Although much has been learned about routes of infection of hepatitis C, it is still difficult to advise patients about their risk of infection following specific exposures. The development of polymerase chain reaction methods for detecting hepatitis C RNA has provided the potential for assessing the infectiousness of people who are positive for the hepatitis C virus. Dore and colleagues reviewed the literature concerning transmission of the hepatitis C virus to assess the role of polymerase chain reaction in estimating the infectiousness of people with the hepatitis C virus.

The authors searched Medline and Embase databases plus bibliographies of published papers to identify all published studies examining the transmission of hepatitis C from patients testing positive for the virus. Only studies using the polymerase chain reaction with or without other tests to establish infection status were included in this review.

Of the 29 studies published between 1992 and 1996, 21 studies examined vertical hepatitis C transmission from pregnant mothers; three studied hepatitis C transmission after bone marrow or solid organ donation; three studied transmission after blood transfusion; and two studies examined transfusion after needlestick exposures among health care workers. Data were available on a total of 2,022 people exposed to sources positive for hepatitis C infection. Among the 1,148 persons exposed to sources testing positive by polymerase chain reaction, 148 cases of confirmed transmission occurred.

The rate of transmission of hepatitis C among the 903 children born to mothers with hepatitis C infection and positive polymerase chain reaction ranged up to 42 percent, with a combined rate from all studies of 6.2 percent. Five studies confirmed higher rates of vertical transmission when the mother had a high level of viremia. Positive human immunodeficiency virus (HIV) infection status was also associated with higher rates of vertical transmission in mothers with hepatitis C antibody and was estimated to be 15.8 percent (range: 11.8 to 19.8 percent). In one study, vertical transmission was significantly increased in infants delivered vaginally compared with infants delivered by cesarean section (32 percent versus 6 percent), but no association could be demonstrated between hepatitis C transmission and breast-feeding. Measures of maternal hepatic function and infection with specific hepatitis C genotypes did not appear to influence the risk of vertical transmission of infection.

Studies of transplantation recipients showed very high rates of transmission of hepatitis C infection (78 percent; range: 72 to 94 percent) when the polymerase chain reaction was positive in the donor. Similarly, studies of recipients of blood transfusions showed transmission rates of 74 to 92 percent, with a pooled estimate of 83 percent. Only two studies examined risk to health care workers following needlestick injuries from patients with confirmed hepatitis C infection. The pooled estimate from these studies was 6.1 percent (range: 2.3 to 9.9 percent).

The authors estimate the current risk for infection when the polymerase chain reaction is positive in the infection source to be 6.2 percent after perinatal exposure, 6.1 percent following needlestick exposure, 78 percent after transplant exposure and 83 percent following transfusion of blood infected with hepatitis C.

The authors conclude that based on the evidence from studies of vertical transmission, the level of viremia and coinfection with HIV are important risk factors for transmission that deserve further study. The authors also believe that the viral genotype could be a factor in the degree of risk of transmission. The probability of infection from a source who has antibodies to hepatitis C but negative polymerase chain reaction is extremely small and may in fact pose “virtually no risk.” This point may be extremely important when counseling patients at risk for transmitting hepatitis C and those exposed to infective sources, particularly health care workers who have antibodies to hepatitis C.