Nephrocalcinosis in Premature Infants: Variability in Ultrasound Detection
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Original Article nnnnnnnnnnnnnn Nephrocalcinosis in Premature Infants: Variability in Ultrasound Detection Thomas Campfield, MD Francis J. Bednarek, MD CONCLUSION: Mariann Pappagallo, MD There is significant variability among radiologists in the ultrasound identification of nephrocalcinosis in premature infants; a 7.5-MHz ul- Frederick Hampf, MD trasound transducer is associated with less variability in recognizing this John Ziewacz, MD lesion. Jacqueline Wellman, MD Gary Rockwell, MD Gregory Braden, MD Renal calcification is an uncommon problem in children, but recent Patrecia Flynn-Valone, RD reports have called attention to the development of nephrocalcinosis Michael Neylan in premature infants. These calcium oxalate deposits may be identi- Antonio Pangan, MD fied on plain films of the abdomen but are more readily detected by ultrasound.1,2 Estimates of the incidence of nephrocalcinosis in pre- 3 OBJECTIVE: mature infants have varied considerably. Jacinto et al. have reported 4 To measure variability among radiologists in the ultrasound diagnosis of a 65% incidence of nephrocalcinosis, whereas Short and Cooke re- nephrocalcinosis in premature infants. ported a 27% incidence in very low birth weight infants. Differences in image quality obtained using different ultrasound transducers or METHODOLOGY: observer variability in image interpretation may have contributed to In this prospective multicenter study, renal ultrasounds were performed this variation in incidence. No data are available regarding these two on 54 very low birth weight infants using a 5.0- and 7.5-MHz transducer, potential sources of variability. It is also possible that improvements and these ultrasounds were read independently by three radiologists. k in neonatal care, leading to a decrease in the severity of chronic lung coefficients were calculated to assess variability in identification of disease and less exposure to diuretic therapy, may have contributed to nephrocalcinosis among the radiologists. a change in the incidence of this lesion. This study was designed to examine the hypothesis that renal RESULTS: ultrasounds performed using a 7.5-MHz transducer are associated The k coefficient (6 confidence intervals) using a 5.0-MHz transducer with less inter-observer variability compared with a 5.0-MHz trans- was 0.143 (0.108, 0.178); using the 7.5-MHz transducer, the k coefficient ducer in the identification of neonatal nephrocalcinosis. was 0.268 (0.243, 0.293). All three radiologists agreed in their identifica- tion of nephrocalcinosis on 3 of 54 ultrasounds using a 5.0-MHz trans- METHODS ducer; a total of 6 of 54 ultrasounds obtained using a 7.5-MHz transducer Patients were read as positive by all three radiologists. This prospective study took place between August 1, 1995 and July 31, 1996 at three neonatal intensive care units. All infants with a birth weight ,1500 gm were considered eligible for the study. Infants were not included in the study for the following reasons: failure to obtain Departments of Pediatrics (T. C., G. R.), Radiology (F. H.), and Medicine (G. B.), Baystate Medical Center, Springfield, MA; Departments of Pediatrics (T. C., G. R.), Radiology (F. H.), informed consent from parents, transfer of the infant to another facil- and Medicine (G. B.), Tufts University School of Medicine, Boston, MA; Division of Neonatol- ity before images could be obtained, or the appearance of hydrone- ogy (F. J. B.), Memorial Health Care, University of Massachusetts Medical Center, Worcester MA; Department of Pediatrics and Obstetrics and Gynecology (F. J. B.), University of Massachu- phrosis or other congenital renal abnormalities on the ultrasound setts School of Medicine (A. P.), Worcester MA; Departments of Pediatrics (M. P.) and Radiology images. In addition, some infants could not be included in the study (J. Z.), University of Connecticut Health Center, Farmington, CT; Diagnostic Radiology (J. W.), Memorial Health Care, Worcester, MA; Department of Nutrition, University of Massachusetts, because one of the ultrasound transducers was broken or unavailable Amherst, MA; and Ross Products Division, Abbott Laboratories, Columbus, OH. when the images were obtained. Renal ultrasounds were performed at This work was supported by Ross Products Division of Abbott Laboratories. 7 to 8 weeks of age using both a 5.0-MHz sector transducer and a Address correspondence and reprint requests to Thomas Campfield, MD, Baystate Medical 7.5-MHz linear array transducer. The ultrasound equipment used in Center, Newborn Medicine, W2810, Springfield MA 01199. this study included a Toshiba SSH 140 A (Tokyo, Japan) and an ATL Journal of Perinatology (1999) 19(7) 498–500 © 1999 Stockton Press. All rights reserved. 0743–8346/99 $12 498 http://www.stockton-press.co.uk Ultrasound Detection of Nephrocalcinosis Campfield et al. 400 Apogee (Bothell, WA). Three longitudinal and three transverse views of each kidney were obtained. Ultrasound images were obtained at the bedside by radiologists and ultrasound technicians, but their bedside impressions of these images were not included in this study. Patient confidentiality was maintained by covering the patient’s name on the ultrasound hard copy image. Image Interpretation Hard copy images of each renal ultrasound were interpreted indepen- dently by three radiologists. Of the three radiologists involved in the study, two had training in ultrasonography; one radiologist had train- ing in pediatric radiology as well as ultrasonography. These radiolo- gists were not aware of the infant’s clinical condition, and they were unaware of other radiologists’ interpretation of these images. The only images shown to these radiologists were those described above; if other views were obtained because of a suspected abnormality, these Figure 1. k coefficients (6 confidence intervals) based on the presence or ab- other images were not included in the study. These criteria were estab- sence of nephrocalcinosis on ultrasounds on 54 premature infants, obtained with 5.0- and 7.5-MHz transducers, read independently by three radiologists. lished as part of the study to minimize bias in interpretation. Images were interpreted as normal or positive for nephrocalcinosis based on the presence of echogenic foci with acoustic shadowing. Positive read- ings were classified as “focal” or “diffuse” nephrocalcinosis. For the obtained using a 5.0-MHz transducer was 0.143 (0.108, 0.178), k purposes of this study, diffuse nephrocalcinosis was defined as whereas a 7.5-MHz transducer resulted in a value of 0.268 (0.243, nephrocalcinosis involving .50% of both kidneys. Nephrocalcinosis 0.293). Thus, ultrasounds obtained using a 7.5-MHz transducer re- involving ,50% of one or both kidneys was classified as focal sulted in significantly less variability in interpretation compared with nephrocalcinosis. a 5.0-MHz transducer (Figure 1). The percentage of ultrasounds interpreted as positive by individ- Data Analysis ual radiologists using a 5.0-MHz transducer was 28%, 6%, and 35% Interobserver variability in the diagnosis of nephrocalcinosis was for radiologists 1, 2, and 3, respectively. The percentage of ultrasounds k k measured using the coefficient. The coefficient measures agree- interpreted as positive using a 7.5-MHz transducer was 35%, 11%, and ment among observers above that caused by chance alone. In general, 39% for radiologists 1, 2, and 3, respectively (Figure 2). These propor- k values between 0.00 and 0.20 indicate minimal agreement; at the tions were significantly different for both transducers by x-squared k other extreme, values between 0.80 and 1.00 indicate nearly perfect analysis. Because radiologists 1 and 3 were similar with regard to k agreement. values were calculated for both the 5.0-MHz and 7.5- their percentage of positive readings, their image interpretations were MHz transducers, and confidence intervals were used to determine compared using the k statistic. This comparison gave a value of 0.433 whether one transducer gave results significantly less variable than with the 5.0-MHz transducer and a k value of 0.400 with the 7.5-MHz the other. transducer, reflecting only moderate agreement between these two To determine whether the variability observed in ultrasound observers. In other words, although the percentages of positive read- interpretation could be attributed to “over-reading” or “under-read- ings were similar, different images were interpreted as positive fre- ing” by an individual radiologist, the proportion of positive interpre- quently enough to show only moderate agreement using the k tations for each transducer by each radiologist was determined. These statistic. x proportions were compared using the -squared analysis. The proportion of ultrasounds interpreted as positive by all three To determine whether either transducer was associated with more radiologists using the 5.0-MHz transducer was 3 of 54 ultrasounds frequent identification of nephrocalcinosis, the proportion of ultra- (5.6%), whereas 6 of 54 ultrasounds (11.1%) were interpreted as posi- sounds read as positive for nephrocalcinosis by all three radiologists tive by all three radiologists using a 7.5-MHz transducer. Although was determined for each transducer, and these proportions were then there was a trend toward more uniformly positive image interpreta- compared using Fisher’s exact test. tion with the 7.5-MHz transducer, this difference did not achieve sig- nificance (Fisher’s exact test). The three ultrasounds read as abnor- RESULTS mal with the 5.0-MHz transducer were also uniformly read as A total of 54 patients were studied. The mean birth weight of these 54 abnormal by all three radiologists with the 7.5-MHz transducer. infants was 976 6 28 gm (mean 6 SEM) and the mean gestational There were a total of 38 positive readings among all three radiol- age was 27.1 6 0.3 weeks; ultrasounds were performed at 49.7 6 1.8 ogists using the 5.0-MHz transducer; only 2 of these 38 (5.3%) were days of age. interpreted as showing diffuse rather than focal nephrocalcinosis. The k statistic (6 confidence intervals) for renal ultrasounds Similarly, of the 46 positive image interpretations with the 7.5-MHz Journal of Perinatology (1999) 19(7) 498–500 499 Campfield et al.