Pediatr Nephrol (2004) 19:153–156 DOI 10.1007/s00467-003-1363-2
ORIGINAL ARTICLE
Ima Moorthy · Deirdre Wheat · Isky Gordon
Ultrasonography in the evaluation of renal scarring using DMSA scan as the gold standard Received: 2 June 2003 / Revised: 6 October 2003 / Accepted: 6 October 2003 / Published online: 11 December 2003 IPNA 2003
Abstract Dimercaptosuccinic acid (DMSA) renal scan is presently the technique of choice for assessing renal scars. Recent advances suggest that ultrasonography could replace DMSA scan for this purpose. This paper describes the experience of a tertiary pediatric referral hospital performing ultrasonography and DMSA scans in the assessment of renal scarring. Investigations were conducted 3–6 months after patients presented with urinary tract infection (UTI). Results were extracted from the radiology information system and recorded for analysis. All children with a UTI who had undergone DMSA and ultrasound examination on the same day between January 1995 and December 1999 were included; 930 kidneys were compared. DMSA scan was utilized as the reference method. When used to detect focal renal scarring, ultrasonography had a sensitivity of 5.2%, specificity of 98.3%, a positive predictive value (PPV) of 50% and a negative predictive value (NPV) of 75.8%. When used to detect diffuse renal scarring, ultrasonography had a sensitivity of 47.2%, specificity of 91.8%, PPV of 60.8% and NPV of 86.6%. Our results demonstrate that although ultrasonography has a good specificity for the detection of renal scarring compared with DMSA, it has low sensitivity, PPV and NPV. Ultrasonography cannot I. Moorthy Department of Radiology, Guy’s and St. Thomas’ NHS Trust, London, UK I. Moorthy · I. Gordon Department of Radiology, Great Ormond Street Hospital for Children NHS Trust, London, UK D. Wheat Department of Clinical Informatics, Great Ormond Street Hospital for Children NHS Trust, London, UK I. Moorthy ()) 47 Alwyne Road, Wimbledon, London SW19 7AE, UK e-mail:
[email protected] Tel.: +44-208-9461076 Fax: +44-207-9605839
be substituted for DMSA scan in the evaluation of focal renal scarring. Keywords Radioisotope scanning · Ultrasonography · Urinary tract infections
Introduction Urinary tract infection (UTI) with or without vesicoureteric reflux can lead to renal scarring [1]. Children with scarred kidneys are predisposed to hypertension, chronic renal failure and toxaemia of pregnancy [2, 3]. For the past decades, dimercaptosuccinic acid (DMSA) renal scans have been considered the most-accurate method for assessment of renal scarring [4, 5, 6]. Recent advances in ultrasound technology have led to the suggestion that ultrasonography could replace DMSA scan for this purpose [7]. This paper describes the experience of a tertiary pediatric referral hospital performing ultrasound and DMSA scans in the assessment of renal scarring.
Materials and methods All children who presented to the radiology department with a proven diagnosis of UTI between January 1995 and December 1999 were included. Only children who underwent ultrasonography and DMSA scan on the same day were included. Imaging data on these children were extracted from the radiology patient information management system and recorded. The age range was 3 months to 16 years. If a child had more than one attendance, only the first attendance was used. Exclusion criteria included children with a single kidney, transplant kidney and imaging undertaken for reasons other than a UTI, e.g., hypertension. All investigations were carried out 3–6 months after patients initially presented with UTI. Patients attended a dedicated pediatric radiology department where there is a dedicated ultrasound unit and nuclear medicine facility. The ultrasound examination immediately preceded the DMSA scan. Ultrasound examinations were performed using Acuson XP10 machines with probes (of varying MHz) depending on the child’s size; probes of 4–13 MHz probes were used. A standardised ultrasound examination protocol was followed. Longitudinal and transverse grey-scale images through both kidneys were obtained ventrally and dorsally. Kidneys were
154 Table 1 Criteria for diagnosis of scarring on dimercaptosuccinic acid (DMSA) scan
Diffuse scarring
Focal scarring
Differential function<45% with homogenous uptake
Diffuse or sharp indentation in contour with thinning of cortex Any shaped defects with loss of renal volume Degree of photopenia more commonly severe or absent activity
Table 2 Criteria for diagnosis of scarring on ultrasound scan Diffuse scarring
Focal scarring
Global cortical thinning
Approximation of sinus echoes to cortical surface with or without underlying calyceal dilatation Irregularity of cortical outline
>10% difference in renal length on prone view
routinely assessed for hydronephrosis, echogenicity, corticomedullary differentiation, lengths (by comparison with standards for age) and regularity of cortical outline. Doppler studies were not carried out routinely in this group of children. The ultrasound examinations were supervised by a consultant pediatric radiologist and undertaken by either the consultant or the trainee radiologist or fully trained pediatric sonographers. Video documentation was not considered necessary as the report of the examination took place before the child left the ultrasound facility. Planar DMSA imaging studies were performed using one of two single-headed gamma cameras and a high-resolution parallel hole collimator. Views in the posterior and both posterior oblique projections were obtained for 300 kilocounts or more. The DMSA studies were performed by pediatric nuclear medicine technicians and were reported by a consultant pediatric radiologist trained in nuclear medicine. Focal scarring on DMSA was defined according to the criteria described by Patel et al. [8] (Table 1). Diffuse scarring on DMSA was defined as a differential function of <45% with homogenous uptake on the posterior view, as proposed by the consensus group on renal cortical scintigraphy in children with UTI [9] (Table 1). Scarring on ultrasonography was defined according to the criteria proposed by Barry et al. [7]. This included focal approximation of sinus echoes to the cortical surface. Underlying calyceal dilatation was not considered essential for the diagnosis of scarring. Generalised approximation of sinus echoes to the cortical surface, denoting generalised cortical thinning, or a 10% difference in prone renal length was taken to denote diffuse scarring on ultrasonography (Table 2). The sensitivity, specificity, positive predictive value and negative predictive value of ultrasonography in indicating focal scarring and diffuse renal scarring was calculated using DMSA scan as the reference method. Ultrasonography was considered a “true positive” if the location of renal scarring exactly matched the location of the defect reported on DMSA scan.
Table 3 Results: focal scarring in kidneys Ultrasonography
Positive Negative Total
DMSA scan Positive
Negative
Total
12 219 231
12 687 699
24 906 930
Sensitivity of ultrasonography=12/231=5.2% Specificity of ultrasonography=687/699=98.3% Positive predictive value of ultrasonography=12/24=50% Negative predictive value of ultrasonography=687/906=75.8% Table 4 Diffuse scarring in kidneys Ultrasonography
Positive Negative Total
DMSA scan Positive
Negative
Total
93 104 197
60 673 733
153 777 930
Sensitivity of ultrasonography=93/197=47.2% Specificity of ultrasonography=673/733=91.8% Positive predictive value of ultrasonography=93/153=60.8% Negative predictive value of ultrasonography=673/777=86.6%
was 47.2%, 91.8%, 60.8%, and 86.6%, respectively (Table 4).
Discussion Results In total 2,322 records were initially examined; 465 patients (79.4%) had both examinations on the same day and had 2 kidneys. Therefore, 930 kidneys could be compared (465 right and 465 left kidneys). The sensitivity, specificity, positive predictive value and negative predictive value for ultrasonography in the detection of focal renal scarring was 5.2%, 98.3%, 50%, and 75.8%, respectively (Table 3). The sensitivity, specificity, positive predictive value and negative predictive value for ultrasonography in the detection of diffuse renal scarring
This is a retrospective study in children with UTI who underwent ultrasonography and DMSA scans on the same day, at least 3 months after the proven UTI. Since both ultrasonography and DMSA scan were carried out on the same day, there was no possibility for new scars to develop in the interval between the two investigations. Any bias that may have been introduced by the inclusion of multiple sets of studies in the same patient has been eliminated through inclusion of only a single set of reports for each patient. At the time of data collation, no attempt was made to coordinate the ultra-
155
sound report with the corresponding DMSA report, thus avoiding investigator bias. Planar DMSA imaging with a high-resolution parallel hole collimator has been reported to be as sensitive as pinhole collimators [10]. Rossleigh et al. [10] found no statistically significant difference in renal scar detection between pinhole imaging, planar imaging or single photon emission computed tomography (SPECT) in their study correlating imaging of renal scarring with pathological findings in the pig model. The consensus group on renal scintigraphy found that planar imaging with a highresolution collimator was adequate for the detection of renal scarring [9]. Calculation of accurate differential function can only be performed by parallel-hole planar imaging and not by pinhole or SPECT imaging [10]. SPECT has a number of drawbacks, which limit its use in pediatrics. It requires the use of heavy sedation, requires longer acquisition times, is prone to movement artefact and a higher incidence of false positives for renal scarring [10, 11, 12]. Our planar imaging protocol, which includes posterior and right and left oblique images, resulting in imaging in three planes, avoids misinterpretation of renal shortening. Previous investigators have found that while focal scarring on DMSA scan was easily defined, diffuse scarring was more difficult to diagnose [13]. However a differential function outside 45%–55% is regarded as abnormal. These kidneys, when no focal defect is seen but the kidney may be considered functionally small, we have termed “diffuse scarring”. We used criteria similar to Barry et al. [7] to define focal scarring on ultrasonography. However since the longitudinal length of the kidney was always measured, a difference in length of >10% was regarded as abnormal even in the absence of a focal defect. Standardised renal length measurements for age are a widely accepted measure of renal size on ultrasonography [14]. The most-frequent causes of “diffuse scarred” kidney with no focal defect include hypoplasia, dysplasia, renal vascular injury, post-obstructive atrophy and the so-called small normal kidney. This latter entity is not well understood and is thought to be the same as the congenital small kidney. Although our criteria for diffuse scarring may have resulted in the mislabelling of some small kidneys, this mislabelling would be expected to affect both the DMSA and ultrasound studies equally, thus increasing the apparent sensitivity of ultrasonography in the detection of diffuse scarring. Furthermore, small kidneys, irrespective of whether failure of growth is due to previous infection or to a vascular insult, are prone to similar long-term complications. Barry et al. [7] found that in the hands of dedicated pediatric radiologists, allowing ample examination time and using state-of-the-art equipment similar to the equipment used in this study, ultrasonography achieved an excellent sensitivity and specificity in the detection of renal scars. These results are not borne out by our study, which looked at the reports of investigations carried out in the same time period as Barry et al. [7]. Although our ultrasonography equipment was state of the art during the
years studied, our operators varied in levels of expertise and experience. Arguably, our circumstances more closely approximated the everyday situation in most hospitals. There have been further advances in ultrasound equipment since the end of our study period. In our own department, the Acuson XP10 has been superseded by newer equipment, which has improved Doppler capabilities, notably allowing more-sensitive power Doppler imaging. However, while power Doppler imaging may show promise in the imaging of the ischaemic changes of acute pyelonephritis, it does not have a role in the imaging of the fibrosis that denotes established scarring [10, 15]. Hence our results, using grey-scale ultrasonography alone, remain valid. A recent meta-analysis highlighted the need for welldesigned studies of large numbers of kidneys to evaluate the accuracy of ultrasonography in the detection of renal scarring [16]. The authors of this meta-analysis identified only ten studies published in the English language literature between 1985 and 1997, containing sufficient information to permit the calculation of the sensitivity and specificity of ultrasonography relative to DMSA scan. They found methodological flaws in all ten studies, which gave a wide range of sensitivities (37%–100%) and specificities (65%–99%) for ultrasonography in the detection of renal scarring using DMSA scan as the gold standard [16]. Our retrospective study aimed to study a large number of kidneys, trying to avoid some of the identified methodological defects; however a retrospective study can not rule out some variables. Our results demonstrate that although ultrasonography has a good specificity in the detection of renal scarring compared with DMSA scan as the gold standard, it has low sensitivity, positive predictive value and negative predictive value. We conclude that, at present, ultrasonography cannot be substituted for DMSA in the evaluation of renal scarring. Acknowledgements We thank Dr. Rose de Bruyn, Consultant Pediatric Radiologist, Great Ormond Street Hospital, for her help in preparing this manuscript. We thank the staff of the Radiology and Nuclear Medicine Departments at Great Ormond Street Hospital for their technical help in extracting relevant data from electronic systems. We thank Dr. Angie Wade, Medical Statistician, The Institute of Child Health, and Ms. Uma Moorthy, Statistician, for their statistical advice. We thank Ms. Subadra Ambikapathy, Information Services Department, Great Ormond Street Hospital, for her assistance with data formatting.
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