## 1 Introduction  
Prevalence of hepatitis C virus (HCV) infection is currently estimated to be 1%, corresponding to about 70 million virenic people worldwide [1]. HCV infection untreated for many years can lead to serious consequences such as liver cirrhosis, hepatocellular carcinoma (HCC), hepatic decompensation and death. Currently available treatment with direct-acting antivirals (DAAs) is pangenotypic, safe, easy, and short, and its efficacy is almost 100% [2; 3]. Diagnosis of HCV infection currently is based on the presence of antibodies followed by HCV RNA detection. According to recent findings, HCV core antigen (HCVcAg) testing can replace HCV RNA detection [4; 5; 6]. Its clearance during DAA therapy can predict sustained virologic response (SVR), which is an indicator of HCV clearance [7]. Quantitative HCV RNA is still a gold standard for monitoring anti-viral treatment efficacy. According to the most recent guidelines of the European Association for the Study of the Liver (EASL) the use of HCVcAg was recommended as an alternative marker of treatment efficacy [8]. Since HCVcAg has not been routinely applied for monitoring of treatment efficacy up to now, we decided to use it simultaneously with HCV RNA in our study. Evaluation of the progression of liver disease is a crucial element of HCV diagnosis, treatment prioritization and its monitoring. Invasiveness, cost and possible side effects of liver biopsy, recognized up to now as a gold standard, have led recently to the rapid development of non-invasive techniques for the evaluation of liver fibrosis [9; 10; 11; 12]. Two major directions in this area are serologic tests, combining a number of laboratory measures with a final calculation of the numeric value and measurement of liver stiffness (LS) with transient elastography (TE) or shear wave elastography (SWE) [13; 14]. Major advantages of LS measurement are safety, non-invasiveness, the possibility of testing in real time, repeatability and low cost of examination [15; 16; 17]. The main weakness of all non-invasive techniques is insufficient differentiation between moderate degrees of fibrosis [18]. However, it is essential to highlight that LS does not represent fibrosis directly; as a matter of fact, the LS value is a resultant of fibrosis, inflammation and blood microcirculation with a possible additional effect of hepatic steatosis or other hepatic conditions, and therefore can serve as an independent measure of liver disease progression [19]. Currently, LS testing is widely used to assess liver disease for possible prioritization of HCV treatment. It can also provide prognostic information during the post-treatment follow-up [20; 21]. Some recent studies confirmed higher diagnostic value of SWE compared to TE, and therefore we applied this technique in our practice and in this study [22; 23]. Currently, three techniques are available for elastography of the liver: one-dimensional transient elastography, (1D-TE), acoustic radiation force impulse (ARFI) that include point shear wave elastography (pSWE), and real-time two-dimensional shear wave elastography (2D-SWE). Depending on the method used, the shear wave measurement takes place perpendicular to the plane of excitation (pSWE; 2D-SWE) or parallel to excitation (1D-TE). In 1D-TE, the mechanical vibrator exerts a controlled vibrating external \"blow\" on the surface of the body, which create shear waves which propagate through the examined tissue. After that, the same probe measures the velocity of the shear wave and, after transformation, we get a measurement of stiffness. In pSWE, acoustic radiation force impulse is used to induce tissue movement in the normal direction in a single focal location, but tissue transposition is not measured itself. Longitudinal waves are converted to shear waves through the absorption of acoustic energy; then, the speed of these waves is measured and transformed to quantify tissue elasticity.