Diagnosis of Smallpox

CDC Smallpox Diagnostic Algorithm (22:V4:3)

The Algorithm was created so that clinicians would have a tool with a high specificity and a minimum of false positives in order to determine variola from other rash illnesses. It is estimated that there will be approximately 1 million cases of varicella (or chickenpox) in the US in 2003 and many more millions of cases of other rash illnesses. With this number of cases, if 1 out of every 1000 varicella cases were suspected to be smallpox, there would be 1000 false alarms per year. Most public health systems cannot easily deal with thousands of false alarms.

The algorithm strategy needed was one that has high specificity to accurately detect the first case of smallpox (should there ever be one). High specificity strategy will serve to minimize unnecessary laboratory testing for smallpox, that would have the potential risk of producing a false positive lab test. The algorithm provides a method to focus resources on the most suspicious cases.

There are two primary goals of the Rash Illness Algorithm. (22:V4:39) The first goal is that the rash illness algorithm is to provide a systemic approach to evaluation of cases of febrile vesicular or pustular rash illness. The second goal is that the algorithm uses the main clinical features of smallpox to establish major and minor criteria and uses these criteria to classify cases of vesicular/pustular rash illness into risk categories (likelihood of being smallpox). 

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