Red blood cell (RBC) aggregation is the reversible adhesion of RBCs among themselves. We previously reported a positive correlation between blood glucose level and the degree of RBC aggregation (the brightness of the B-mode image). In the present study, we investigated the contribution to the brightness according to the deviation from the central axis in measurements along with the long-axis view of the vein. The results show that the brightness changed significantly for a slight change in the lateral position in the short-axis image. We found that the stability of the measurements was not guaranteed in the long-axis view and estimated the correct analysis window range for the short-axis view.