No methods have been validated to distinguish acute from chronic HCV by now. Some test such as HCV Ab-IgM test, an ELISA for the detection of HCV core antigen, and HCV Avidity test have been introduced but they have not been proved practically useful yet. The only method to conclusively diagnose acute HCV infection is detection of HCV RNA without HCV Ab which can be confirmed by subsequent seroconversion. However, these cases are not common in the practice. HCV Ab or HCV RNA positive plus symptoms can also be diagnostic but again it is not common in practice ( 400 IU/L, and one of HCV Ab or HCV RNA positive tests. However, exacerbation of chronic HCV infection, and some other conditions in a person with chronic HCV infection such as alcoholic hepatitis and drug-induced liver dysfunction can interfere using these criteria for diagnosis of acute HCV hepatitis. We have done a systematic review of acute hepatitis C studies which revealed the variation of criteria used by different studies for inclusion of acute HCV infection cases. While a majority of studies required seroconversion to HCV antibody and/or HCV RNA, and abnormal ALT levels, the cut-off for abnormal ALT ranged from 2 to 20 times the upper limit of normal.

I am not a clinician but I think the main indicators are the clinical symptoms, and usually RNA and antibody assays are also required. I cite from the AASLD guidelines (http://www.ncbi.nlm.nih.gov/pubmed/19330875) :

"The differentiation of acute from chronic HCV infec- tion depends on the clinical presentation: namely the presence of symptoms or jaundice, and whether or not there was a prior history of ALT elevation and its dura- tion. After acute exposure, HCV RNA is usually detected in serum before antibody; HCV RNA can be identified as early as 2 weeks following exposure whereas anti-HCV is generally not detectable before 8-12 weeks."

You can also check the CDC guidelines for additional information (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6218a5.htm).

Thanks for your answer, Unfortunately the acute HCV infection is asymptomatic inst cases and absence of symptoms can not exclude the acute HCV infection.

No methods have been validated to distinguish acute from chronic HCV by now. Some test such as HCV Ab-IgM test, an ELISA for the detection of HCV core antigen, and HCV Avidity test have been introduced but they have not been proved practically useful yet. The only method to conclusively diagnose acute HCV infection is detection of HCV RNA without HCV Ab which can be confirmed by subsequent seroconversion. However, these cases are not common in the practice. HCV Ab or HCV RNA positive plus symptoms can also be diagnostic but again it is not common in practice ( 400 IU/L, and one of HCV Ab or HCV RNA positive tests. However, exacerbation of chronic HCV infection, and some other conditions in a person with chronic HCV infection such as alcoholic hepatitis and drug-induced liver dysfunction can interfere using these criteria for diagnosis of acute HCV hepatitis. We have done a systematic review of acute hepatitis C studies which revealed the variation of criteria used by different studies for inclusion of acute HCV infection cases. While a majority of studies required seroconversion to HCV antibody and/or HCV RNA, and abnormal ALT levels, the cut-off for abnormal ALT ranged from 2 to 20 times the upper limit of normal.