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Table 1.  
Demographic Characteristics of Patients With Hidradenitis Suppurativa in the United States
Demographic Characteristics of Patients With Hidradenitis Suppurativa in the United States
Table 2.  
Overall and Adjusted Group-Specific Prevalence Estimates for HS in the United States
Overall and Adjusted Group-Specific Prevalence Estimates for HS in the United States
Table 3.  
Age-Specific Prevalence of HS by Racea
Age-Specific Prevalence of HS by Racea
1.
Lookingbill  DP.  Yield from a complete skin examination: findings in 1157 new dermatology patients.  J Am Acad Dermatol. 1988;18(1, pt 1):31-37.PubMedGoogle ScholarCrossref
2.
Jemec  GB, Heidenheim  M, Nielsen  NH.  The prevalence of hidradenitis suppurativa and its potential precursor lesions.  J Am Acad Dermatol. 1996;35(2, pt 1):191-194.PubMedGoogle ScholarCrossref
3.
Jemec  GB.  The symptomatology of hidradenitis suppurativa in women.  Br J Dermatol. 1988;119(3):345-350.PubMedGoogle ScholarCrossref
4.
von der Werth  JM, Williams  HC.  The natural history of hidradenitis suppurativa.  J Eur Acad Dermatol Venereol. 2000;14(5):389-392.PubMedGoogle ScholarCrossref
5.
Revuz  JE, Canoui-Poitrine  F, Wolkenstein  P,  et al.  Prevalence and factors associated with hidradenitis suppurativa: results from two case-control studies.  J Am Acad Dermatol. 2008;59(4):596-601.PubMedGoogle ScholarCrossref
6.
Jemec  GB.  Clinical practice: hidradenitis suppurativa.  N Engl J Med. 2012;366(2):158-164.PubMedGoogle ScholarCrossref
7.
Vinding  GR, Miller  IM, Zarchi  K, Ibler  KS, Ellervik  C, Jemec  GB.  The prevalence of inverse recurrent suppuration: a population-based study of possible hidradenitis suppurativa.  Br J Dermatol. 2014;170(4):884-889.PubMedGoogle ScholarCrossref
8.
Cosmatos  I, Matcho  A, Weinstein  R, Montgomery  MO, Stang  P.  Analysis of patient claims data to determine the prevalence of hidradenitis suppurativa in the United States.  J Am Acad Dermatol. 2013;69(5):819.PubMedGoogle ScholarCrossref
9.
Shlyankevich  J, Chen  AJ, Kim  GE, Kimball  AB.  Hidradenitis suppurativa is a systemic disease with substantial comorbidity burden: a chart-verified case-control analysis.  J Am Acad Dermatol. 2014;71(6):1144-1150.PubMedGoogle ScholarCrossref
10.
Shahi  V, Alikhan  A, Vazquez  BG, Weaver  AL, Davis  MD.  Prevalence of hidradenitis suppurativa: a population-based study in Olmsted County, Minnesota.  Dermatology. 2014;229(2):154-158.PubMedGoogle ScholarCrossref
11.
Explorys Inc. The IBM Explorys Platform. Somers, NY: IBM Corporation; June 2016. https://public.dhe.ibm.com/common/ssi/ecm/hp/en/hps03052usen/HPS03052USEN.PDF. Accessed January 18, 2016.
12.
US National Library of Medicine Unified Medical Language System (UMLS). Systematized Nomenclature of Medicine—ClinicalsTerms (SNOMED CT). https://www.nlm.nih.gov/research/umls/Snomed/snomed_main.html. Published March 1, 2016. Accessed October 10, 2016.
13.
Nelson  SJ, Zeng  K, Kilbourne  J, Powell  T, Moore  R.  Normalized names for clinical drugs: RxNorm at 6 years.  J Am Med Inform Assoc. 2011;18(4):441-448.PubMedGoogle ScholarCrossref
14.
McDonald  CJ, Huff  SM, Suico  JG,  et al.  LOINC, a universal standard for identifying laboratory observations: a 5-year update.  Clin Chem. 2003;49(4):624-633.PubMedGoogle ScholarCrossref
15.
Kaelber  DC, Foster  W, Gilder  J, Love  TE, Jain  AK.  Patient characteristics associated with venous thromboembolic events: a cohort study using pooled electronic health record data.  J Am Med Inform Assoc. 2012;19(6):965-972.PubMedGoogle ScholarCrossref
16.
Kim  CH, Al-Kindi  SG, Jandali  B, Askari  AD, Zacharias  M, Oliveira  GH.  Incidence and risk of heart failure in systemic lupus erythematosus.  Heart. 2017;103(3):227-233.PubMedGoogle ScholarCrossref
17.
US Census Bureau. Projections of the Total Resident Population by 5-Year Age Groups, and Sex with Special Age Categories: Middle Series, 1999 to 2000 (NP-T3-A): http://www.census.gov/population/projections/files/natproj/summary/np-t3-a.pdf. Published January 13, 2000. Accessed October 31, 2016.
18.
Wilson  EB.  Probable inference, the law of succession, and statistical inference.  J Am Stat Assoc. 1927;22(158):209-212.Google ScholarCrossref
19.
Fay  MP, Feuer  EJ.  Confidence intervals for directly standardized rates: a method based on the gamma distribution.  Stat Med. 1997;16(7):791-801.PubMedGoogle ScholarCrossref
20.
Khalsa  A, Liu  G, Kirby  JS.  Increased utilization of emergency department and inpatient care by patients with hidradenitis suppurativa.  J Am Acad Dermatol. 2015;73(4):609-614.PubMedGoogle ScholarCrossref
21.
Kirby  JS, Miller  JJ, Adams  DR, Leslie  D.  Health care utilization patterns and costs for patients with hidradenitis suppurativa.  JAMA Dermatol. 2014;150(9):937-944.PubMedGoogle ScholarCrossref
Original Investigation
August 2017

Sex- and Age-Adjusted Population Analysis of Prevalence Estimates for Hidradenitis Suppurativa in the United States

Author Affiliations
  • 1Department of Dermatology, Hofstra Northwell School of Medicine, New Hyde Park, New York
  • 2Department of Dermatology, Penn State Milton S. Hershey Medical Center, Hershey, Pennsylvania
JAMA Dermatol. 2017;153(8):760-764. doi:10.1001/jamadermatol.2017.0201
Key Points

Questions  What is the prevalence of hidradenitis suppurativa in the United States, and does prevalence vary by demographic group?

Findings  In this population-based analysis, the overall point prevalence of hidradenitis suppurativa was 0.10%, or 98 per 100 000 persons. Prevalence was highest among women (137 per 100 000), those aged 30 to 39 years (172 per 100 000), and African American (296 per 100 000) and biracial (218 per 100 000) patient groups.

Meaning  Hidradenitis suppurativa is an uncommon, but not rare, disease in the United States that disproportionately affects female patients, young adults, and African American and biracial patients.

Abstract

Importance  The true prevalence of hidradenitis suppurativa (HS) is unknown.

Objective  To establish standardized overall and group-specific prevalence estimates for HS in the United States.

Design, Setting, and Participants  This retrospective analysis included a demographically heterogeneous population-based sample of more than 48 million unique patients across all US census regions. As of October 27, 2016, a total of 47 690 patients with HS were identified using electronic health record data.

Main Outcomes and Measures  Standardized overall point prevalence for HS and sex-, age-, and race-specific prevalence estimates of HS in the general US population.

Results  Of the 47 690 patients with HS (26.2% men and 73.8% women), the overall HS prevalence in the US population sample was 0.10%, or 98 per 100 000 persons (95% CI, 97-99 per 100 000 persons). The adjusted prevalence in women was 137 per 100 000 (95% CI, 136-139 per 100 000), more than twice that of men (58 per 100 000; 95% CI, 57-59 per 100 000; P < .001). The prevalence of HS was highest among patients aged 30 to 39 years (172 per 100 000; 95% CI, 169-275 per 100 000) compared with all other age groups (range, 15-150 per 100 000; P < .001). Adjusted HS prevalences among African American (296 per 100 000; 95% CI, 291-300 per 100 000) and biracial (218 per 100 000; 95% CI, 202-235 per 100 000) patients were more than 3-fold and 2-fold greater, respectively, than that among white patients (95 per 100 000; 95% CI, 94-96 per 100 000; P < .001).

Conclusions and Relevance  Hidradenitis suppurativa is an uncommon, but not rare, disease in the United States that disproportionately affects female patients, young adults, and African American and biracial patients.

Introduction

Hidradenitis suppurativa (HS) is a debilitating, painful chronic inflammatory disease for which the inherent unpredictability with respect to the course of disease and response to treatment poses significant challenges for patients. With sparse epidemiologic data for HS, disease burden has not yet been established. The prevalence of HS has been estimated using claims analyses, examinations, medical record reviews, or patient-administered questionnaires. Most prevalence studies have involved selected samples and/or small cohorts, which limit generalizability. Because of the different data acquisition methods, study settings, and geographic samples in these analyses, prevalence estimates have, not surprisingly, varied significantly across the world. Overall prevalence estimates for HS in Europe and in the United States have ranged from 0.00033% to as high as 4.1%.1-10 Few adjusted population analyses on which HS prevalence may be accurately estimated have been performed. The true overall prevalence of HS is therefore unknown. The purpose of our study was to establish standardized prevalence estimates for HS in the overall US population and in specific demographic subgroups.

Methods
Case Identification

We performed a retrospective case cohort analysis by querying a multi-institutional data analytics and research platform (Explorys) developed by IBM Watson Health.11 Clinical information from electronic medical records, laboratories, practice management systems, and claims systems is matched using the single set of Unified Medical Language System ontologies to create longitudinal records for each unique patient. Human Subjects Committee review was waived by the Feinstein Institute for Medical Research at Northwell Health because no idendifiers are associated with the data.

The data were standardized and curated according to common controlled vocabularies and classifications systems, including International Classification of Diseases, Ninth Revision (ICD-9); Systemized Nomenclature of Medicine–Clinical Terms (SNOMED-CT)12; RxNorm13; Logical Observation Identifiers Names and Codes (LOINC)14; and Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. At present, the database encompasses 27 participating integrated health care organizations. More than 48 million unique lives, representing approximately 15% of the population across all 4 census regions of the United States, are captured. Patients with all types of insurance and those who self-pay are represented. The earliest clinical observations in the data system are from 1999. The database is dynamic and performs periodic automatic updates. Population counts are reported by the database to the nearest 10 or represented as less than 10 if they range from 0 to 9 to prevent patient identification. The platform has been applied to validation and epidemiologic studies across other disciplines.15,16

In this analysis, all active nondeceased patients in the database at the time of our query (October 1, 2016) were included in this study. The SNOMED-CT term hidradenitis has one-to-one mapping to the ICD-9 code for HS (705.83) and was used to identify patients with the ICD-9 code. To validate use of this administrative coding to identify the HS case cohort in the database, we reviewed a random sample of 150 patient records linked to the ICD-9 diagnosis code for HS at Northwell Health, which is a member of the data consortium. Two trained medical students extracted clinical information documented by the health care professionals in each medical record. Two dermatologists experienced in evaluating HS (including A.G.) independently reconciled the clinical information for each patient against a criterion checklist for the case definition adopted by the 2nd International Conference on Hidradenitis Suppurativa. Cases with disagreement on HS diagnosis were reconciled by discussion between the dermatologists. In this cohort, a single ICD-9 code for HS yielded a positive predictive value of 79.3% for diagnosis. This case identification method was also validated previously and independently validated, and it was shown to have a positive predictive value of 77%.9

Statistical Analysis

We calculated frequencies describing the demographic characteristics of the HS cohort identified from January 1, 1999, through October 1, 2016. Sex- and age-specific HS prevalences were calculated for the following 4 racial groups: white, African American, biracial (ie, white and African American), and other (including Asian, Pacific Islander, Native American, Alaskan Native, Latin American, Native Hawaiian, and unknown). The entire nondeceased population for each racial group was considered at risk. To calculate sex- and age-specific HS prevalences for the overall population, the number of HS cases and total population size were collapsed across the 4 racial groups. We applied direct standardization to allow for subgroup comparisons using the projected US population in 2000 as the standard population and the following 6 age groups: 0 to 17, 18 to 29, 30 to 39, 40 to 49, 50 to 59, and 60 years or older.17 Estimates were age adjusted for sex comparisons, sex adjusted for age group comparisons, and sex and age adjusted for race comparisons. Crude and standardized prevalence estimates were calculated based on patients with available race, sex, and age information. Confidence intervals for crude and standardized prevalences were computed using the Wilson score method18 and gamma distribution,19 respectively. Standardized prevalences were compared assuming the prevalence ratio follows a log-normal distribution. Age-specific prevalences were compared using the χ2 test. A 2-sided α level of .05 was applied to determine statistical significance. All analyses were performed using SAS software (version 9.4; SAS Institute, Inc).

Results

Table 1 lists the demographic characteristics of patients with HS in the United States. We identified 47 690 patients with HS as of October 27, 2016. Women with HS outnumbered men with HS by nearly 3 to 1 (73.8% female and 26.2% male). Patients with HS were most frequently in the fourth decade of life. Patients aged 0 to 17 years represented 2.2% of the HS cohort. Of note, 30.1% of the HS cohort was 50 years or older. Patients with HS were most frequently white (54.1%), and of importance, nearly one-third were African American.

Overall and adjusted group-specific HS prevalences in the United States are listed in Table 2. We observed an overall standardized point prevalence of 0.10%, or 98 patients with HS per 100 000 persons in the United States (95% CI, 97-99 per 100 000 persons). Female patients had a greater than 2-fold prevalence of HS (137 per 100 000; 95% CI, 136-139 per 100 000) than male patients (58 per 100 000; 95% CI, 57-59 per 100 000; P < .001). The prevalence of HS among patients aged 30 to 39 years (172 per 100 000; 95% CI, 169-175 per 100 000) was significantly higher than that among all other age groups (range, 15-150 per 100 000; P < .001 for all pairwise comparisons). The standardized prevalence of HS in the pediatric age group was 15 per 100 000 (95% CI, 14-16 per 100 000). The standardized prevalences of HS among African American (296 per 100 000; 95% CI, 291-300 per 100 000) and biracial (218 per 100 000; 95% CI, 202-235 per 100 000) patients were 3-fold and 2-fold greater, respectively, than that among white patients (95 per 100 000; 95% CI, 94-96 per 100 000; P < .001 for both comparisons).

Age-specific HS prevalences by race are listed in Table 3. The prevalence of HS was greater among African American (range, 40-541 per 100 000) and biracial (range, 20-428 per 100 000) patients than among white patients (range, 14-176 per 100 000) across all age-groups (P < .001), except for biracial patients aged 0 to 17 years (20 per 100 000; 95% CI, 11-37 per 100 000; P = .20). The highest HS prevalence among African American (541 per 100 000; 95% CI, 525-559 per 100 000) and biracial (428 per 100 000; 95% CI, 370-495 per 100 000) patients occurred in the group aged 30 to 39 years.

Discussion

In this study, we have determined overall and group-specific adjusted prevalence estimates for HS in the United States. On the basis of sample size, demographic diversity of the prevalence cohort, and standardization of prevalence estimates using 12 age- and sex-specific strata, we believe that these results may be generalized to the US population.

The overall point prevalence of HS is 0.10%, or 98 HS patients per 100 000 persons in the United States. The prevalence of HS among female patients was more than 2-fold greater than in male patients. The prevalence of HS was greatest among patients aged 30 to 39 years and similar to the prevalence in patients aged 18 to 29 years and 40 to 49 years. The standardized prevalence of HS in the pediatric population was significantly lower than in the overall population or in all other age groups. An additional finding in this study is that HS disproportionately affects African American and biracial patients, who have 3-fold and 2-fold greater prevalences, respectively, than white patients. Although this disparity was observed across all age groups except biracial patients aged 0 to 17 years, the highest prevalence for African American and biracial patients was in the group aged 30 to 39 years. To date, we are not aware of other population-based prevalence estimates of HS among nonwhite patients.

Few adjusted population-based studies evaluating HS prevalence in the United States have been performed. One claims-based study8 identified 7927 patients with HS among a population of 15 million persons in 2007. The overall prevalence of HS was estimated to be 53 per 100 000, or 0.053%, which was similar to prevalence estimates adjusted for sex (0.052%) and age (0.051%). The prevalence of HS among women (75 per 100 000; 0.075%) was 3 times that among men (29 per 100 000; 0.029%). Prevalence estimates for patients with HS aged 18 to 29 years was 85 per 100 000 (0.085%); for patients aged 30 to 44 years, 87 per 100 000 (0.087%). Prevalence in the pediatric population was 17 per 100 000 (0.017%).8 However, that analysis estimated prevalence based on 4 case definition criteria, 3 of which included Current Procedural Terminology codes for procedures that were not all specific to a diagnosis of HS. The case cohort captured only commercially insured patients, and thus many demographic groups may have been underrepresented in their sample. Moreover, their analysis did not include a description of race.8

Among 178 HS cases in Olmstead County,10 the total sex- and age-adjusted point prevalence was 127.8 per 100 000 (0.13%). Disease prevalence among women (189.5 per 100 000; 0.19%) was roughly 3 times that among men (65.5 per 100 000; 0.066%). The age range with the highest overall prevalence was 60 to 69 years (201.8 per 100 000; 0.20%), with only 24 cases represented in this subgroup. The highest subgroup prevalence was among women aged 20 to 29 years (307.7 per 100 000; 0.31%). With only 2 cases in the group aged 0 to 19 years, the observed prevalence was 5.1 per 100 000 (0.005%).10 These results may have limited generalizability because the analysis was limited to patients in southeastern Minnesota and nearly 88% of the patients with HS were white. Small case cohort size may also have limited subgroup analyses by age and race.10

Population-based prevalence estimates from Europe are generally 1 order of magnitude higher than those from the United States. Two study distinctions include smaller sample sizes and use of interviews or questionnaires to identify cases in European analyses. In a French study,5 1 survey question that had not been previously validated was used to identify cases in a representative sample of the population. Of the 10 000 persons surveyed, 68.9% completed the survey. Sixty-seven cases of HS were identified, yielding an adjusted prevalence of 1%.5 In a Danish analysis,2 prevalence was evaluated by survey of a representative sample of 8000 persons. Among 585 respondents, 6 self-reported a history compatible with HS, yielding a 1-year prevalence of 1%. In another Danish population analysis,7 a validated self-administered questionnaire was used to identify HS cases among 17 454 representative participants aged 30 to 89 years. The overall prevalence in that analysis was observed to be 2.1%. The prevalence of HS among women (2.56%) was roughly 1.5 times that among men (1.58%). Prevalence was greatest among patients aged 30 to 39 years (3.91%) and declined thereafter by the decade.7 Data for patients younger than 30 years were not included, and almost all patients with HS were white. In another cohort in the Danish analysis,2 the prevalence of HS was observed to be 4.1%. Cases were identified by examination of 507 consecutive patients evaluated in a screening clinic for sexually transmitted diseases.2

Limitations

Some limitations to our analysis bear consideration. We could not capture patients with undiagnosed HS or who did not seek care in health systems included in the database. Diagnosis of HS could not be verified in medical records for the entire universe of data because they are deidentified. The extent to which our analysis may have overestimated or underestimated true prevalence is not known. However, we are reassured that our overall prevalence approximates that of the Olmstead County analysis in which cases were validated.10

Conclusions

We have estimated that HS is an uncommon, but not rare, disease in the United States that disproportionately affects female patients, young adults, and African American and biracial patients. However, costs and utilization of health care services among patients with HS rival or exceed those of patients with psoriasis, a common skin disease.20,21

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Article Information

Corresponding Author: Amit Garg, MD, Hofstra Northwell School of Medicine, 1991 Marcus Ave, Ste 300, New Hyde Park, NY 11042 (amgarg@northwell.edu).

Accepted for Publication: January 21, 2017.

Published Online: May 10, 2017. doi:10.1001/jamadermatol.2017.0201

Author Contributions: Mr Lavian and Ms Lin contributed equally to the investigation and the manuscript. Dr Garg and Mr Strunk had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Garg, Kirby, Lavian, Strunk.

Acquisition, analysis, or interpretation of data: Garg, Lavian, Lin, Strunk.

Drafting of the manuscript: Garg, Lavian, Lin, Strunk.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: Garg, Strunk.

Administrative, technical, or material support: Kirby, Lavian, Lin.

Study supervision: Garg.

Conflict of Interest Disclosures: Drs Garg and Kirby report serving as advisors for and receiving honoraria from AbbVie. No other disclosures were reported.

References
1.
Lookingbill  DP.  Yield from a complete skin examination: findings in 1157 new dermatology patients.  J Am Acad Dermatol. 1988;18(1, pt 1):31-37.PubMedGoogle ScholarCrossref
2.
Jemec  GB, Heidenheim  M, Nielsen  NH.  The prevalence of hidradenitis suppurativa and its potential precursor lesions.  J Am Acad Dermatol. 1996;35(2, pt 1):191-194.PubMedGoogle ScholarCrossref
3.
Jemec  GB.  The symptomatology of hidradenitis suppurativa in women.  Br J Dermatol. 1988;119(3):345-350.PubMedGoogle ScholarCrossref
4.
von der Werth  JM, Williams  HC.  The natural history of hidradenitis suppurativa.  J Eur Acad Dermatol Venereol. 2000;14(5):389-392.PubMedGoogle ScholarCrossref
5.
Revuz  JE, Canoui-Poitrine  F, Wolkenstein  P,  et al.  Prevalence and factors associated with hidradenitis suppurativa: results from two case-control studies.  J Am Acad Dermatol. 2008;59(4):596-601.PubMedGoogle ScholarCrossref
6.
Jemec  GB.  Clinical practice: hidradenitis suppurativa.  N Engl J Med. 2012;366(2):158-164.PubMedGoogle ScholarCrossref
7.
Vinding  GR, Miller  IM, Zarchi  K, Ibler  KS, Ellervik  C, Jemec  GB.  The prevalence of inverse recurrent suppuration: a population-based study of possible hidradenitis suppurativa.  Br J Dermatol. 2014;170(4):884-889.PubMedGoogle ScholarCrossref
8.
Cosmatos  I, Matcho  A, Weinstein  R, Montgomery  MO, Stang  P.  Analysis of patient claims data to determine the prevalence of hidradenitis suppurativa in the United States.  J Am Acad Dermatol. 2013;69(5):819.PubMedGoogle ScholarCrossref
9.
Shlyankevich  J, Chen  AJ, Kim  GE, Kimball  AB.  Hidradenitis suppurativa is a systemic disease with substantial comorbidity burden: a chart-verified case-control analysis.  J Am Acad Dermatol. 2014;71(6):1144-1150.PubMedGoogle ScholarCrossref
10.
Shahi  V, Alikhan  A, Vazquez  BG, Weaver  AL, Davis  MD.  Prevalence of hidradenitis suppurativa: a population-based study in Olmsted County, Minnesota.  Dermatology. 2014;229(2):154-158.PubMedGoogle ScholarCrossref
11.
Explorys Inc. The IBM Explorys Platform. Somers, NY: IBM Corporation; June 2016. https://public.dhe.ibm.com/common/ssi/ecm/hp/en/hps03052usen/HPS03052USEN.PDF. Accessed January 18, 2016.
12.
US National Library of Medicine Unified Medical Language System (UMLS). Systematized Nomenclature of Medicine—ClinicalsTerms (SNOMED CT). https://www.nlm.nih.gov/research/umls/Snomed/snomed_main.html. Published March 1, 2016. Accessed October 10, 2016.
13.
Nelson  SJ, Zeng  K, Kilbourne  J, Powell  T, Moore  R.  Normalized names for clinical drugs: RxNorm at 6 years.  J Am Med Inform Assoc. 2011;18(4):441-448.PubMedGoogle ScholarCrossref
14.
McDonald  CJ, Huff  SM, Suico  JG,  et al.  LOINC, a universal standard for identifying laboratory observations: a 5-year update.  Clin Chem. 2003;49(4):624-633.PubMedGoogle ScholarCrossref
15.
Kaelber  DC, Foster  W, Gilder  J, Love  TE, Jain  AK.  Patient characteristics associated with venous thromboembolic events: a cohort study using pooled electronic health record data.  J Am Med Inform Assoc. 2012;19(6):965-972.PubMedGoogle ScholarCrossref
16.
Kim  CH, Al-Kindi  SG, Jandali  B, Askari  AD, Zacharias  M, Oliveira  GH.  Incidence and risk of heart failure in systemic lupus erythematosus.  Heart. 2017;103(3):227-233.PubMedGoogle ScholarCrossref
17.
US Census Bureau. Projections of the Total Resident Population by 5-Year Age Groups, and Sex with Special Age Categories: Middle Series, 1999 to 2000 (NP-T3-A): http://www.census.gov/population/projections/files/natproj/summary/np-t3-a.pdf. Published January 13, 2000. Accessed October 31, 2016.
18.
Wilson  EB.  Probable inference, the law of succession, and statistical inference.  J Am Stat Assoc. 1927;22(158):209-212.Google ScholarCrossref
19.
Fay  MP, Feuer  EJ.  Confidence intervals for directly standardized rates: a method based on the gamma distribution.  Stat Med. 1997;16(7):791-801.PubMedGoogle ScholarCrossref
20.
Khalsa  A, Liu  G, Kirby  JS.  Increased utilization of emergency department and inpatient care by patients with hidradenitis suppurativa.  J Am Acad Dermatol. 2015;73(4):609-614.PubMedGoogle ScholarCrossref
21.
Kirby  JS, Miller  JJ, Adams  DR, Leslie  D.  Health care utilization patterns and costs for patients with hidradenitis suppurativa.  JAMA Dermatol. 2014;150(9):937-944.PubMedGoogle ScholarCrossref
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