Sonographic assessment of renal length in the first year of life: the problem of "spurious nephromegaly".

PURPOSE

Interest in the potential diagnostic or prognostic implications of nephromegaly as evidence for compensatory renal hypertrophy has recently been emphasized in a variety of clinical settings. This project was designed to compare the results of linear and nonlinear sonographic models in the interpretation of renal size and growth during the first year of life.

MATERIALS AND METHODS

We identified all renal and abdominal ultrasound examinations that were performed between March 1994 and October 1997 in full-term infants under age 1 year during which (1) both renal lengths were measured and (2) both kidneys appeared anatomically normal. Using three different computerized algorithms based on published standards for sonographic renal length in relation to age, we calculated z-scores for the renal lengths and compared the results of the three methods: in method A the standards at birth, 1 week, 4 months, 8 months, and 1 year were all used; in method B the 1-week standard was omitted; in method C the standards at 1 week, 4 months, and 8 months were omitted.

RESULTS

We evaluated 1,234 renal measurements in 617 patients (293 boys, 324 girls; mean age 0.24 year). Compared with method A, z-scores were significantly increased when either method B or C was used (P < 0. 0001). The mean increment in z-score was + 0.433 for method B and + 1.135 for method C. The prevalence of "nephromegaly" (z > + 2) was significantly increased when subannual standards were omitted (P < 0. 0001): using method A, 20 (1.6 %) kidneys were large for age compared with 74 (6.0 %) using method B, and 214 (17.3 %) using method C. All kidneys that were large for age based on method A were also large for age using both methods B and C. The rate of false-positive diagnosis of nephromegaly was 73 % (54/74 kidneys) with method B and 91 % (194/214 kidneys) with method C.

CONCLUSIONS

Although the use of multiple subannual standards for renal length in infants less than 1 year of age is time consuming and mathematically more complicated, omission of these standards results in a statistically significant increase in the frequency of "spurious" nephromegaly.

LEARNING OBJECTIVES

Precise application of published standards is important in the interpretation of sonographic measurements of renal length. Omission of the subannual standards for renal length in children who are less than 1 year of age can result in an incorrect impression of nephromegaly.