Erythema induratum represents a chronic skin disease most often caused by Mycobacterium tuberculosis, consisting of chronic inflammatory nodules commonly located on the lower limbs of the patient. Similarities exist between this condition and nodular vasculitis, but at this time the two are considered separate entities.
Erythema Induratum (EI) more often affects middle-aged women , but men and children can also acquire this illness.
An EI case usually suffers from pulmonary tuberculosis, but other sites of infection, like cervical lymphadenitis , renal parenchyma , epididymo-orchitis  or aortic valvular lesions of the same etiology  are possible. One study  even describes EI after Bacillus Calmette–Guérin (BCG) vaccine administration. In addition, it is important to evaluate the existence of human immunodeficiency virus (HIV) concomitant infection clinical traits .
The patients show erythematous or violaceous nodules, 1 - 4 cm in size, or plaques with a scaly surface located on the lower limbs, that may be tender and have a recurrent character, reappearing once every 3 or 4 months. Nodules sometimes cause shallow ulcers that heal, leaving scars and hyper pigmented areas. The affected regions, like calves, ankles, thighs, buttocks, but also trunk and arms are edematous. Disseminated forms have been described . Pruritus is absent. The disease may worsen with cold weather. The clinical picture may be completed by the presence of distal painful peripheral neuropathy , cutis marmorata or erythrocyanosis. Systemic complaints are usually absent, but other dermal manifestations of tuberculosis, such as papulonecrotic tuberculids may coexist.
Workup should include a complete cell blood count with differential, inflammatory markers (such as the erythrocyte sedimentation rate) and liver function tests. M. tuberculosis infection can be diagnosed in may ways: interferon-gamma release assays  such as QuantiFERON--tuberculosis (TB) Gold In-Tube test and the T-SPOT-TB test . The manifestation of pathognomonic lesions and a positive QuantiFERON exam establish the diagnosis, even if acid-fast bacilli are not detected or the thoracic radiography is normal . The purified protein derivative tuberculosis skin test is positive in these patients and an exaggerated host immunologic response and hypersensitivity reaction are to be expected after administration. In case an underlying active disease is suspected, the physician should order urine, early morning sputum, and gastric aspirates microbiological examination.
Imaging modalities include thoracic radiographs (posteroanterior and lateral) that may show aspects of active or inactive tuberculosis. If the diagnosis cannot be established otherwise, an excisional biopsy should be done. The tissue will then be analyzed using hematoxylin-eosin, specific stains for acid-fast bacilli, but also for bacteria and fungi. In cases with negative tuberculosis findings, hepatitis C and HIV infection should be searched for. Furthermore, a histological analysis will depict a granulomatous panniculitis with possible inflammatory vasculitis , noncaseating granulomas and caseation-like necrosis. Other cell types, like giant, epithelioid cells or histiocytes are also encountered in late stages, while fresh lesions mainly contain lymphocytes. The presence of M. tuberculosis deoxyribonucleic acid in the excised tissue can be proven using polymerase chain reaction . However, its absence does not confirm the non-existence of the ailment.