NORMAL.html
Normal variance in renal size in relation to body habitus
Werner S Harmse, MB ChB
Department of Diagnostic Radiology, Faculty of Health Sciences, University of the Free State, Bloemfontein
Corresponding author: W Harmse (wsharmse@mweb.co.za)
Abstract
Objective.
Renal length determination is common in everyday radiology practice.
However, a normal range of kidney sizes may not apply to people of all
body habitus. This study investigates this relationship in order to
determine normal ranges in relation to body habitus. A secondary aim
was to evaluate the relationship of renal size to gender and race.
Methods.
Kidney lengths were measured on oblique coronal reformatted CT images
of 514 patients who received routine abdominal CT scans for conditions
unrelated to renal pathology. The patients had normal serum creatinine
levels, no history of renal disease, no renal masses, and
normal-appearing kidneys on CT. Weight, height, race and gender of the
patients were recorded.
Results.
The mean renal length was 108 mm with a standard deviation of 9.82 mm.
Statistical analysis demonstrated a relationship between kidney size
and body weight and height, both individually and collectively. The
most accurate prediction model was ‘kidney size = 49.18 + 0.21 x weight
+ 0.27 x height’, with a R2-value of 0.32. Additionally, kidneys were
generally larger in the white population than in the black, and also in
males than females.
Conclusion.
Normal renal size varies according to patients’ body habitus. This
variation can be expressed as a function of body weight and height,
which can be represented by a nomogram and used as an easy reference in
clinical practice.
Renal size is an important parameter for clinical
assessment of patients with diabetes, renal artery stenosis or chronic
renal failure, and for assessment of kidney transplant candidates.
Renal sizes facilitate differentiation between chronic and acute renal
failure, and when a decision has to be made on whether to take renal
biopsies or not.
It is therefore
imperative to have accurate data regarding normal renal sizes. Numerous
studies have established normal renal lengths for the average adult
population, which is approximately 11 cm ± 1 cm,1
with only slight variation among different authors. However, limited
research has been done on the variation of normal renal size in
relationship to body habitus, as well as gender and race. In a study
performed in 1991 with a volunteer Danish population in Copenhagen,4
a definite association in adults between renal size and body habitus
was found. However, no specific model was developed, as this was not
the primary aim of the investigation. This study was also on a single
racial group.4 Fernandes et al.5
found differences in measurements between different population groups
in a study performed in Brazil. Most previous studies also used
sonography or intravenous urography to measure renal dimensions. In
2007, Kang et al.6 evaluated different
radiological methods of estimating renal size, and concluded that
coronal CT scans were the most accurate radiological method for doing
so. A clinical dilemma facing radiologists and clinicians may arise in
deciding whether larger kidney sizes may be acceptable for a big
patient or, conversely, if a small kidney can be accepted as normal for
a smaller adult patient. The aim of this study was (i)
to evaluate the relationship between renal length and different body
habitus indices, including weight, height, body mass index (BMI) and
body surface area (BSA) by using CT images; (ii)
to determine whether a relationship exists and, if so, to establish a
normal range for renal lengths in relation to body habitus; and (iii) to establish a possible relationship between renal size and age, gender and/or race.
Ethical
approval to conduct the investigation was granted by the Ethics
Committee of the Faculty of Health Sciences, University of the Free
State.
Methods
An analytical cross-sectional study was performed
on patients attending the diagnostic radiology department at the
National District Hospital, Bloemfontein. All patients receiving
routine CT scans of the abdomen from July 2009 to June 2010 were
evaluated for inclusion in the study. These patients were scanned on a
GE (General Electric) 8-slice Brightspeed CT scanner at a section
thickness of 2.5 mm.
Patients were
limited to adults between 18 and 70 years of age. The patients were
from central South Africa and represented different racial groups,
including black, white, coloured and Asian.
Patients with
diseases or pathology that might influence renal size were excluded
from the study. Exclusion criteria applied to the patients on arrival
were (i) known chronic renal disease, (ii) previous renal surgery, (iii) patient too ill to allow weighing and measuring, and (iv) elevated serum creatinine >100 µmol/l (upper limit of normal at the local laboratory).
Patients
included in the study were weighed and measured, and their age, gender
and race recorded. After receiving their scans, their images were
evaluated and a further 4 exclusion criteria were applied, namely (i)
incidental finding of renal masses on CT scan, including a solitary
cyst >4 cm, multiple cysts (>4), polycystic renal disease,4 (ii) hydronephrosis, (iii) a single kidney, and (iv) congenital abnormalities including ectopic and fused kidneys.
Both the left
and right kidneys of the remaining patients were measured, which was
done by using multiplanar reformations on Philips iSite Advanced
Visualisation Software. Oblique coronal planes were used to measure the
maximum diameter of the kidneys. Standard coronal reformatted images
were individually tilted along the longitudinal axes for both left and
right kidneys, using a reference line on sagittal images. The maximum
length of the kidneys was then measured on these oblique coronal images
(see Fig. 1).
Results
A total of 677 patients from July 2009 to June 2010
with no known renal disease, no previous renal surgery and a serum
creatinine level <100 µmol/l received abdominal CT scans at
National Hospital in Bloemfontein. One hundred and sixty-three of these
patients were excluded owing to hydronephrosis, visible renal atrophy,
renal masses, multiple cysts or congenital renal variants, giving a
remainder of 514 patients who were included in the study.
On average, the left kidney was 2.06 mm (p
<0.001) larger than the right kidney, with differences ranging from
23 mm larger to 17 mm smaller. For each patient, the mean kidney size
between left and right was determined and used for further analysis,
being referred to as ‘kidney size’ unless stated otherwise.
Kidney sizes varied from 80 mm to 134 mm, with a mean size of 108 mm.
As illustrated in Fig. 2, the kidney sizes showed a symmetric
distribution with a standard deviation of 9.82 mm.
Firstly the
relationship between kidney size and weight was explored. The
regression model (see Fig. 3) was indicative of a significant
relationship between kidney size and weight, with a unit increase in
the patient’s weight being associated with an increase of 0.2562
units in kidney size. The R2 of the regression model is 0.2464, indicating that weight can explain 24.64% of the variation in kidney size.
The
relationship between kidney size and the patient’s height is
shown in Fig. 4 with the estimates of a regression model. The
regression model indicates a significant relationship between kidney
size and length, with a unit increase in length being associated with
an increase of 0.39868 units in kidney size. The R2 of the model is 0.1801.
Fig. 5
illustrates the relationship between kidney size and the
patient’s body mass index (BMI = weight in kg divided by height
in meters squared) with the estimates of a linear regression model. BMI
is significant in the regression model, with one unit increase in BMI
being associated with an increase in kidney size of 0.42180 units.
However, the R2 of the model is only 0.08708, indicating that BMI can explain only 8.7% of the variation in kidney sizes.
The
relationship between kidney size and the patient’s body surface
area (BSA) is shown in Fig. 6 with the estimates of a linear regression
model. BSA was calculated using the Mosteller formula,7 being BSA (m²) = ([height (cm) x weight (kg)]/3600)½. BSA
is significant in the regression model, with one unit increase in BSA
being associated with an increase in kidney size of 20.359 units. The R2 of the model is 0.2975.
Regarding these
variables (weight, height, BMI and BSA), BSA best explains the variance
in kidney size. However, a model was also fitted with both weight and
length, and both these parameters were significant in the model. The
estimated model is:
Kidney size (mm) = 49.18109 + 0.20605 x weight (kg) + 0.27360 x height (cm) (standard error = 8.1 mm)
When comparing this model with the model using only BSA, we see that ‘weight + height’ has an adjusted R2 of 0.3191, while the model with only BSA has an adjusted R2
of 0.2962. This difference indicates that ‘weight + length’
is a better model than using only BSA. For further discussion,
‘0.20605 x weight (kg) + 0.27360 x height (cm)’ is referred
to as the ‘body habitus constant’.
Further
analyses of data were done regarding differences in renal size related
to age, gender and race. No clear relationship between age and kidney
size was found (see Fig. 7). Table I illustrates the gender and racial
distribution, with the average renal sizes, as well as body habitus
constants
for each group.
Owing to the
coloured and Asian groups being too small to make a meaningful
contribution to the findings, their results were excluded from specific
race group analyses. Kidney sizes were generally larger in males than
females (3.2 mm, p <0.05) and also in whites than blacks (9.1 mm, p
<0.05). Body habitus may be regarded as a confounding factor in
interpreting these results when considering our initial analysis. The
mean body habitus constant
of
males was 61.0, and 57.7 in females, which is similar to the findings
on gender-related differences in renal sizes. This finding suggests
that the difference in renal sizes between gender groups is most likely
due to the difference in body habitus between male and female, rather
than a true difference due to gender. When correcting for this
difference in body habitus, a much smaller difference in renal sizes of
only 0.1 mm (p=0.6) is found between male and female patients, which is not clinically significant.
Similar results
were found comparing the renal sizes and body habitus constants of
blacks and whites. Blacks had on average smaller renal sizes (9.1 mm),
but also smaller body habitus constants than whites. This finding
therefore suggests that the difference in renal sizes found between
different racial groups may also be due to the difference in body
habitus, rather than a true racial difference. When correcting renal
sizes for the differences in body habitus, a difference of only 3.2 mm (p
<0.05) is observed, which is substantially smaller than the 9.1 mm
and, although statistically significant, unlikely to be clinically
significant. The differences in body habitus may be attributed to a
number of different genetic, cultural and socio-economic factors.
Discussion
Kidney sizes
are important in the evaluation of renal disease for both radiologists
and clinicians. Coronal reformatted CT scans were used for measurement
purposes as this approach was previously proven to be the most accurate
way of measuring kidney sizes on imaging studies.6
Furthermore, CT scans are more reproducible and less operator- and
patient-dependent than ultrasound. In clinical practice, however,
ultrasound is more readily available and free of radiation, and will
most likely remain the preferred method for evaluating renal sizes. The
mean renal size is 110 mm (±10 mm),1
with only minimal variation reported by different authors. In our study
population, mean renal sizes were fairly similar, with a mean size of
108.2 mm and a standard deviation of 9.82 mm. A clinical dilemma might
arise in patients with a kidney size bigger or smaller than this; one
then needs to decide whether this can be accepted as normal for the
particular patient owing to his/her body habitus. As expected, I found
a relationship between renal sizes and body habitus, which supports the
findings of Emamian et al.,4
who demonstrated a correlation between renal sizes and BSA and height.
From a physiological perspective, this finding would make sense, as
patients with a bigger body habitus will have a larger blood volume
requiring larger kidneys for filtration. Glodny et al.8
also demonstrated a relationship between body height and renal length.
They, however, found a strong relationship between renal length and
BMI, which differed from my results where the relationship with BMI was
the weakest of all the factors studied. They also did not study the
influence of body weight, BSA or other combinations of weight and
height.8
The most
accurate model proposed by the author for predicting renal sizes is
‘Kidney size (mm) = 49.18109 + 0.20605 x weight (kg) + 0.27360 x
height (cm)’. This model had a standard error of 8.1 mm. It is a
cumbersome formula to use in daily practice, and I consequently
constructed a nomogram (Fig. 8) to allow easy reference in the clinical
situation.
When a patient’s weight and height is known,
a straight line can be drawn between these values on the scales
representing weight (left) and height (right). This line will intersect
the central scale indicating the approximate kidney size expected for
this patient.
No relationship between age and renal size was
found, although variation between different genders and races was
observed. The variation in kidney sizes, however, was similar to the
differences in body habitus, and more likely due to body habitus rather
than inherent differences. It could further be argued that the
differences in body habitus are in all probability the result of
numerous socio-economic and cultural factors.
A few limitations were experienced during this
study and need to be noted. The measurements were made by a single
observer, which was mostly because of lack of personnel. Reformations
were also done on 2.5 mm slices which were the limit on routine
abdominal scans on the available equipment. These factors limited the
accuracy of the results, leading to a larger variation of normal
values. The standard error of estimation of our model was 8.1 mm. By
increasing sample size and improving the accuracy of the measurements,
it should be possible to decrease this error. Nevertheless, the results
were regarded as valid, especially since normal renal sizes do have a
range of values spanning approximately 20 mm in variation, rather than
a single specific value.
Conclusion
Normal renal sizes vary according to
patients’ body habitus, and can be expressed as a function of
body weight and height. This function can be represented by a nomogram
that can be used as an easy reference in clinical practice. No
relationship was found between renal sizes and age. Variations were
found in renal sizes of different genders and races, although these
appeared to be related to differences in body habitus and not true
inherent differences.
Acknowledgements. Professor
Gina Joubert, Department of Biostatistics, University of the Free
State, and Christiaan Marais are thanked for the statistical analysis
of data, and Daleen Struwig, medical writer, Faculty of Health
Sciences, University of the Free State, for technical and editorial
preparation of the manuscript for publication.
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Diagnostic Radiology and Imaging. 7th ed. London: Churchill
Livingstone, 2003:828-885.
3. Dahnert W, ed. Radiology Review Manual. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2007.
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L. Kidney dimensions at sonography: correlation with age, sex, and
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Fig. 1. Oblique coronal images used to measure maximum renal length.
Fig. 2. Distribution of kidney sizes.
Fig. 3. Kidney size in relation to body weight.
Fig. 4. Kidney size in relation to body height.
Fig. 5. Kidney size in relation to body mass index.
Fig. 6. Kidney size in relation to body surface area.
Fig. 7. Kidney size in relation to age.
Fig. 8. Renal size nomogram in relation to weight and height.
Table I. Mean renal size and body habitus constant related to gender and race.
Number of participants
Mean renal size (mm)
Body habitus constant
Total
514
108.2
59.01
Gender
Female
306
106.9
57.7
Male
208
110.1
61.0
Race
African
342
105.5
57.4
Caucasian
146
114.6
63.2
Coloured
25
108
56.5
Asian
1
107.5
58.8