Measurement and Comparison of Kidney Dimension Using Ultrasonography and Computed Tomography Scan

¹ Resident of Radio-Oncology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran

² Department of Radiology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran

³ Department of Radiology, School of Medicine, Shahrekord University of Medical Sciences, Shahrekord, Iran

*Corresponding author: Sam Mirfendereski, Department of Radiology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran. Email: doctorsam1396@gmail.com

EXTENDED ABSTRACT

Background

Kidney dimensions are clinically important in the diagnosis and management of renal disease. In adults, renal size varies with age and sex, and deviations from the expected range may accompany different renal disorders [1]. In patients with azotemia, distinguishing acute from chronic renal failure can be difficult when physical examination and routine laboratory findings are insufficient; reduced kidney size can support chronicity and may influence timely management [1]. Renal enlargement has also been described in insulin-dependent diabetes, together with higher glomerular filtration rate and renal plasma flow [2]. Consequently, accurate and reproducible assessment of kidney size is relevant both for characterization of renal disease and for longitudinal clinical decision-making.

Ultrasonography is commonly used for renal measurement because it is noninvasive, inexpensive, widely available, and free of ionizing radiation. However, renal length estimated by ultrasonography can be influenced by bowel gas, suboptimal patient positioning, anatomic variation, respiratory motion, operator technique, and failure to obtain the true maximal longitudinal axis [3]. Computed tomography (CT), in contrast, permits multiplanar image reconstruction and is extensively used for evaluating renal disorders, stones, tumors, vascular anatomy, trauma, and pretransplant anatomy [4,5]. The authors therefore considered CT a suitable reference method for comparing renal measurements. Because comprehensive comparative data on kidney dimensions measured by ultrasonography and CT were limited, this study aimed to determine renal dimensions by both methods, quantify their correlation and agreement, and evaluate the diagnostic performance of ultrasonography for identifying normal kidney size.

Methods

This cross-sectional study enrolled adults older than 18 years who presented to Hajar Hospital in Shahrekord, Iran, and had an indication for ultrasonography of the kidneys or abdomen. Participants were included by census sampling. Renal dimension was defined as the greatest kidney size obtained in the longitudinal plane. Measurements were first performed by transabdominal ultrasonography using a Mindray DC-60 Exp system with a 2-5 MHz probe, with patients examined in the supine position and, when needed, in an oblique position. Among these patients, individuals who also underwent abdominal CT for any clinical indication within less than one week were selected for paired comparison.

CT measurements were obtained using a 16-slice Siemens scanner. Images were reconstructed in multiple planes with thin 1.5-mm sections to identify the maximal renal dimension accurately. Ultrasonographic and CT measurements of the right and left kidneys were then compared. Data were analyzed in SPSS using the t test and Pearson correlation. Agreement between methods was assessed using Bland-Altman analysis, and receiver operating characteristic (ROC) analysis was used to evaluate the ability of ultrasonography to classify kidney size as normal using CT as the reference. The study also calculated sensitivity, specificity, false-positive and false-negative rates, positive predictive value, and negative predictive value. The study was approved by the Ethics Committee of Shahrekord University of Medical Sciences (IR.SKUMS.REC.1396.130), and informed consent was obtained from all participants.

Results

A total of 253 patients underwent renal measurement with both ultrasonography and CT. Mean age was 45.59±12.21 years, with a reported age range of 26-72 years. Twenty-six patients were younger than 30 years, 74 were 30-39 years old, 50 were 40-49 years old, 70 were 50-59 years old, and 33 were 60 years or older. There were 143 men (56.5%) and 110 women (43.5%). Mean age was 45.04±11.71 years in men and 46.32±12.81 years in women, with no significant difference between the sexes (P=0.41).

On ultrasonography, mean right-kidney length was 103.46±13.5 mm and mean left-kidney length was 107.48±12.15 mm. On CT, the corresponding values were 103.6±15.37 mm for the right kidney and 108.81±14.77 mm for the left kidney. Pearson analysis showed a significant direct correlation between ultrasonography and CT for both kidneys: r=0.81 for the right kidney and r=0.69 for the left kidney (both P<0.001). Bland-Altman analysis also demonstrated high agreement. For the right kidney, the mean difference between methods was 0.1 mm, with limits of agreement approximately -17.6 to 17.9 mm, and 96.7% of measurements were within the reported 95% agreement range. For the left kidney, the mean difference was -1.3 mm, with limits of agreement approximately -22.6 to 19.9 mm; the article reports 95% agreement between the two methods.

Figure 1. Correlation between ultrasonography and computed tomography in determining right-kidney size.

Figure 2. Correlation between ultrasonography and computed tomography in determining left-kidney size.

Figure 3. Bland-Altman agreement between ultrasonography and computed tomography for right-kidney size.

Figure 4. Bland-Altman agreement between ultrasonography and computed tomography for left-kidney size.

Using CT findings, right-kidney size was classified as normal in 202 patients (79.8%) and abnormal in 51 (20.2%). Left-kidney size was normal in 232 patients (91.7%) and abnormal in 21 (8.3%). ROC analysis identified ultrasonographic cutoffs greater than 93 mm for a normal right kidney and greater than 95 mm for a normal left kidney. The area under the ROC curve was 0.93 (0.87-0.99) for the right kidney and 0.97 (0.94-0.99) for the left kidney, indicating high discrimination relative to the CT classification.

Figure 5. Receiver operating characteristic (ROC) curves for ultrasonographic classification of normal kidney size using computed tomography as the reference: (A) right kidney and (B) left kidney.

For the right kidney, ultrasonography had sensitivity of 82.4%, specificity of 92.6%, a false-positive rate of 7.4%, a false-negative rate of 17.6%, positive predictive value of 73.7%, and negative predictive value of 95.4%. For the left kidney, sensitivity was 88%, specificity 88.6%, false-positive rate 11.4%, false-negative rate 11%, positive predictive value 45.8%, and negative predictive value 98.5%. Thus, ultrasonography showed strong overall agreement with CT and particularly high negative predictive values, but the lower positive predictive value, especially for the left kidney, indicated that an ultrasonographic classification of reduced or abnormal renal size did not always correspond to the CT reference.

Conclusion

Ultrasonography and CT produced closely similar mean renal dimensions and showed significant correlation and high agreement for both kidneys. Ultrasonography therefore remains a useful, noninvasive method for routine renal-size assessment. Nevertheless, its diagnostic performance was not perfect, and the positive predictive value for an abnormal left-kidney measurement was only 45.8%. Given the clinical importance of renal size in suspected kidney failure, the study supports additional CT assessment when the ultrasonographic measurement is uncertain or when confirmation of reduced renal size could materially affect interpretation of renal failure.

Keywords: Kidney Failure, Computed Tomography, Ultrasonography

REFERENCES

1. Ninan V, Koshi KT, Niyamthullah M, Jacob C, Gopalakrishnan G, Pandey A, et al. A comparative study of methods of estimating renal size in normal adults. Nephrol Dial Transplant. 1990;5(10):851-4. PMID: 2128379 DOI: 10.1093/ndt/5.10.851
2. Christiansen JS, Gammelgaard J, Frandsen M, Parving H-H. Increased kidney size, glomerular filtration rate and renal plasma flow in short-term insulin-dependent diabetics. Diabetologia. 1981;20(4):451-6. PMID: 7016638 DOI: 10.1007/BF00253406
3. Bakker J, Olree M, Kaatee R, de Lange EE, Beek FJ. In vitro measurement of kidney size: comparison of ultrasonography and MRI. Ultrasound Med Biol. 1998;24(5):683-8. PMID: 9695271 DOI: 10.1016/s0301-5629(97)00212-3
4. Webb WR, Brant WE, Major NM. Fundamentals of Body CT E-Book: Elsevier Health Sciences. 2014.
5. Ghafoori M, Zahiri Z. Assessment of the renal length measurement accuracy on spiral CT scan with multiplanar reconstruction of images (MPR) in Hashemi Nejad Hospital in 2004. RJMS. 2006;13(50):127-34.
6. Brandt TD, Neiman H, Dragowski M, Bulawa W, Claykamp G. Ultrasound assessment of normal renal dimensions. J Ultrasound Med. 1982;1(2):49-52. PMID: 6152926 DOI: 10.7863/jum.1982.1.2.49
7. Kang K-Y, Lee YJ, Park SC, Yang CW, Kim Y-S, Moon IS, et al. A comparative study of methods of estimating kidney length in kidney transplantation donors. Nephrol Dial Transplant. 2007;22(8):2322-7. PMID: 17452412 DOI: 10.1093/ndt/gfm192
8. Van NDN, Velghe A, Petrovic M, Vandewiele C, Lameire N, Voet D, et al. The role of ultrasonography in the assessment of renal function in the elderly. J Nephrol. 2003;16(5):658-62. PMID: 14733411
9. Miletić D, Fučkar Ž, Šustić A, Mozetič V, Štimac D, Žauhar G. Sonographic measurement of absolute and relative renal length in adults. J Clin Ultrasound. 1998;26(4):185-9. PMID: 9572380 DOI: 10.1002/(sici)1097-0096(199805)26:4<185::aid-jcu1>3.0.co;2-9
10. Wang F, Cheok S, Kuan B. Renal size in healthy Malaysian adults by ultrasonography. Med J Malaysia. 1989;(1):45-51. PMID: 2696868
11. Kotre C, Owen J. Method for the evaluation of renal parenchymal volume by X-ray computed tomography. Med Biol Eng Comput. 1994;32(3):338-41. PMID: 7934262 DOI: 10.1007/BF02512534
12. Lerman LO, Bentley MD, Bell MR, Rumberger JA, Romero JC. Quantitation of the in vivo kidney volume with cine computed tomography. Invest Radiol. 1990;25(11):1206-11. PMID: 2254054 DOI: 10.1097/00004424-199011000-00009
13. Yokoyama M, Watanabe K, Inatsuki S, Ochi K, Takeuchi M. Measurement of renal parenchymal volume using computed tomography. J Comput Assist Tomogr. 1982;6(5):975-7. PMID: 7142515 DOI: 10.1097/00004728-198210000-00019