The World Health Organization's (WHO) International Classification of Diseases 10 (ICD-10) code for HS is L73.2, in the category of “Other Follicular Disorders” within “Disease of the Skin and Subcutaneous Tissue,” but some countries still utilize the older
ICD-9 code, which classifies HS
as a skin disorder of the sweat glands (705.83). The ICD-9 classification does not reflect what is now known about this disease.
Diagnosis & Classification
The diagnosis of Hidradenitis
Suppurativa is usually based on a clinical examination and a patient history. HS is therefore frequently misdiagnosed, and the time to accurate diagnosis may be measured in years, if not decades. Although a collaborative diagnostic methodology was published in 2000 specifically devised for a clinical investigation regarding the familial transfer of HS , there are no universally accepted diagnostic criteria for HS, which presents a significant challenge for patients, physicians, and researchers.
Global consensus regarding diagnosis should be a priority in order to facilitate the advancement of HS research and to assist physicians in making more accurate and timely diagnoses. Until such a consensus
is made, the following represents a summary of the most current expert medical opinion regarding the clinical diagnosis of HS.
Expert Medical Perspectives Clinical Features: Establishing the diagnosis of HS relies on three main features: Typical lesions, i.e., deep-seated nodules (blind boils) and/or fibrosis; Typical localizations, i.e., armpit and groin; and Relapses and chronicity. A family history has been found in approximately 1 in 3 of those affected, which may aid in the diagnosis of the disease. A symmetrical presentation of lesions also indicates a systemic disease rather than local infection.
Eight Questions to Ask When Taking a Patient History Which May Aid the Diagnosis: Questions where a positive answer supports the diagnosis of HS: 1. Does anyone in your family suffer from the same symptoms? 2. Do your boils recur at the same spot every time? 3. Do you smoke tobacco? 4. Do you experience a premenstrual flare of your boils regularly? Questions where the negative answer supports the diagnosis of HS: 5. Do you get random boils on your skin, i.e. on the thighs or abdomen as well? 6. Has the treatment offered by your doctor helped?
7. Do you suffer from infections elsewhere? 8. Do you get a fever when your boils appear? 
Classification & Differentiation: HS is multifactorial. Follicular occlusion and disruption are predisposing factors, but other factors, including bacterial colonization and an as yet unknown pro-inflammatory mechanism are at work. HS can be described as inverse recurrent suppuration according to topography, clinical evolution, and morphology. Looking at the parameters of etiology, morphology, pathogenesis, and therapy, HS can be differentiated from the acnes and from simple folliculitis. .
 von der Werth JM, Williams HC, Raeburn JA. The clinical genetics of hidradenitis suppurativa revisited.
Br J Dermatol. 2000; 142(5):947-953. LINK