This study was designed to investigate interobserver variability in the measurement of internal carotid artery (ICA) peak systolic velocity (PSV). We hypothesize that the reproducibility of repeated duplex scanning parameters, in the hands of very experienced vascular technologists in a laboratory accredited by the Intersocietal Commission for Accreditation of Vascular Laboratories, would be excellent.
Methods
Thirty-one patients underwent carotid duplex scanning by three vascular technologists using the same duplex scanning system. They examined patients with the laboratory's standard protocol. Statistical analysis of the sources of variation was carried out with two-way analysis of variance. The Altman-Bland method was used to detect bias and evaluate the interval of agreement between technologists for the ICA PSV on a continuous scale. The κ statistic enabled measurement of agreement for ICA PSV on a categorical scale of stenosis (<50%, 50%-70%, >70%).
Results
Patient variability was responsible for 97.2% of the total variance, with only 0.58% (P < .005) attributed to the technologists. The level of agreement on a continuous scale between the measurements of ICA PSV by our technologists is wide. For individual patients it ranged from −25% to 43% between technologists A and B, −27% to 43% between technologists A and C, and −27% to 31% between technologists B and C. When we compared the three technologists, no systematic overestimation or underestimation of the ICA PSV was found (ie, no fixed bias). The level of agreement between the technologists did not depend on the value of the PSV (ie, no proportional bias). However, analysis of ICA PSV agreement on a categorical scale revealed almost perfect agreement (κ >0.8).
Conclusion
From measurements of PSV, the severity of carotid stenosis can be reproducibly categorized into ranges (<50%, 50%-70%, >70). However, the unacceptably wide interobserver variation of ICA PSV on a continuous scale makes the interchangeability of our technologists' measurements problematic for clinical use, as in determination of progression of severity of stenosis. When an ICA PSV measurement is in the vicinity of a cutoff value, the diagnostic accuracy may be improved with the use of additional diagnostic testing.
