Reliability of transabdominal ultrasound in the measurement of prostate size

Thirty-eight patients underwent transabdominal sonographic measurement of their prostate by two different sonographers. Prostatic weight was estimated by the ellipsoid method. In 47% of the cases the second estimate of weight was within 5g of the first and in 84% of cases the second estimate was within 10g of the first. 95% of the time the second estimate was within 16g of the first. The differences in estimated weight between the two observers ranged from 1g to 22g with a mean of 6g. The estimated prostatic weight differed from the mean prostatic weight by more than 20% in 58% of cases. (Mean prostatic weight = the mean of the two weights estimated for each patient). Measurement error was less significant in the larger prostates. The accuracy is probably satisfactory in routine clinical practice when the aim is to demonstrate the presence and degree of prostatic enlargement but the error may be significant in pre-operative assessment of prostate size and measurement prior to stenting the prostatic urethra. The error, if not accounted for, could compromise a clinical trial involving serial prostatic measurements.