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Paraprotein is a laboratory biomarker of plasma cell tumors and other lymphoproliferative diseases. Its determination is necessary for diagnosing, monitoring and assessment of therapy effectiveness. The lecture presents the main methods of qualitative and quantative analysis of monoclonal proteins: gel electrophoresis, capillary electrophoresis, immunofixation and nephelometry features, possibilities and limitations are reviewed. The main sources of errors and artifacts during these studies are considered. Also the difficulties in the diagnosis and interpretation of the results of serum and urine tests are highlighted.

Introduction. Several viruses including hepatitis C virus (HCV) and human immunodeficiency virus (HIV) can induce vasculitis.Aim. We aimed to study the incidence, risk factors, and severity of skin vasculitis in patients with HCV and HCV/HIV coinfection.Patients and Methods. The study group included 331 patients (254 patients with HCV and 77 patients with HCV/HIV coinfection) referred to a specialized hepatology center for antiviral therapy of HCV infection.Results. Skin vasculitis was found in 21% (95% CI: 17–25%) of cases, n = 69/331. Skin vasculitis was observed in in 20% of patients infected with HCV (95% CI: 15–25%) and in 25% of HIV/HCV co-infected patients (95% CI; 16–35%), χ2 = 0,892, р = 0,345. Most patients with vasculitis infected with HCV had cryoglobulinemia (94%, n = 47/50), meanwhile, in HIV/HCV co-infected patients, cryoglobulinemia was found in 63% cases (n = 12/19), χ2 = 10.568, р = 0.001. Multiple regression analysis showed that skin vasculitis was related with cryoglobulinemia (OR = 6,928, 95% CI: 3.245–14.790, р < 0.001), liver cirrhosis (OR = 2.015, 95% CI: 1.062–3.824, р = 0.032), duration of overt HCV infection (OR = 1.057, 95% CI: 1.021–1.094, р = 0.002), patients age (OR = 1.029, 95% CI: 1.002–1.057, р = 0.033) and inversely related with plasma alanine transaminase (OR 0.433, 95% CI: 0.229–0.820, р = 0.010). The statistical model was normalized for gender, HIV-positivity and bilirubin levels, and regression equation constant was 4.398 (p < 0.001). The intensity of skin rashes was comparable in HCV infected and HCV/HIV co-infected patients (χ2 = 6.741, р = 0.081), and was highly correlated with cryoglobulin levels both in HCV infected (r = 0.788, p < 0.001), and HCV/HIV co-infected patients (r = 0.909, p = 0.001).Conclusion. Skin vasculitis was found in 20–25% of cases among patients with HCV infection and HCV/HIV co-infection. Cryoglobulinemia was the main factor associated with skin vasculitis, and severity of skin lesions was closely related with cryoglobulin levels.