['Hepatitis C (HCV) and hepatitis B (HBV) virus infection can lead to serious complications if left untreated, but often remain undetected in primary care. Mild alanine aminotransferase (ALT) elevations (30-100 IU/l) are commonly found and could be associated  with viral hepatitis; unfortunately, these findings frequently remain without follow-up.', 'To determine if and how mild ALT elevation can be used to identify hidden HCV and HBV infection in primary care.', 'Primary care patients referred  for liver enzyme testing were selected by a large primary care Diagnostic Centre (Saltro).', 'First, 750 anonymous samples were collected in three categories of ALT elevation (30-50 IU/l, 50-70 IU/l, and 70-100 IU/l) and tested for HCV and HBV. Second,  the national prevalence of each ALT elevation was estimated by analysing all annual ALT tests performed at Saltro.', 'HCV prevalence was 1.6% and 1.2% in patients with an ALT of 50-70 IU/l and 70-100 IU/l respectively. In patients with an ALT of 30-50  IU/l, HCV prevalence was normal (≤0.1%). HBV prevalence was normal (≤0.4%) in all groups. The estimated number of ALT tests performed nationally each year in primary care was 1.1 million. An ALT of 30-50 IU/l was found in 21.1%, an ALT of 50-70 IU/l in 5.6%, and 2.6% had  an ALT of 70-100 IU/l.'] Does follow-up of mild alanine aminotransferase elevation identify hidden hepatitis C in primary care?