State health officials have confirmed four more cases of hepatitis C as they continue to investigate an outbreak linked to Exeter Hospital’s cardiac catheterization lab. Altogether, 31 people have tested positive for the same strain of the disease since the investigation began in late May. State officials suspect that a worker’s misuse of drugs led to the outbreak. Anyone who was treated at the lab since October 2010 has been asked to get tested, either at the hospital or at alternate sites in Hampton and Portsmouth. Hepatitis C is a blood-borne viral infection that can cause liver disease and chronic health issues.