Diagnostic imaging methods are commonly used to estimate kidney size. The most commonly used measurement for this purpose in clinical practice is the ratio of kidney length to the length of the L2 vertebral body on a ventrodorsal radiograph; this ratio should be between 2.5 and 3.5 in healthy adult dogs.1 The introduction of routine ultrasonographic examination of the abdomen in dogs has led to multiple methods for estimation of kidney size and volume with this modality, including a kidney length-to-L5 vertebral body ratio and the use of volumetric formulae based on maximum length of the kidney.2–4 It has been shown that although there is a positive correlation between kidney length or volume and body weight, the range of expected kidney size in healthy dogs is wide.5 The authors of a previous study6 attempted to correct for variations in kidney length attributable to body size by calculating KL:Aos from measurements in the sagittal plane for healthy adult dogs and proposed that values < 5.5 or > 9.1 indicate that kidneys are abnormally small or enlarged, respectively.6
Renal imaging studies of juvenile dogs are limited. Authors of one radiographic study7 found a greater kidney size in dogs < 1 year of age than in dogs > 1 year of age, whereas investigators of another study8 found no effect of age on renal size. A kidney length-to-L2 ratio is also more difficult to determine in young dogs owing to the radiographic appearance of the vertebral endplate.9 Ultrasonographic assessments of a litter of 5 Irish Setters from birth to 6 months of age revealed that kidney size relative to other body size variables was greater in puppies < 12 weeks of age than in adult dogs (between 3 and 7 years of age) of the same breed.9 After 12 weeks of age, relative kidney size was not significantly different from that for dogs > 1 year of age.9
Renal disease of any cause has an estimated incidence of 1.6% in the general canine population, and acute or chronic conditions can cause a change in overall kidney size.10 As the previous investigation of ultrasonographic kidney size in neonatal and juvenile dogs included a very small number of related animals, it is not appropriate to apply those results to a more general population of young dogs.9 The reproducibility of the KL:Ao has also not been established for dogs < 1 year of age, and it is difficult to determine whether kidney dimensions in young dogs with clinical signs of renal disease should be interpreted as normal or abnormal.
The purpose of the study reported here was to determine whether the previously characterized cutoffs for ultrasonographically determined KL:Ao are applicable to dogs 1 to 18 months of age. We also aimed to compare ultrasonographic kidney and aorta size measurements performed by observers of various experience levels when the same dogs were segregated into 3 age groups (approx 1, 6, and 12 to 18 months of age). We hypothesized that the KL:Aos for 1-month-old dogs would be significantly greater than those for the 2 older groups. We also hypothesized that intraobserver variability of KL:Aos would be low and that no significant interobserver variability would be found for any of the 3 age groups.
Materials and Methods
Animals
A convenience sample of 88 client- and staff-owned dogs from the patient population of the Michigan State University Veterinary Medical Center and university-owned, vivarium-housed dogs from an on-site breeding colony involved in a research project unrelated to the urogenital system was recruited for the study. The initial sample size was selected on the basis of a study by Mareschal et al6 that included 20 healthy adult dogs used to establish KL:Aos while accounting for factors that might exclude some dogs from analysis. Dogs that were approximately 1 month, 6 months, and 12 to 18 months of age were selected for the study. To be eligible for study enrollment, dogs were required to be examined by a veterinarian and assessed as clinically normal, have no clinical signs attributable to the urinary system, and have a kidney structure deemed normal on ultrasonographic examination. In addition, dogs of the 2 older age groups were required to have BUN and serum creatinine concentrations within the laboratory reference ranges. All dogs in the 1-month age group were weighed with the same digital tabletop scale and dogs of the 2 older groups were weighed with the same digital floor scale immediately prior to the ultrasonographic examination. Informed consent was obtained for the inclusion of all client- and staff-owned dogs. The study was approved by the Michigan State University Institutional Animal Care and Use Committee.
Ultrasonographic examination
Animals were placed in dorsal, oblique dorsal, or lateral recumbency for abdominal ultrasonography and were gently physically restrained. Positioning was at the discretion of the ultrasonographer and handler and was determined by subjective assessment of the dog's comfort level. No sedative agents were administered. Three observers (including 2 study authors [NCN and LTK]) obtained short, B-mode cine loops including sagittal and transverse plane images of the right and left kidneys and transverse plane images of the aorta at the level of the right renal artery by use of a portable ultrasound unit with a microconvex (frequency range, 5 to 8 MHz) transducer.a Observer 1 was a second-year radiology resident, observer 2 was a veterinary technician with 9 years of experience performing abdominal ultrasonography, and observer 3 was a board-certified veterinary radiologist with 13 years of experience performing ultrasonography. Each observer independently obtained 3 cine loops of the described structures in the planes of interest during a single imaging session, resulting in 15 cine loops/observer and 45 cine loops/dog. Cine loops were stored on an image processing and storage systemb and retrieved at a later date to perform measurements. Acquisition of sagittal images of the aorta at the level of the right kidney, as described in a previous report6 was also attempted. However, excessive patient motion (primarily for the 2 younger groups of dogs), prevented all observers from obtaining images of adequate quality for analysis.
Image analysis
Observers measured the following 5 variables on the cine loops with an in-program toolb: right and left kidney length, defined as the maximum distance between the cranial and caudal borders of the kidney in the sagittal plane; right and left kidney width, defined as the maximum distance between the medial and lateral borders of the kidney in the transverse plane at the level of the renal pelvis; and aorta diameter, defined as the maximum luminal diameter of the aorta (determined by watching the cine loop and obtaining the measurement during greatest distension of the vessel; Figure 1). Observers made 2 subsequent sets of measurements on a different set of cine loops each time, with a minimum 2-day gap between measurements, for a total of 15 data points/observer and 45 data points/patient. Observers were not blinded to patient identification or group assignment. Each observer only performed measurements on images they had personally acquired.
Still B-mode ultrasonographic images of the left kidney of a 6-month-old dog depicting measurements used in a study to determine whether previously described cutoffs for ultrasonographically determined KL:Ao for assessment of kidney size in adult dogs is applicable to healthy dogs 1 to 18 months of age and to assess interobserver and intraobserver variability for related measurements in dogs grouped according to age. A—Measurement of kidney length in the sagittal plane. B—Measurement of kidney width in the transverse plane. C—Measurement of aorta diameter in the transverse plane at the level of the origin of the right renal artery. Lines drawn in each image represent the described measurement. Values on the right margin of each image indicate measurement in centimeters.
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.764
Still B-mode ultrasonographic images of the left kidney of a 6-month-old dog depicting measurements used in a study to determine whether previously described cutoffs for ultrasonographically determined KL:Ao for assessment of kidney size in adult dogs is applicable to healthy dogs 1 to 18 months of age and to assess interobserver and intraobserver variability for related measurements in dogs grouped according to age. A—Measurement of kidney length in the sagittal plane. B—Measurement of kidney width in the transverse plane. C—Measurement of aorta diameter in the transverse plane at the level of the origin of the right renal artery. Lines drawn in each image represent the described measurement. Values on the right margin of each image indicate measurement in centimeters.
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.764
Still B-mode ultrasonographic images of the left kidney of a 6-month-old dog depicting measurements used in a study to determine whether previously described cutoffs for ultrasonographically determined KL:Ao for assessment of kidney size in adult dogs is applicable to healthy dogs 1 to 18 months of age and to assess interobserver and intraobserver variability for related measurements in dogs grouped according to age. A—Measurement of kidney length in the sagittal plane. B—Measurement of kidney width in the transverse plane. C—Measurement of aorta diameter in the transverse plane at the level of the origin of the right renal artery. Lines drawn in each image represent the described measurement. Values on the right margin of each image indicate measurement in centimeters.
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.764
The KL:Ao was calculated as the mean of right and left kidney lengths divided by the aorta diameter. The KL:BW was calculated as the mean of right and left kidney length divided by body weight in kilograms. These ratios were calculated for each observer measurement for each dog.
Statistical analysis
Distribution of the data was assessed by a Shapiro-Wilk test, and results indicated normal distribution of data. Right and left kidney measurements were compared within age groups by use of a paired Student t test. All measurements and calculated ratios were pooled for each age group and compared among groups with a 3-way ANOVA accounting for the effects of age group, left versus right kidney, and observer. Interobserver variability was assessed with the same 3-way ANOVA with least square means estimated and compared between pairs of observers for each age group; Tukey-adjusted P values were reported. Intraobserver variability was measured by calculation of the CV for triplicate measurements of kidney length, kidney width, and aorta diameter with the following formula:
Variation in kidney length, kidney width, and aorta diameter measurements in each age group for each observer was then calculated with the following formula:
Measurement variation (mm) = Mean measurement of structure (mm) × observer CV for the structure.
Statistical analysis was performed with commercially available software.c,d Values of P < 0.05 were considered significant.
Results
Animals
Of 88 dogs recruited for study enrollment, 5 were excluded (1 for unilateral renal pelvic dilation and 4 for BUN or serum creatinine concentrations [or both] above the applicable reference range). One dog was subsequently removed from the study because an intrahepatic shunt was detected while scanning the right kidney. A total of 82 dogs were included in the analyses, with 28, 29, and 25 dogs categorized as 1, 6, and 12 to 18 months of age, respectively. Ages ranged from 30 to 33 days, from 173 to 187 days, and from 360 to 545 days for dogs grouped as 1, 6, and 12 to 18 months of age, respectively.
The 28 dogs in the 1-month age group were enrolled from 6 different litters at the university vivarium; these included Beagle-Corgi (n = 9), Beagle-Briard (8), and Beagle-Papillion (4) mixed-breed dogs and Curly-Coated Retrievers (7). Mean ± SD body weight for this group was 1.6 ± 0.6 kg. Thirteen of 29 dogs in the 6-month age group were enrolled from 3 different vivarium litters (Beagle-Corgi [n = 8] and Beagle-Papillion [5] mixes). Mean ± SD body weight for this group was 13.0 ± 6.9 kg. The remaining 16 dogs in the 6-month age group were client- and staff-owned dogs, and breeds included Australian Shepherd (n = 5), Golden Retriever (4), German Shepherd Dog (2), French Bulldog (2), Rat Terrier (1), Boxer (1), and Great Dane (1). Mean ± SD body weight for this group was 18.6 ± 10.6 kg. All 25 dogs in the 12 to 18-month age group were client- and staff-owned dogs. This group comprised Labrador Retriever–mixed breed dogs (n = 3), medium-sized unknown mixed-breed dogs (3) and Beagle-Papillion mixed-breed dog (1) as well as Dachshunds (6), Golden Retrievers (2), Doberman Pinschers (2), and 1 each of the following breeds: English Springer Spaniel, Norwegian Buhund, American Eskimo Dog, Boxer, English Bulldog, Bloodhound, and Border Collie. Mean ± SD body weight for this group was 18.6 ± 10.6 kg.
Image analysis
Right kidney length was significantly (P < 0.001) greater than the left for dogs of the 1-month age group (mean, 37.6 ± 5.7 vs 36.4 ± 5.3 mm); no other with-in-group measurement differences were found. The mean of right and left kidney measurements was reported and used for calculation of KL:Ao and KL:BW.
Mean kidney length, kidney width, and aorta diameter were significantly smaller, and mean KL:Ao and KL:BW were significantly greater, for dogs in the 1-month age group than for dogs in the 6-month and 12 to 18–month groups (Table 1). No significant differences in any measured variables or calculated ratios were found between the latter 2 groups.
Mean ± SD kidney length, kidney width, and aorta diameter as determined by B-mode ultrasonographic measurements and calculated KL:Ao and KL:BW for 82 healthy young (≤ 18-month-old) dogs grouped by age.
| Age group (mo) | |||
|---|---|---|---|
| Variable | 1 (n = 28) | 6 (n = 29) | 12 to 18 (n = 25) |
| Kidney length (mm) | 37.1 ± 5.6* | 54.5 ± 12.2 | 58.4 ± 12.3 |
| Kidney width (mm) | 18.2 ± 3.1* | 28.0 ± 6.1 | 29.8 ± 6.3 |
| Aorta diameter (mm) | 3.4 ± 0.6* | 8.2 ± 1.6 | 9.0 ± 2.0 |
| KL:Ao | 11.1 ± 1.6* | 6.6 ± 0.8 | 6.6 ± 0.8 |
| KL:BW | 25.3 ± 8.1* | 5.0 ± 1.8 | 4.3 ± 2.6 |
Dogs in the 1-month, 6-month, and 12 to 18–month age groups were 30 to 33 days, 173 to 187 days, and 360 to 545 days of age, respectively. Three observers with various experience levels (a second-year radiology resident, a veterinary technician with 9 years of experience performing abdominal ultrasonography, and a board-certified veterinary radiologist with 13 years of experience performing ultrasonography) performed ultrasonography and obtained measurements in triplicate for gently restrained, unsedated dogs. The mean of right and left kidney measurements was reported and used for calculation of KL:Ao and KL:BW.
Value is significantly (adjusted P < 0.05) different from that for the other 2 age groups.
Significant (P < 0.05) interobserver variability was found for ultrasonographic measurements of kidney length and aorta diameter in dogs of the 1-month age group (Table 2). Kidney length measurements differed between observers 1 and 2 and between observers 2 and 3, and aorta diameter measurements differed between observers 1 and 3. The KL:Aos for dogs in this group differed between observers 1 and 2, 1 and 3, and 2 and 3 (P < 0.001 for all comparisons), and the KL:BWs differed between observers 1 and 2 and between observers 2 and 3 (P < 0.001 for both comparisons).
Interobserver variability for ultrasonographically determined kidney length, kidney width, aorta diameter, KL:Ao, and KL:BW for the same dogs as in Table 1.
| Variable | Age group (mo) | Observer comparison | Least square mean difference | Adjusted P value |
|---|---|---|---|---|
| Kidney length | 1 | 1 vs 2 | 0.72 | 0.034 |
| 1 vs 3 | –0.02 | < 0.999 | ||
| 2 vs 3 | –0.74 | 0.026 | ||
| 6 | 1 vs 2 | 0.12 | < 0.999 | |
| 1 vs 3 | –0.17 | 0.996 | ||
| 2 vs 3 | –0.29 | 0.971 | ||
| 12 to 18 | 1 vs 2 | 0.94 | 0.003 | |
| 1 vs 3 | 0.44 | 0.647 | ||
| 2 vs 3 | –0.50 | 0.464 | ||
| Kidney width | 1 | 1 vs 2 | 0.25 | 0.965 |
| 1 vs 3 | 0.22 | 0.981 | ||
| 2 vs 3 | –0.02 | < 0.999 | ||
| 6 | 1 vs 2 | 0.40 | < 0.999 | |
| 1 vs 3 | –0.51 | 0.090 | ||
| 2 vs 3 | –0.91 | 0.039 | ||
| 12 to 18 | 1 vs 2 | 0.25 | 0.976 | |
| 1 vs 3 | –0.03 | < 0.999 | ||
| 2 vs 3 | –0.28 | 0.948 | ||
| Aorta diameter | 1 | 1 vs 2 | 0.18 | 0.140 |
| 1 vs 3 | 0.25 | 0.007 | ||
| 2 vs 3 | 0.06 | 0.988 | ||
| 6 | 1 vs 2 | 0.08 | 0.987 | |
| 1 vs 3 | 0.19 | < 0.999 | ||
| 2 vs 3 | 0.11 | < 0.999 | ||
| 12 to 18 | 1 vs 2 | 0.02 | < 0.999 | |
| 1 vs 3 | –0.05 | 0.999 | ||
| 2 vs 3 | –0.06 | 0.994 | ||
| KL:Ao | 1 | 1 vs 2 | –0.37 | < 0.001 |
| 1 vs 3 | –0.82 | < 0.001 | ||
| 2 vs 3 | –0.44 | < 0.001 | ||
| 6 | 1 vs 2 | –0.04 | < 0.999 | |
| 1 vs 3 | –0.18 | < 0.999 | ||
| 2 vs 3 | –0.14 | < 0.999 | ||
| 12 to 18 | 1 vs 2 | 0.06 | 0.999 | |
| 1 vs 3 | 0.07 | 0.995 | ||
| 2 vs 3 | 0.01 | < 0.999 | ||
| KL:BW | 1 | 1 vs 2 | 0.56 | < 0.001 |
| 1 vs 3 | 0.10 | 0.913 | ||
| 2 vs 3 | –0.46 | < 0.001 | ||
| 6 | 1 vs 2 | 0.04 | < 0.999 | |
| 1 vs 3 | –0.03 | < 0.999 | ||
| 2 vs 3 | –0.07 | < 0.999 | ||
| 12 to 18 | 1 vs 2 | 0.06 | < 0.999 | |
| 1 vs 3 | 0.01 | < 0.999 | ||
| 2 vs 3 | –0.05 | < 0.999 |
Values of P < 0.05 were considered significant.
There was a significant (P < 0.05) difference in the kidney width measurements between observers 2 and 3 for dogs of the 6-month age group and in kidney length measurements between observers 1 and 2 for dogs of the 12 to 18–month age group (Table 2). The remaining measured variables and calculated ratios were not different between observers for either group of dogs.
Intraobserver variability was considered acceptable for all performed measurements (Table 3). Aorta diameter had the highest CV of the 3 measurement variables in all groups of dogs for all observers, ranging from 3.7% to 7.8%, and was also the smallest measurement performed; the largest variation in this measurement was 0.4 mm. All CVs for kidney length measurement were < 2.3%, with a maximum measurement variation of 1.0 mm. Kidney width CVs ranged from 2.7% to 4.7%, with a maximum measurement variation of 1.3 mm.
Intraobserver variability (CV) and variation in measurements for kidney length, kidney width, and aorta diameter for the same dogs as in Table 1.
| Kidney length | Kidney width | Aorta diameter | |||||
|---|---|---|---|---|---|---|---|
| Observer | Age group (mo) | CV (%) | Variation (mm) | CV (%) | Variation (mm) | CV (%) | Variation (mm) |
| 1 | 1 | 1.4 | 0.5 | 3.5 | 0.6 | 5.5 | 0.2 |
| 6 | 1.4 | 0.8 | 4.7 | 1.3 | 4.2 | 0.3 | |
| 12 to 18 | 1.7 | 1.0 | 3.5 | 1.0 | 3.7 | 0.3 | |
| 2 | 1 | 2.2 | 0.8 | 2.7 | 0.5 | 7.7 | 0.3 |
| 6 | 1.7 | 0.9 | 3.9 | 1.1 | 4.4 | 0.4 | |
| 12 to 18 | 1.8 | 1.0 | 3.3 | 1.0 | 4.0 | 0.4 | |
| 3 | 1 | 1.7 | 0.6 | 2.9 | 0.5 | 7.8 | 0.3 |
| 6 | 1.4 | 0.8 | 3.2 | 0.9 | 4.1 | 0.3 | |
| 12 to 18 | 1.6 | 1.0 | 2.7 | 0.8 | 3.9 | 0.4 | |
Within a category, percentage CV for the observer was calculated as (SD/mean of measurements) × 100, and measurement variation was calculated as mean measurement of structure (mm) × observer CV for the structure.
Discussion
The present study investigated measurements by 3 observers for assessment of kidney size in dogs approximately 1 month (30 to 33 days), 6 months (173 to 187 days), and 12 to 18 months (360 to 545 days) of age. Neonatal dogs (in the 1-month age group) had significantly lower mean kidney and aorta measurements and significantly higher KL:Aos and KL:BWs than did older dogs. These results supported our first hypothesis as well as previous reports7,9 that kidney size relative to body size is greater in very young dogs than in more mature dogs. This finding was consistent with the fact that in the late stages of fetal development, the area of the metanephros, the organ that will develop into the kidney, increases in size at a higher rate than does the length of the fetus, suggesting that animals will have comparatively large kidneys at birth.11 This was also supported by the significantly higher KL:BW for dogs of the 1-month age group in our study, compared with both groups of older dogs, and a slightly but nonsignificantly lower KL:BW for dogs in the 12 to 18–month group than for dogs in the 6-month group. Similar findings have been described for healthy children, with as much as a 15% to 20% total increase in kidney length developing in the first few weeks after birth, and a subsequent slowing of this growth rate throughout the remainder of the first year of life.12,13 Interestingly, this pattern of early kidney growth has been recognized as a source of potential false-positive diagnosis of renomegaly, with some radiologists recommending subannual measurements of kidney length as well as correspondence with clinical circumstance to help prevent overdiagnosis of the condition in human patients.14 In the present study, the KL:Aos were within the proposed range of values previously described for healthy adult dogs6 by 6 months of age, with similar values found for dogs 12 to 18 months of age. It is possible, on the basis of our findings, that normal-sized kidneys in 1-month-old dogs have been misinterpreted as renomegaly, which could lead to unnecessary treatments or procedures. Future studies of neonatal and juvenile dogs with renal abnormalities are needed to investigate whether there is an associated change in kidney size outside of the values reported here.
Significant differences in kidney measurements and calculated ratios for normalization to body size were found between observers for all age groups of dogs in the present study, causing us to reject our hypothesis that there would be no significant interobserver variability detected for KL:Ao values. For the 1-month age group in particular, multiple interobserver differences were found, including differences in measurements of kidney length, aorta diameter, and KL:BW between some pairs of observers and differences in the KL:Ao between all observer pairs. This indicates poor reproducibility. However, interobserver variability for dogs in the 6-month age group was found exclusively in measurement of kidney width, a value not used in calculation of KL:Ao. For dogs in the 12 to 18–month age group, although there was interobserver variability in measurement of kidney length for 1 pair of reviewers, the KL:Ao and KL:BW did not differ between any 2 observers. This led us to conclude that these calculated ratios may still be useful in a clinical setting with canine patients ≥ 6 months of age. Patient motion may have been 1 reason for this discrepancy between groups, with younger animals struggling more often during the examinations resulting in increased difficulty obtaining high-quality images and measurements. The smaller size of the aorta in the youngest group of dogs also meant increased difficulty in identifying aortic margins, potentially leading to errors when placing measurement calipers on the images.
Interestingly, the right kidney measured slightly (1.2 mm) longer than the left kidney in the 1-month age group, similar to findings in a previous study.15 This was most likely attributable to a combination of scanning technique and the limitations of ultrasound technology. Length of the right kidney can be more difficult to measure than that of the left owing to its cranial position in the abdomen and many potentially gas-filled surrounding structures such as the pylorus, descending duodenum, and colon that can obscure the cranial pole. This may have led to slight overestimation of kidney length by all observers. This difference was only found in the age group with the smallest organs, where measurement errors have a greater chance of being statistically significant because they are proportionally larger relative to the organ size. However, in practice, such a small difference in kidney size is unlikely to be clinically relevant. Previously reported difficulties in acquiring sagittal images of the right kidney were not encountered in this study, which we attributed to use of a curvilinear instead of linear-array ultrasound transducer.9 A curvilinear transducer has a reduced area of contact with the animal and could be placed in the right costochondral region to evaluate the right kidney without inducing signs of discomfort in the dogs.
Measurements of aorta diameter in the transverse plane have been reported to be less repeatable, compared with those obtained in the sagittal plane.6 We considered the intraobserver variability acceptable (CVs < 7.9%) for aorta diameter measurements in the transverse plane for all observers, regardless of the age groups of dogs. However, in addition to the previously mentioned interobserver variability, this measurement had subjectively higher intraobserver variability than did measurements of kidney length and width. However, on the basis of mean aorta diameter measurements for each age group, intraobserver variability resulted in measurement variations ≤ 0.4 mm, a discrepancy unlikely to be of any clinical importance. As previously mentioned, the reason for these discrepancies is most likely excessive patient motion, so that obtaining high-quality sagittal images similar to those obtained by Mareschal et al6 was extremely difficult. Transverse images were obtained with relative ease, as maintaining the aorta within the scan field was much easier than it would have been with the vessel in the sagittal plane. However, although efforts were made to ensure a truly transverse section of the aorta was imaged, motion may have resulted in some slightly oblique images of the aorta, affecting the diameter measurement. In practice, sedative agents are commonly used to prevent patient motion and improve accuracy of measurements. However, these were not used in the present study because of a concern about possible effects on vessel diameter. The impact of sedation on vessel diameter and accuracy of vascular measurements are areas of possible future study. Less motion encountered with the older groups of dogs likely contributed to apparently lower intraobserver variability for measurements of those animals, and ultimately suggested that the transverse aorta view may also be useful for measurement of aorta diameter in older animals.
The present study had limitations. Regarding patient selection, clinicopathologic data indicative of renal function were only available for the 2 older groups of dogs. Collection of blood samples can be a difficult procedure in neonates and was thought to represent an unnecessary stress to 1-month-old dogs. All dogs in the youngest group were part of a university breeding colony and were under careful observation; no health problems, including urinary system signs, were detected in any of these dogs in the year following the ultrasonographic evaluations in this study, making it unlikely that any were affected with pathological conditions that could cause changes to the kidneys. For dogs of the 6-month and 12 to 18–month age groups, BUN and serum creatinine concentrations within the respective reference ranges and lack of clinical signs referable to the urinary tract were the basis for inclusion. Certain pathological conditions, such as portosystemic shunts, can affect kidney size without always increasing BUN and creatinine concentrations. While the incidence of such abnormalities is low in the general dog population, it is possible a dog with such a disease might not have been appropriately excluded from the study.
Regarding the patient population, large- and giant-breed dogs were underrepresented, especially in the 1-month age group. Also, several dogs in the present study were related, with the youngest dogs obtained from 6 litters and more than half of the 6-month-old dogs obtained from 3 litters in the same breeding colony. This may have caused a size bias, with smaller-breed dogs overrepresented. The inclusion of these dogs might also have introduced the presence of abnormally small or large kidneys or aortas within a particular genetic lineage. This was unlikely, as older (> 6-month-old) dogs from the breeding colony had organ sizes similar to those of client- and staff-owned dogs. Finally, the number of dogs in the present study (82) was small relative to the general population of companion dogs. However, considering the variety of included breeds and overall differences for the 1-month age group, compared with older dogs, the conclusions were thought to be valid. Specific breed differences in kidney dimensions have also been previously noted,16 but the number of dogs of each breed in the study were too low to draw any specific conclusions.
Our results revealed that healthy 1-month-old dogs have larger kidneys relative to body weight and aorta diameter than do healthy older dogs. Overall, significant interobserver variability for kidney length and aorta diameter measurements in neonatal dogs in the present study suggested that the use of these measurements and resulting ratios is limited for such patients in clinical practice. Dogs with renal abnormalities were not included in this study and thus, it remains unknown whether detectable differences could exist for such patients. However, the fact that interobserver differences in measurements did not impact the KL:Ao or KL:BW, and that KL:Ao values were similar to those previously published for adult dogs,6 suggested that these ratios may prove useful for evaluation of dogs ≥ 6 months of age. Further research in this area with a larger population of dogs is needed, and veterinarians are encouraged to consider all clinical, biochemical, and imaging data when assessing renal function in dogs.
Acknowledgments
No third-party funding or support was received in connection with this study or the writing or publication of this manuscript. The authors declare that there were no conflicts of interest.
Presented in part as an oral presentation at the American College of Veterinary Radiologists Annual Scientific Conference, Phoenix, Ariz, October 2017.
The authors thank Bea VanKampen for technical assistance in acquiring ultrasonographic images.
ABBREVIATIONS
| CV | Coefficient of variation |
| KL:Ao | Kidney length-to-aorta diameter ratio |
| KL:BW | Kidney length-to-body weight ratio |
Footnotes
Logiq e, GE Medical Systems, Milwaukee, Wis.
McKesson Radiology, version 12.1.1, McKesson Technology Solutions, Alpharetta, Ga.
SAS, version 9.4, SAS Institute Inc, Cary, NC.
SigmaPlot for Windows, version 11.0, Build 11.2.0.5, Systat Software Inc, San Jose, Calif.
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