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Case 1: Discussion

Risk and Incidence of Seroconversion

The average risk of HIV transmission following accidental percutaneous injury (needlestick) involving an HIV-infected source patient is approximately 0.3%, assuming that no postexposure chemoprophylaxis is given to the health care worker. The risk with a mucous-membrane exposure is approximately 0.09%[1,2]. These risks are significantly lower than the risk of acquiring hepatitis C virus or hepatitis B virus from a similar injury (Figure 1)[1]. As of December 2001, the Centers for Disease Control and Prevention (CDC) had reported 57 documented cases in which health care personnel became infected with HIV following an occupational exposure[3]; most of these cases involved a percutaneous injury (Figure 2)[4]. In an analysis of the first 56 cases, 49 of these health care workers were exposed to HIV-infected blood, 3 to concentrated HIV in a laboratory, 1 to visibly bloody fluid, and 3 to an unspecified fluid (Figure 3)[4]. One hundred and thirty-seven other cases of HIV infection have been recorded in health care workers with a history of occupational exposure, but the documentation in these cases was insufficient to clearly establish occupational exposure as the source of transmission[4]. The CDC estimates that more than 360,000 percutaneous injuries occur each year in United States hospitals, with more than 60% of these involving hollow-bore needles[3].

Zidovudine for Postexposure Prophylaxis

In 1997, the CDC Needlestick Surveillance Group published findings from their retrospective, case-controlled investigation of factors that influence the risk of HIV transmission among health care workers who sustain a needlestick injury (Figure 4)[5]. In this study, the investigators compared 33 cases (health care workers who contracted HIV following an occupational percutaneous needlestick injury) with 665 controls (health care personnel who failed to contract HIV despite suffering a similar percutaneous injury involving an HIV-infected source patient). Many of the health care workers among both cases and controls had used zidovudine (Retrovir) for postexposure prophylaxis, allowing for an estimate of the efficacy of this intervention. Most of the health care workers who took zidovudine received their first dose within 4 hours and most took at least 1000 mg/day. The investigators found that zidovudine reduced the risk of HIV transmission by 81%. The study has several limitations, including the small sample size of HIV transmission cases, the use of cases and controls drawn from different populations, and the biases that can occur with a retrospective, non-randomized trial. Nevertheless, if zidovudine had provided no protective effect, one might have expected a higher proportion of zidovudine recipients to have become infected compared with those who did not receive zidovudine, since at the time of this study postexposure prophylaxis was not routinely given and zidovudine use might have served as a marker for higher-risk injuries. The CDC did initiate a prospective, randomized, placebo-controlled trial to examine the efficacy of zidovudine postexposure prophylaxis, but investigators discontinued the trial because of insufficient enrollment.

Risk Factors for Seroconversion

Multiple other factors appear to affect the risk of seroconversion following percutaneous injury, all of which relate to the amount of virus in the inoculum. High-risk injuries include needlesticks involving a needle that had been used in an artery or vein of the source patient, and those in which there is visible blood on the needle or device immediately prior to the injury. Injuries involving large-diameter hollow-bore needles probably enhance the risk of HIV transmission when compared with solid-bore needlestick injuries, presumably because they involve exposure to a larger volume of blood. Indeed, there have been no well documented infections as a result of needlesticks with solid-bore needles, despite the relative frequent injuries involving solid-bore needles. In the CDC Needlestick Surveillance Group study, a large-diameter, hollow-bore needlestick injury was associated with an increased risk of HIV transmission, but this association did not reach statistical significance in a multivariate analysis (p = 0.08)[5]. The HIV RNA level (viral load) of the source patient probably also correlates with risk of transmission, as reflected by the increased risk of transmission among cases where the source patient has end-stage AIDS, in which viral loads are typically high.

The health care worker's use of gloves and the drying time of the needle may also affect the risk of transmission. Although absence of glove use was not documented as a risk factor by the CDC Needlestick Surveillance Group, a study of simulated needlestick injuries using an animal model has shown that glove use reduces the volume of blood transmitted to the underlying skin by approximately 50%. Given that the risk of transmission appears to increase with higher volumes of blood, any measure that decreases the volume of blood, such as wearing gloves, would likely reduce this risk. Laboratory experiments suggest that HIV infectivity decreases approximately tenfold every 9 hours when it is exposed to drying conditions[7]. Other experiments, however, suggest that in a cool, humid climate, viable HIV may persist for up to 6 weeks within a syringe[8]. Hence, dried blood at the tip of a discarded needle probably carries a very low risk of HIV transmission, but the liquid contents within a needle could contain infectious HIV for a long period of time.

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    Figure 1. Estimated Pathogen-Specific Seroconversion Rate Per Exposure for Occupational Needlestick Injury

    Abbreviations: HCV = hepatitis C virus; HBsAg = hepatitis B surface antigen; HBeAg= hepatitis B e antigen

    The risk of transmission of HIV, hepatitis C virus (HCV), and hepatitis B virus (HBV) to a health care worker following a percutaneous injury involving an infected source patient varies considerably by pathogen. For HBV, the highest risk occurs when the source patient has evidence of hepatitis B e antigen. Source: Centers for Disease Control & Prevention. MMWR 2001;50(RR-11):1-52.

    Figure 1
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    Figure 2. Type of Exposure Involved in Transmission of HIV to Health Care Workers

    Among 56 clearly documented cases of transmission of HIV from an infected patient to a health care worker, percutaneous injury was by far the most common type of exposure. Mucocutaneous exposure was the next most common type of exposure. Source: Centers for Disease Control & Prevention. MMWR 2001;50(RR-11):1-52.

    Figure 2
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    Figure 3. Source of HIV Involved in HIV Transmission to Health Care Workers

    Blood is the most common source of HIV involved in cases of HIV transmission to health care workers, but other bodily fluids (especially if visibly bloody) and laboratory samples of HIV also carry a risk of transmission. Source: Centers for Disease Control & Prevention. MMWR 2001;50(RR-11):1-52.

    Figure 3
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    Figure 4. Risk Factors for HIV Transmission with Occupational Exposure to HIV-Infected Blood

    *The authors defined terminally ill as disease leading to death of the source patient from AIDS within 2 months after the health care worker's exposure.

    This table shows a logistic-regression analysis of risk factors for HIV transmission to a health care worker after exposure to HIV-infected blood in the occupational setting. A deep injury was the strongest risk factor. Data from Cardo DM, Culver DH, Ciesielski CA, et al. A case-control study of HIV seroconversion in health care workers after percutaneous exposure. Centers for Disease Control and Prevention Needlestick Surveillance Group. N Engl J Med 1997;337:1485-90.

    Figure 4