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By Carol A. Kemper, MD
False-Positive HIV-1 RNA Results
Source: Rich JD, et al. Ann Intern Med 1999;130:37-39.
Rich and colleagues describe three probable cases of falsely-positive HIV RNA test results. Levels of plasma HIV-1 were low in all cases, ranging from 1254-1574 copies/mL. Case 1 was a healthy 12-year-old boy who had previously tested HIV-negative on multiple occasions following perinatal exposure to HIV. Following an episode of herpes zoster, a single positive HIV-1 RNA (using the BDNA assay) was obtained, but all further studies and repeat RNAs were negative. Case 2 was a healthy 40-year-old woman who had a positive HIV RNA test result (using BDNA) following unprotected sex with an HIV-positive male partner. All other studies remained negative and three subsequent HIV-RNA tests were negative. Case 3 was a bit more complex—a 20-year-old woman whose only risk factor was heterosexual sex, had positive HIV ELISA and indeterminate western blot test results on at least two occasions. Repeat studies five months later were negative, but a single HIV-RNA was positive (using RT PCR). Repeat studies remained negative six months later.
None of these patients were believed to be HIV-infected, although there were no further attempts to investigate this possibility. Rich et al argue that only one of these patients had recent significant HIV risk factors and each had only one RNA test result that was positive, raising the possibility of laboratory error or specimen mix-up. In addition, low viral test results are inconsistent with those typically observed in primary HIV infection or in long-standing disease.
On the other hand, consideration should be given to the possibility that these patients may have had transient viremia. Six patients were recently described with HIV-1 viremia despite persistent seronegativity (Sixth Retrovirus Conference. Chicago, IL. Abstract #52). Peripheral blood mononuclear cells from all six patients failed to produce antibodies to HIV-1 in vitro.
While it may be tempting to use HIV RNA as a "screening" tool, and there are certainly instances when it may seem desirable, consideration of the pre-test probability and a certain degree of circumspection is required when evaluating discrepant test results.
Infected Bite Wounds
Source: Talan DA, et al. N Engl J Med 1999;340:84-92.
A surgeon friend was recently bit on the right hand by a small (but territorial) dachshund, and wondered if he should take prophylactic antibiotics. He sustained only three small but relatively deep puncture wounds, which were promptly washed with soap and water. Should he have been given antibiotics preemptively and, if so, with what?
Talan and colleagues describe an amazing constellation of bacteria found in infected bites wounds inflicted by cats and dogs. Fifty patients with dog bites and 57 patients with cat bites were evaluated by 18 different emergency rooms throughout the United States. More than one-half of the bites occurred on the hands. About two-thirds were puncture wounds, 3% were lacerations, and the remainder were both. Tendon involvement occurred in 19% and one infection involved the finger joint. Lymphangitis was present in 44%, and nearly one-half of the wounds required incision and drainage and/or debridement. One-third of the patients were initially hospitalized for parenteral antibiotics.
Mixed aerobic and anaerobic infections were found in 63% of cat wounds and 48% of dog bites, with an average of five different organisms found in each wound. Even nonpurulent dog wounds grew an average of two different organisms (range, 0-9), and nonpurulent cat wounds grew five (range, 0-12). Only 7% of the cultures had no growth. Pasteurella species were the most frequently identified isolates from both dog (50%) and cat bites (75%). Common aerobic species found in 10% or more of wounds included various Streptococcus, Staphylococcus, Neisseria, and Moraxella species. Fusobacterium, Bacteroides, Porphyromonas, Prevotella, and Propionibacterium were common anaerobic isolates (> 10%). Capnocytophaga was isolated from 7% of cat bites and 2% of dog bites. A number of organisms not previously associated with bites or human infection were identified, including Reimerellea anatipestifer—a pathogen related to weeksella and Capnocytophaga which causes sepsis in birds (one can only imagine what the bird looked like).
The reference laboratory was much more successful than local laboratories at isolating organisms: while the reference lab was able to isolate a median of five organisms per wound, the local laboratories cultured an average of only one and negative cultures were twice as frequent.
All but four patients improved with antimicrobial therapy, each of whom initially received a first-generation cephalosporin. The most successful regimens included a beta-lactam plus a beta-lactamase inhibitor, a combination of penicillin plus a first-generation cephalosporin, or clindamycin plus a flouroquinolone. Although the microbial flora of bite wounds is much more varied and complex than previously understood, appropriate antibiotics such as augmentin or trovafloxacin coupled with aggressive debridement as needed, are adequate for most infected dog and cat bites. We are also reminded that prophylactic antibiotics are recommended for any high-risk wounds (especially deep puncture wounds from cat bites) or wounds that require surgical repair—or those that involve the hands!
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