Can you tell me how accurate is portable ultrasound screenings of carotid arteries?

Very. As long as the portable ultrasound screening program is icavl accredited, the results will be 90%-98% accurate. Ultrasounds are operator dependent, that is why it is important that the ultrasound lab follow standard protocols to optimize results. Icavl is an acronym for... Intersocietal commission for the accreditation of vascular laboratories.
Dependable. Most ultrasound screening programs are reliable. This technology is dependent on the technician and the speciality of the lab. An accredited lab that has registered technologists should be reliable. In any event a confirmatory test such as a ct angiogram may be indicated if the results i the ultrasound are questionable.