Comparison of 2 A-scans

To compare 2 A-scan instruments with regard to differences in measured results for the same patient sample.

St. Erik's Eye Hospital, Stockholm, Sweden.

In a study to evaluate the lens-haptic plane concept of intraocular lens (IOL) power calculation, 148 patients eligible for cataract surgery were measured with 2 different A-scan instruments (BVI Axis and Sonomed 1500). The axial length (AL), anterior chamber depth (ACD), and lens thickness (LT) results were analyzed for systematic differences and random errors.

The Sonomed 1500 measured systematically longer than the BVI Axis for AL (0.41 mm) and ACD (0.28 mm), although the correlation was good (r = 0.99 and r = 0.87, respectively). The LT correlated poorly (r = 0.18) and showed no systematic trend. The relative random errors (standard deviations) in ACD (7.2%) and LT (18.6%) were larger than that of the AL (0.8%). The systematic difference in the AL corresponds to a 1.0 diopter difference in the A-constant.

The large random errors in the ACD and LT reduce their value as predictors of postoperative IOL position in formulas that use them. Systematic differences in AL can be large enough to require separate formula constants for different pieces of equipment. If this is the situation in 1 setting, there is a risk of mistakes. This confusion could be avoided if there were an agreed standard and a universal calibration procedure for instruments intended for AL measurement.