Tuberculosis on the skin takes several distinct forms, ranging from painless reddish-brown bumps to open ulcers, warty plaques, and firm nodules that drain pus. Unlike the lung disease most people associate with TB, skin tuberculosis (accounting for only 1–2% of all TB cases) can look dramatically different depending on how the bacteria reached the skin and how strong your immune system is. Some forms resemble common skin conditions like warts or eczema, which makes them easy to overlook.
How TB Reaches the Skin
TB bacteria can affect the skin through three main routes, and the route largely determines what the lesion looks like. First, bacteria can enter directly through a cut or wound, especially in people who handle infected tissue. Second, an existing TB infection in a lymph node, bone, or organ can spread outward to the skin above it. Third, bacteria traveling through the bloodstream from the lungs or another site can seed the skin from within.
There’s also a fourth category that doesn’t involve live bacteria in the skin at all. In these cases, called tuberculids, the immune system overreacts to TB proteins circulating from an infection elsewhere in the body, producing a skin rash as a hypersensitivity response.
Tuberculous Chancre: The Entry-Point Lesion
When TB bacteria enter through broken skin in someone who has never been exposed before, a tuberculous chancre can form. This is rare, making up about 1% of skin TB cases. It appears as a red to brown bump at the wound site, typically 2 to 4 weeks after exposure. Over time, the bump breaks open into an ulcer with undermined, overhanging edges. The lesion is generally painless, which is one reason people may not seek care right away.
Lupus Vulgaris: The “Apple Jelly” Plaque
Lupus vulgaris is the most recognizable form of skin TB and has nothing to do with the autoimmune disease lupus. It typically appears on the face and neck, especially in younger adults. The lesion starts as small, soft, reddish-brown bumps that merge into a scaly plaque. New bumps continue to develop at the edges, causing the plaque to slowly expand outward while the center may thin out and show signs of tissue loss.
The hallmark feature is what doctors call the “apple jelly” sign. If you press a glass slide firmly against the plaque (a technique called diascopy), the lesion takes on a light brownish-yellow, translucent appearance, like apple jelly held up to light. Under magnification, fine small blood vessels are visible against this golden background. The plaques are usually not painful or itchy, and they grow very slowly over months to years, which often delays diagnosis.
Warty TB: Rough, Raised Plaques
Tuberculosis verrucosa cutis looks strikingly like a wart. It develops when TB bacteria are inoculated into the skin of someone who already has partial immunity from a previous TB exposure. The lesion begins as a small, painless, warty bump and gradually expands into a rough, raised plaque with an irregular, wavy outline and finger-like projections at the border. The texture is firm and the surface is hard and crusted.
Location varies by geography. In Europe, these plaques most commonly appear on the hands (historically earning the name “prosector’s wart” among pathologists and lab workers). In Asia, the knees, ankles, and buttocks are more frequently affected. Because these lesions closely resemble common warts, fungal infections, or even precancerous skin growths, they are frequently misdiagnosed.
Scrofuloderma: Draining Skin Over Infected Nodes
Scrofuloderma is one of the more visually dramatic forms. It happens when a TB-infected lymph node, bone, or joint pushes outward and breaks through the overlying skin. The process starts as a firm, painless lump under the skin, most often on the neck over a swollen lymph node. Over weeks to months, the lump softens into a fluctuant abscess.
Eventually the skin above it breaks down, forming a depressed ulcer with irregular, dusky, purplish borders and a yellowish, grainy base. Sinus tracts, or tunnels under the skin, develop and drain thick, purulent or cheesy material. When these ulcers finally heal, they leave behind contracted, rope-like scars or raised keloid scars. The entire process, from initial lump to scarring, can take many months.
Papulonecrotic Tuberculid: Symmetrical Bumps That Scar
This form results from the immune system’s reaction to TB proteins rather than from bacteria directly invading the skin. It appears as crops of small, dusky red to brown bumps distributed symmetrically on the body, with a preference for the knees, elbows, legs, hands, and feet. Each bump develops a darkened, necrotic center that crusts over.
The distinctive pattern is cyclical: new crops appear, develop central crusting, then heal relatively quickly, leaving behind small, pitted, pockmark-like scars. The bumps themselves are typically painless. Because tuberculids are a hypersensitivity reaction, their presence signals an active TB infection somewhere else in the body, usually in the lungs.
Miliary TB on the Skin
Acute miliary skin TB is a rare, serious form that develops almost exclusively in people with severely weakened immune systems, including those with advanced HIV (typically when immune cell counts drop below 100 cells per microliter). It can also affect young children. The skin eruption consists of widespread red to purplish papules, sometimes topped with tiny blisters that break open, flatten in the center, and crust over. These lesions tend to clear within 1 to 4 weeks, leaving behind depressed, lighter-colored scars.
People with miliary skin TB are usually visibly unwell, with fever, weight loss, fatigue, and poor appetite. This form is not subtle or isolated the way other types of skin TB can be.
TB Gumma: Deep Abscesses
TB gumma, or metastatic tuberculous abscess, forms when bacteria spread through the bloodstream and lodge in the deep tissue beneath the skin. It primarily affects malnourished children and immunocompromised adults. The lesions present as firm nodules deep under the skin that eventually soften, break open, and form draining ulcers, similar in appearance to scrofuloderma but without an obvious underlying infected lymph node or bone.
Why Skin TB Is Difficult to Diagnose
Many forms of skin TB mimic other, more common conditions. Warty TB can look like psoriasis, warts, or precancerous skin patches. Lupus vulgaris is often confused with sarcoidosis, chronic discoid lupus, or fungal infections. Scrofuloderma can resemble hidradenitis suppurativa or deep fungal infections. Even syphilis and leishmaniasis enter the list of look-alikes.
Standard lab tests are not always reliable. In one study of skin biopsy specimens, the classic acid-fast stain detected TB bacteria in only 30% of cases. Culturing the bacteria performed better at 60%, but cultures take weeks to grow. DNA-based testing (PCR) was the most sensitive at 85%, catching nearly a third of cases that both staining and culture missed. A skin biopsy is typically necessary, and often multiple tests are run in combination to reach a diagnosis.
What Treatment Looks Like
Skin TB is treated with the same multi-drug antibiotic regimens used for pulmonary TB, typically lasting 6 months or longer. The same combination of antibiotics used for lung TB targets the bacteria in skin lesions. Most forms of skin TB respond well to treatment, though healing often leaves scars, particularly with scrofuloderma and papulonecrotic tuberculid. Tuberculid forms also improve because treatment eliminates the underlying TB infection driving the immune reaction.
Because skin TB lesions grow slowly and are often painless, people sometimes live with them for months or years before getting a diagnosis. The appearance alone is rarely enough for a definitive answer, so if you notice a persistent, slowly expanding plaque, a non-healing ulcer, or draining nodules over a lymph node, those are patterns worth getting evaluated, particularly if you have risk factors like prior TB exposure or a weakened immune system.