Background. This clinical case demonstrates the difficulty of diagnosing systemic vasculitis and the effectiveness of immunosuppressive therapy.
Description of the clinical case. The article presents a clinical case of microscopic polyangiitis in a 61-year-old man. The disease proceeded with predominant kidney damage associated with antineutrophil cytoplasmic antibodies targeting proteinase-3, febrile fever, myalgia, arthralgia, weight loss, and polyneuropathy. The material obtained during the primary kidney biopsy was of little information, since it contained less than 10 glomeruli without crescents. The patient abstained from the proposed repeated nephrobiopsy. Taking into account clinical and laboratory data, ANCA-glomerulonephritis was diagnosed, presumably within the framework of microscopic polyangiitis. Immunosuppressive therapy with glucocorticosteroids and cyclophosphamide made it possible to obtain a rapid remission of the disease. Anti-relapse therapy was not carried out. After 2 years, a relapse of the disease with a decrease in kidney function developed. The patient underwent repeated nephrobopsy, signs of ANCA-associated glomerulonephritis with the presence of crescents in 63%
of the glomeruli were found. The prescribed immunosuppressive therapy with glucocorticosteroids and cyclophosphamide led to remission of the disease with restoration of kidney function.
Conclusion. The clinical case illustrates the difficulty of early verification of the diagnosis of microscopic polyangiitis, the high efficiency of immunosuppressive therapy, and the need for long-term anti-relapse therapy.

Keywords: antineutrophil cytoplasmic antibodies, microscopic polyangiitis, systemic vasculitis, immunosuppressive therapy, kidney biopsy, rapidly progressive glomerulonephritis