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Clinical Decisionmaking

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Guidelines can help to distinguish anthrax from influenza in patients with flu-like illness and possible exposure to anthrax

In the case of intentional release of anthrax over a metropolitan area, there may be a well-defined plume with estimable risk of exposure according to geographic location. Early symptoms of inhalational anthrax are similar to those of influenza, so it is important for clinicians to be able to distinguish between the two. A new study recommends that when anthrax exposure is a possibility in a patient with influenza symptoms during flu season, doctors should use rapid influenza testing. Patients who test negative for influenza should receive short-term antibiotics for anthrax pending anthrax blood culture results.

This recommendation is the result of a decision model developed to evaluate the risks and benefits of six testing and treatment strategies in an emergency department for a hypothetical group of 100,000 patients with influenza-like illnesses who had probabilities of inhalational anthrax of 0.01 percent, 0.1 percent, 1 percent, and 10 percent. Based on the decision model, for any probability of anthrax exposure, patient mortality was always lowest when all patients were treated with antibiotics for anthrax either for 60 days or for three days pending blood culture results. These strategies, however, were associated with more morbidity (for example, adverse reactions to antibiotics) than were strategies without antibiotic treatment. The number of adverse events and antibiotic patient-days were reduced substantially with the two-test strategy, in which patients with influenza were identified early and not treated.

In general, for probabilities of anthrax equaling or exceeding 2 percent, treating all patients with antibiotics for 60 days was best, but for probabilities between 0.1 percent and 2 percent, the sensitivity of blood culture for anthrax determined the optimal strategy. When the sensitivity exceeded 95 percent, a short course of ciprofloxacin until blood culture results became available was best, but for sensitivities below 95 percent, more aggressive antibiotic use was warranted. The study was supported in part by the Agency for Healthcare Research and Quality (contract 290-00-0020).

See "Is it influenza or anthrax? A decision analytic approach to the treatment of patients with influenza-like illnesses," by Andrew M. Fine, M.D., M.P.H., John B. Wong, M.D., Hamisch S.F. Fraser, M.B., Ch.B., M.Sc., and others, in the March 2004 Annals of Emergency Medicine 43(3), pp. 318-328.

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