Sandbox:Mashal

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mashal Awais, M.D.[2]

Tuberculosis may be classified into 6 major classes based on exposure, clinical symptoms, and adjunct diagnostic testing. The classification ranges from Class 0, in individuals with no previous exposure to TB and negative tuberculin skin testing and/or interferon-gamma release assays (2 methods of screening for TB), to Class 3 for active TB and Class 5 for suspected TB based on signs and symptoms of the disease. The U.S. Citizenship and Immigration Services has also established a special classification for immigrants and refugees based on the risk of infection.

Classification systems for cutaneous TB vary. Commonly, cutaneous TB is divided into two major groups: true cutaneous TB and tuberculids [10]. (See 'True cutaneous tuberculosis' below and 'Tuberculids' below.)

True cutaneous TB includes conditions in which cutaneous lesions are a direct manifestation of infection at the site of the skin lesions. In true cutaneous TB, mycobacteria can be identified in the sites of skin involvement using tests such as smears, in situ hybridization of pathology specimens, or cultures [10]. Detection may be more difficult in some forms of true cutaneous TB. (See 'Bacterial load' below.)

In contrast, tuberculids are considered to result from a hypersensitivity reaction to mycobacterial antigens. Mycobacteria typically are not detectable via smears, histopathologic techniques, or cultures from tuberculid lesions. (See 'Tuberculids' below.)

True cutaneous tuberculosis — The characteristic most often used to subdivide true cutaneous TB is the mode of infection. The quantity of organisms typically present in the skin (bacterial load) is another method of classification. (See 'Mode of infection' below and 'Bacterial load' below.)

Mode of infection — Most cases of true cutaneous TB arise from contiguous spread or hematogenous dissemination to the skin from an endogenous focus of infection. Direct entry of M. tuberculosis into the skin (ie, inoculation from an exogenous source) occasionally results in localized infection [15,16]. (See 'True cutaneous tuberculosis' below.)

Classification according to mode of infection:

●Contiguous spread or autoinoculation (see 'Contiguous spread' below)

•Scrofuloderma

•TB cutis orificialis

•Lupus vulgaris

●Hematogenous spread to the skin (see 'Hematogenous spread' below)

•Lupus vulgaris

•Acute miliary TB

•Metastatic tuberculous abscess

●Inoculation from an exogenous source (see 'Exogenous inoculation' below)

•Primary inoculation TB

•TB verrucosa cutis

Bacterial load — True cutaneous TB may also be classified according to the bacterial load in the skin [17]:

●Multibacillary forms (bacilli easily detectable in tissue or exudate):

•Primary inoculation TB

•Scrofuloderma

•TB cutis orificialis

•Acute miliary TB

•Metastatic tuberculous abscess

●Paucibacillary forms (bacilli hard to detect in tissue or exudate):

•TB verrucosa cutis

•Lupus vulgaris

Tuberculids — The tuberculids are TB-associated cutaneous disorders that likely represent hypersensitivity reactions to mycobacterial antigens, rather than manifestations of local skin infection. (See 'Tuberculids' below.)

The tuberculids include:

●Papulonecrotic tuberculid

●Lichen scrofulosorum

●Erythema induratum of Bazin (EIB)

Practice here.

High-power field of peripheral blood smear revealing a large, atypical B cell with mild cytoplasmic expansion, coarse chromatin, multiple distinct nucleoli and peripheral vacuolation.Source: Charakidis M. et al, Department of Haematology-Oncology, Royal Hobart Hospital, Tasmania, 7000, Australia.

Classification of Waldenstrom macroglobulinemia (WM) and Related Disorders

Criteria	Symptomatic WM	Asymptomatic WM	IgM-Related Disorders	MGUS
IgM monoclonal protein	+	+	+	+
Bone marrow infiltration	+	+	-	-
Symptoms attributable to IgM	+	-	+	-
Symptoms attributable to tumor infiltration	+	-	-	-