn from patients seeking care.

As awareness of the diagnosis rises, there may also be an increase in incidence. Only two studies have reported HS incidence (see Table 2.1 ). Through a US dataset, the 1-year incidence of HS in the United States was reported as 11.4 per 100,000 (0.01%), with the 10-year incidence as 8.6 per 100,000. In Germany, the incidence was estimated at 0.03%.

Demographic Factors

When evaluating the epidemiology of a disease, it is important to consider the factors that influence disease prevalence, as they may give insight into the types of populations more susceptible to the disease and help determine populations that should receive increased screening or for whom clinicians should have a higher baseline suspicion of disease. Prevalence of HS has been evaluated against different demographic factors and found to vary based on gender, age group, and race ( Table 2.2 ). Several studies examining this demography in more detail have provided insight as to the types of populations likely to contain the highest disease burden.