Recent regional and insurance database studies indicate that diagnoses of hidradenitis suppurativa (HS) are rare, with fewer than 200,000 affected patients in the United States. These findings are at odds with the generally accepted prevalence of approximately 1%. To estimate the number of patients diagnosed with HS, including patients enrolled in public health plans and the uninsured, we performed an investigation using National Center for Health Statistics databases of visits to health care providers. Survey data from physician offices and clinics, hospital outpatient departments, and emergency departments in the United States were analyzed. There were 254,000 (95% confidence interval: 180,000, 328,000) health care visits by patients with HS per year from 2002 to 2010. An annualized estimate of 118,000 (95% confidence interval: 83,000, 152,000) patients with HS diagnoses was determined. Excluding emergency department data, the annualized estimate was 100,000 (95% confidence interval: 67,000, 134,000) patients. Adults between 18 and 64 years of age comprised 92.8% (standard error, 2.6%) of HS patients. Of that working-age population, 31.5% (standard error, 5.6%) were enrolled in Medicare or Medicaid, which is a significantly higher proportion than for those in the same age group in the general population. A review of earlier surveys of HS prevalence was performed. HS is a rarely diagnosed disease, and epidemiologic data to support undiagnosed HS as a common disease are lacking.
Hidradenitis suppurativa (HS) is an inflammatory follicular skin disease that is diagnosed by its clinical features and its chronic, recurring nature. Its prevalence is widely cited as approximately 1%. HS has a substantial negative effect on quality of life and can be disabling. Recently, 2 important epidemiologic studies of this disease were published. Vazquez et al. analyzed medical records from Olmsted County, Minnesota, to find an age- and sex-adjusted annual incidence of 9.6 per 100,000 person-years from 2000 to 2008 and concluded that HS may be a rare disease. From an analysis of commercial health insurance reimbursement data for 2007 from a large group of patients in the United States, Cosmatos et al. determined a prevalence of 0.053% for clinically detected HS, or an estimated 146,000–162,000 patients. The Rare Diseases Act of 2002 defines a rare disease as any disease that affects fewer than 200,000 persons in the United States.
Earlier studies that placed the prevalence of HS at 1% relied on self-reporting using questionnaires or screening interviews. In 2005, Revuz et al. surveyed a sample of 10,000 people representative of the French population and found that 1% of respondents indicated that they had HS within the previous year. This finding is in agreement with the 1-year prevalence determined in a survey of a representative Danish population. A validation study of these questionnaires has been reported. The Office of Rare Diseases Research of the National Institutes of Health does not classify HS as a rare disease.
For a better understanding of the epidemiology of HS, national survey databases compiled by the National Center for Health Statistics (NCHS) of the Centers for Disease Control and Prevention (Atlanta, Georgia) have been used here to provide estimates of health care utilization not limited to patients with private insurance or to a single region of the country. The National Ambulatory Medical Care Survey (NAMCS) contains data from patient visits to physician offices, and the National Hospital Ambulatory Medical Care Survey (NHAMCS) contains data from visits to hospital outpatient departments (OPDs) and emergency departments (EDs). The use of this national database provides new insights into the demographic characteristics of HS patients, the health care they receive, and the disability associated with their condition.
NAMCS and NHAMCS are event-based, rather than population-based, surveys, and are designed to provide estimates of the number of patient-physician encounters, or visits. Detailed documentation for the NAMCS and NHAMCS surveys are provided by the NCHS. NAMCS microdata files are derived annually by the NCHS from patient record forms documenting visits by patients to physicians at private offices and clinics. Annual NHAMCS data are divided into OPD and ED files. NHAMCS OPD files are organized similarly to NAMCS files, with many identical or similar variables.
NAMCS and NHAMCS data beginning in 2002 include sample design variables intended for use with software using ultimate cluster variance methods. Visit weights are included in each record in the public microdata file, so that national estimates of patient visits can be obtained from the number of records for selected variables. The most recent available public data are for visits in 2010. For the analyses performed here, the annual NAMCS, NHAMCS OPD, and NHAMCS ED microdata files from 2002 to 2010 were merged to produce a single file with records from physician offices and hospital OPDs and EDs.
Patient visits for treatment of HS were identified within the merged file by selecting records in which any of the 3 listed diagnoses was for HS, as identified by the International Classification of Diseases, Ninth Revision, Clinical Modification, code 705.83. The first listed diagnosis is the physician’s primary diagnosis for the visit.
The patient’s primary expected payment source for a visit is recorded in the data file on the basis of a hierarchy of expected sources of payment, with Medicare or Medicaid being first, followed by workers compensation, private insurance, self-payment, and no charge. Here, the variables for the primary expected payment source listed for each visit were combined and recoded into the following 3 groups: public (combining Medicare and Medicaid), private (private insurance and workers compensation), and uninsured (self-pay, no charge, other, unknown, or blank). Thus, if a patient had both Medicare and private insurance, the visit would be coded to indicate the primary expected payment source was public insurance. Patient age was recoded into 4 groups. The original race categories of black, white, and other were recoded as black or white/other, because the number of records with HS diagnosis and race category of “other” was too small for analysis. These characteristics of patients at visits recording a diagnosis of HS were compared with those of patients in records of visits with any diagnosis within the merged data file.
The method of Burt and Hing was used to estimate the number of patients with HS. The visit weight variable in the microdata file was divided by the total number of visits within the previous 12 months to the same physician to yield a patient weight variable. Use of the method of Burt and Hing will overestimate patient numbers when patients see more than 1 physician. However, when using records for visits in which a single diagnosis (e.g., HS) is clinically relevant, overestimation is less likely because most patients will have a single physician treating a particular condition in a given year. Estimation of the number of patients with HS was performed using NAMCS and NHAMCS OPD data only, as well as with the inclusion of NHAMCS ED data. For ED records before 2007 that did not include numbers of past visits, it was assumed that there were no other ED visits during the past 12 months. Patient numbers estimated without ED data are lower limits, because they exclude those diagnosed with HS who were seen only in the ED. Numbers estimated with ED data are upper limits, because a patient who is seen in the ED and at an office in the same 12-month period may be counted twice. Also, assuming no prior ED visits when ED patient record forms do not include the past visit variable will lead to overestimation if a patient has made multiple ED visits. However, because EDs frequently see patients with HS, and because some uninsured patients may obtain treatment only at EDs, that setting is considered in the analysis.
When analyzing the merged file containing all records of patient visits with any diagnosis, the method of Burt and Hing is inaccurate because of the likelihood that patients will have more than 1 physician. To compare the HS patient population to the population of all persons who visited a physician within a 12-month period from 2002 to 2010, we estimated the latter using National Health Interview Survey databases. The use of NHIS databases is described in the Web Appendix available at http://aje.oxfordjournals.org/.
Treatments prescribed or performed at visits in which there was a diagnosis of HS were analyzed. In survey years 2006–2010, the medication variables included codes from the Cerner Multum Lexicon Plus therapeutic classification system (Cerner Multum, Inc., Denver, Colorado). For each medication, which may be a single- or multiple-ingredient drug, the Cerner Multum Lexicon lists up to 4 therapeutic categories with up to 3 levels of increasing detail per category. In survey years 2002–2005, medications had been coded using the US Food and Drug Administration’s National Drug Code Directory. For the analysis performed here, the listed medications for those years were recoded according to the Cerner Multum system. Estimates were then made for systemic antibiotics (i.e., drug categories 008-018, 240, and 315), systemic analgesics (i.e, drug category 058), biological agents (i.e., drug category 028), and topical dermatological agents (i.e., drug category 136) among any of the 8 listed medications included in each visit record (6 listed medications in records in survey year 2002).
The number of visits with an HS diagnosis in which at least 1 procedure was performed was estimated by selecting records for which an excision or incision and drainage was indicated on the patient record form, or the International Classification of Diseases, Ninth Revision, Clinical Modification, code 85.21, 86.01, 86.04, 86.09, 86.22, or 86.30 was included among the listed procedures. Also included were records for which a wound care procedure or procedures coded 93.57 or 93.59 were indicated.
Analysis was performed using the Complex Samples Module of SPSS, version 21, software (IBM Corp., Somers, New York). Estimates are reported with standard errors. NCHS criteria (number of records n ≥ 30 and relative standard error ≤ 30%) for statistical reliability for estimates of visits were followed unless otherwise noted. Reweighting increases standard error; here, estimates of patient numbers were considered reliable if the number of records was 30 or more and relative standard error was 35% or less. P values were calculated from Rao-Scott Pearson χ2 test statistics using the Crosstabs function of SPSS when comparing within the merged NAMCS and NHAMCS file. When comparing those data to National Health Interview Survey data, we implemented the Pearson χ2 test as described in the Web Appendix. Significance was examined at the 0.05 level.
Of the 886,124 records of patient visits in the combined NAMCS and NHAMCS microdata file, 236 were identified with a diagnosis of HS within any of the 3 listed diagnoses. Estimates from records for single years did not meet statistical reliability criteria. All analyses were performed on the combined data for 2002–2010. The 236 records correspond to 2,289,000 (standard error (SE), 339,000) patient visits in the 9-year period and an annualized value of 254,000 (SE, 38,000) visits (95% confidence interval: 180,000, 328,000).
In all visits recording a diagnosis of HS, HS was the first-listed diagnosis in 82.4% (SE, 6.2%) (n = 185). Of these, in 67.3% (SE, 5.8%) (n = 122) there were no additional diagnoses listed. Of all visits with HS diagnosis, 77.8% (SE, 4.0%) (n = 61) occurred in physician offices, 14.2% (SE, 3.3%) (n = 122) in OPDs, and 8.0% (SE, 1.7%) (n = 53) in EDs. The physician was the patient’s primary care physician at 32.1% (SE, 7.9%) (n = 34) of visits in offices and OPDs. Of OPD visits, 78.1% (SE, 5.4%) (n = 44) and 18.4% (SE, 4.8%) (n = 69) were to general medicine and surgery departments, respectively. Visits with HS diagnosis were divided over several specialties at physician offices, such that estimates for any single specialty were unreliable. A total of 90.1% (SE, 4.0%) of all visits with an HS diagnosis involved a patient between the ages of 18 and 64 years.
Table 1 shows the distribution of patient visits according to patient sex, age, race, and primary expected source of payment for visits with a diagnosis of HS and for visits with any diagnosis. Results are shown for the analysis when including visits for patients of any age, as well as for only those visits in which the patient was between the ages of 18 and 64 years. The percentage of visits with HS diagnosis in which the patient was black was significantly higher than the percentage of all visits by black patients, regardless of the age group analyzed. The percentage of visits by female patients with HS was not significantly different from the percentage of all visits by women. In records of visits of patients aged 18–64 years, a highly significantly difference was seen in the primary expected payment source. A total of 37.3% (SE, 7.5%) of visits with a diagnosis of HS recorded a public health plan as the primary expected payment source compared with only 17.5% (SE, 0.4%) of all visits with patients in the working-age group. Visits with an HS diagnosis are less likely to involve patients over the age of 45 years when analysis includes all ages.
Procedures were listed in only 22% (SE, 6%) of visits with HS diagnosis. The number of records with procedures was insufficient for further analysis. The majority of medications prescribed at visits with HS diagnosis were systemic analgesics or antibiotics. Estimates for topical dermatological agents did not meet reliability criteria, and there were no records of biological medications. An antibiotic prescription was included in 54.1% (SE, 5.4%) of all visits. No single antibiotic or class of antibiotic predominated. Of all visits, 25.1% (SE, 5.3%) involved an analgesic prescription, 82.1% (SE, 6.7%) of which were for opioids. The most prescribed analgesics were oxycodone and hydrocodone.
There was no association between sex, age, race, or payment type with prescription of any medication (antibiotic, analgesic, or topical dermatological agent) or with prescription of an antibiotic. When considering analgesics, there was a significant association (P < 0.05) within each variable examined, with the exception of race. Male sex, age over 35 years, and primary expected source of payment other than private insurance were all associated with more analgesic prescriptions.
When the complete data file of NAMCS, NHAMCS OPD, and NHAMCS ED records was analyzed using patient weights instead of visit weights, the number of patients with a recorded diagnosis of HS from 2002 to 2010 was found to be 1,058,000 (SE, 157,000). On an annual basis, there were 118,000 (SE, 17,000) patients. When records of ED visits were excluded from the analysis, the annualized estimated number of patients with a HS diagnosis was 100,000 (SE, 17,000). Totals of 100,000 (SE, 17,000) and 118,000 (SE, 17,000) represent lower and upper limits, respectively, to the estimated number of diagnosed HS patients who visited a physician at a facility within the scope of the NAMCS and NHAMCS surveys.
Additional patient demographic characteristics are shown in Table 3 with the analyses performed with and without ED data. There is no significant difference in the distribution by sex when comparing patients with HS to all patients. Although most patients diagnosed with HS were not black, black patients were significantly overrepresented in the HS population when ED visits were considered. Adults between 18 and 64 years of age who were diagnosed with HS were more likely to have public health care as their expected source of payment than were all adults in this age range.
The analysis of NAMCS and NHAMCS data from 2002 to 2010 provides a lower limit of 100,000 (SE, 17,000) (95% confidence interval: 67,000, 134,000) and an upper limit of 118,000 (SE, 17,000) (95% confidence interval: 83,000, 152,000) to the estimated number of patients with a diagnosis of HS who visited a physician during a single year within that time period. This range, which includes patients with private insurance, public health care plans, and the uninsured, is lower than the range of 146,000–162,000 determined by Cosmatos et al.Those researchers obtained their estimates by multiplying the HS rate for age groups within the insured population by the number of persons in those age groups in the general population using census data. The accuracy of that method depends on the general population having the same characteristics as the insured population.
The reweighting method of Burt and Hing used here to obtain patient-level estimates from event-based surveys has been used by others and has shown agreement with results of population-based surveys and Medicare claims analyses. It has the limitation of not controlling for multiple visits by a patient to more than 1 physician. Also, to include ED data in the reweighted estimates, we needed to make assumptions about the number of past visits in survey years prior to 2007. These limitations are addressed here by reweighting only records of visits with diagnoses of HS and by identifying upper and lower limits to the estimates.
The present analysis of NCHS data provides the first overview of treatment patterns for HS in the United States. Treatment is fragmented over multiple specialties and is predominantly medical. Systemic antibiotics are the mainstay of treatment, but there is no single antibiotic or class of antibiotics that is prescribed most frequently.
A new finding is that a remarkably high proportion (31.5% (SE, 5.6%)) of HS patients of working age is covered by public health care plans. It can be inferred that many of these patients are covered as a result of disability. NAMCS and NHAMCS microdata files indicate a diagnosis of HS but not disease severity. However, there are significant differences in treatment between patients with private insurance and those with government insurance. Physicians were unlikely to prescribe an analgesic during a visit from a patient with private insurance, whereas half of patients enrolled in Medicare or Medicaid received analgesics, primarily opioids. Pain has been shown to be the most disabling factor in HS.
Given the rarity of an HS diagnosis among patients receiving medical treatment in this and the other recent database studies, the question remains as to whether HS is a rare disease. The present study has the limitation that people with HS who either did not visit a physician or who were undiagnosed would not have been counted. However, the number of diagnoses found here is so far below 1% of the general population that it is appropriate to reexamine the earlier epidemiologic data.
The Danish and French studies that established the generally accepted prevalence of 1% sampled populations that were demographically representative of their countries. Jemec et al. interviewed 585 persons and found 6 with a positive response to a screening question, although when subsequently examined, none had signs of disease. Revuz et al. sent questionnaires to 10,000 persons asking about the repeated presence of large painful nodules in the axillae or groin. Of those who responded, 1% replied positively. Esmann et al. tested the query used by Revuz et al. in a group of patients with HS matched with a control group of patients from a dermatology department. The sensitivity was reported to be 0.92, and the specificity was 0.86.
Given this specificity, the prevalence of HS determined from the survey of Revuz et al. will have overestimated the prevalence. The positive predictive value of a test depends on sensitivity and specificity, as well as the prevalence of the disease in the population being tested. For a sensitivity of 0.92, a specificity of 0.86, and an HS prevalence of 1%, the positive predictive value will be 0.062. The interview questionnaire used by Jemec et al. had a specificity of 0.85 according to Esmann et al., and the true positives would also have been far outnumbered by false positives.
The report by Jemec et al. describing the prevalence of HS in the general population of Denmark also described the examination of a sample of 507 patients at a sexually transmitted disease clinic. The point prevalence of HS was placed at 4.1% on the basis of the clinical findings of active HS and possible precursor lesions. There have been previous reports of HS diagnosed in large series of patients by physicians at dermatology clinics: Lookingbill found 1 patient with HS in a series of 1,106 (0.09%), and Mahe et al. found 3 patients in 10,575 (0.03%). The prevalence rates found in those studies, which were performed as inventories of all skin diseases and were not focused on HS, are of the same order of magnitude as the prevalence found by Cosmatos et al.. There is a positive family history in approximately 40% of HS cases, and prevalence may vary by area. Harrison et al. were prompted by the observation of a large number of referred patients with a diagnosis of HS to perform a retrospective analysis of all registered patients at 2 general practices in Wales. A total of 12,571 records were reviewed to identify all patients who had either been diagnosed with HS or who could have undiagnosed HS, and patients were reexamined. A total of 21 patients were determined to have HS, for a prevalence of 0.17%. Notably, a number of previously undiagnosed cases of HS were diagnosed on reexamination, but a similar number were found to have originally been diagnosed with HS in error. Criteria for HS diagnosis included either past or present disease, which may account for their prevalence being somewhat higher than that of others that counted only patients with current or past-year diagnoses of HS.
The recent studies by Vazquez et al. and Cosmatos et al. and the results presented here are based on large independent databases and analyses and are consistent in finding that the diagnosis of HS is rare. The existence of a larger population with untreated or undiagnosed HS cannot be addressed by these studies. It is commonly stated that HS patients are often misdiagnosed or undiagnosed, or that they quit seeking medical care because of ineffective results. Long delays between disease onset and diagnosis have been cited. The importance of an accurate determination of the prevalence of HS that includes possible undiagnosed or untreated patients is 2-fold: first, allocation of resources to the development of new therapeutic agents requires knowledge of the number of patients. Larger numbers of potential patients favor development efforts; however, for diseases that are recognized by the Office of Rare Diseases Research (Bethesda, Maryland), advantageous regulatory pathways are available. Secondly, because of the severe impact that HS has on those with the disease, it is important to develop strategies to inform physicians of the disease, identify those patients affected, and possibly prevent progression, disfigurement, pain, and disability.
In summary, empirical evidence from database studies is consistent in showing that diagnoses of HS are rare in the United States. Previous epidemiologic studies based on questionnaires that placed the prevalence at 1% have significant methodological problems. HS is rarely diagnosed; further research is needed to determine the prevalence of undiagnosed HS.
This study can also be seen on Medscape