The chances that an emergency physician will demonstrate three abdominal aortic aneurysms by ultrasound in the emergency department for credentialing: A statistician's view.

The Australasian College for Emergency Medicine requires 15 proctored examinations of the aorta for credentialing in ultrasonography for abdominal aortic aneurysm (AAA). Furthermore, at least three examinations need to be positive for an aneurysm. In the ED where AAA presentations are sporadic, what are the chances that an emergency physician (EP) will have the opportunity to demonstrate three AAAs in the next 12 months? The probability of an event occurring within a given time-frame can be modelled by the Poisson distribution. Central to the Poisson distribution is the infrequency of the event such as encountering an AAA in the ED. An EP working 30 clinical hours/week in our tertiary-referral hospital ED can be expected to encounter 15.6 (3.6 symptomatic + 12 asymptomatic) AAA in the next 12 months. The probability of seeing three or more cases during this time is 99.9%. Assuming a proctor can be found for half the cases, the probability of an EP performing proctored ultrasound examinations in at least three AAAs is 98%. This probability drops to 89% if a proctor can be found for only one-third of cases. For an EP to be almost 100% certain of meeting the credentialing requirements, he/she would need almost 10 proctored ultrasound cases of AAA to be available within his/her shifts during the year. The Poisson distribution has enabled us to model the probability of encountering a given number of AAA in the ED. Analysis such as this may help rationalise the numbers needed for credentialing.