This study presents ultrasound-based normative values for kidney volume in Central European children aged 0–19 years old. The authors also develop age-, height- and BSA-related percentiles and LMS-derived z-scores. To the best of the authors’ knowledge, the presented normative values are based on the largest cohort of paediatric patients ever analysed. This is also the first study to assess kidney function and exclude patients in whom kidney function is impaired. This study is also the first to report LMS values for kidney volume which can be utilised to calculate kidney volume z-scores and/or percentiles of the volume. It presents the data as percentile tables and curves (2.5th, 10th, 25th, 50th, 75th, 90th and 97.5th ), with formulas for kidney volume percentiles and z-score calculations.
Boys/girls
Previously published studies reported discrepant results regarding differences in kidney volume between males and females. In their longitudinal cohort study of infants aged 0–18 months, Schmidt et al. showed that male infants had significantly larger kidney volumes than females at all ages, which was not entirely due to differences in body size, as boys had larger BMI-related kidney volumes (cm3/m2) [25]. The same was true for weight-related kidney volume (cm3/kg) at 0 and 3 months of age, but the difference disappeared at 18 months of age.
A study by Ezeofor et al. also reported larger kidney volumes in males after adjusting for height, however, no differences were found after adjusting for other anthropometric parameters [16]. The majority of studies showed no significant differences between boys and girls [1, 18, 26–28]. In the present study, no sex-related differences were observed in kidney volume according to height. There were, however, differences in age-related kidney volume between males and females in most age groups, with males having larger kidney volumes than females. This is in agreement with previously published studies by Schmidt et al. [25] and Ezeofor et al. [16] and represents another argument for adjusting kidney measurements for height rather than age.
Prone/supine position
Kidney measurements can vary significantly depending on the positioning of a patient. In a study by Michel et al., it was reported that longitudinal measurements of kidney length are significantly larger when performed in the supine position [29]. Therefore, the authors recommended measurements in both positions. Although justified, the application of this recommendation may be problematic in daily practice, as follow-up measurements can only be compared for the same patient position.
In the current study, no significant differences were observed in kidney size between measurements in the prone and supine positions. Also, the authors could not find such differences in other studies on kidney volume measurements. Therefore, it seems reasonable to recommend measuring kidney volume in a position that provides the best visualisation of the kidney length and width.
Left/right kidney
The differences between left and right kidney size have been widely discussed in the published literature. In a previous study, the present authors found a significant difference between left and right kidney length (mean paired difference = 1.03 mm, p < 0.001), however, the value of absolute difference and the effect size were marginal (Cohen’s D = 0.06) and did not correlate with age and height [30, 31]. Some studies described larger volumes of the left kidney [1, 16, 18, 25]. In three studies, the reported kidney volume was higher for the left kidney [1, 16, 18]. In a study by Shi et al., mean left and right kidney volumes were 61.68 mL and 53.39 mL, respectively (t = 30.023, p < 0.001) [1]. Significant asymmetry was also described by Ezeofor et al. with greater left kidney volume in all the age-, weight- and height-matched categories (p < 0.01) [16]. Dinkel et al. also observed this difference, but it was small (+ 2.5 mL) [18]. Interestingly, Schmidt et al. reported that the right kidney had a slightly but significantly larger volume in both boys and girls (p < 0.001 and p < 0.01, respectively) [25]. However, this was the only study in which the right kidney was found to be larger.
In contrast, other studies did not find significant lateral differences in kidney volume [13, 27, 32, 33]. The present authors also did not report differences between left and right kidneys, suggesting that such differences, if they exist, are clinically irrelevant.
Predictors of kidney volume
Previously published studies reported correlations between kidney volume and several anthropometric parameters (height, weight, BSA, age). In a study by Tsehay et al., left kidney volume correlated best with weight (r = 0.628, p < 0.001) whereas right kidney volume had the highest correlation with height (r = 0.560, N = 403, P < 0.001) [34]. Overall, in this study, the best predictors of all kidney dimensions were BSA and height. A strong correlation between kidney volume and weight and BSA was also described in studies by Dinkel et al. (r = 0.971 for the left and r = 0.968 for the right kidney for weight; r = 0.971 for the left and r = 0.969 for the right kidney for BSA) [18] and Ezeofor et al. (r = 0.790 for left and r = 0.646 for right kidney for weight; r = 0.788 for left and r = 0.639 for right kidney for BSA) [16]. BSA was also described as a good correlate with kidney volume in a study by Scholbach et al. in which BSA-related kidney volume was proved to be a good parameter to evaluate kidney volume [32]. In studies by Shi et al. [1] and Chandra et al. [27], the strongest correlation was found for age (r = 0.472 for left and r = 0.399 for right kidneys in the study by Shi et al.; and r = 0.860 and r = 0.850 for left kidneys in girls and boys, respectively and r = 0.890 and r = 0.870 for right kidneys in girls and boys, respectively). In both studies, other correlates included height/length and weight, with BSA additionally described in the study by Chandra et al. In one study, the strongest correlation was observed between kidney volume and lean body mass [25].
The results of the presented study are generally in line with those published before. However, in this study, kidney volume correlated best with BSA (r = 0.94, followed by weight (r = 0.92), height (r = 0.91), age (r = 0.91), and body mass index (r = 0.67; p < 0.001 for all), which is different from findings by Shi et al. and Chandra et al. who reported that age is the strongest correlate.
Comparison vs. other normative values
Figure 6 shows a graphical comparison of the kidney volume normative values obtained in this study (by body weight) with the commonly used European norms published by Dinkel et al. (separately for the right and left kidney). As documented in Fig. 6, children up to approx. 50 kg in this study had comparable median kidney volume as those reported by Dinkel et al. [18] In children with a higher body weight, the median volume of the kidneys in this study was slightly smaller. The apparent differences in median kidney volume in children with higher body weighs may be due to increasing BMI in the pediatric population over the last few decades, most notably seen in the rising rates of overweight and obesity. This trend means that, on average, individuals with higher body weigth have lower kidney volume, as body height is the best predictor of kidney volume [34].
Due to the lack of raw data, it was impossible to compare the kidney volume normative values (curves) presented by Chandra et al. [27] statistically. Visual analysis of the curves reveals slight differences – in the population aged 0–5 years in the current study, kidney volume seems to be slightly greater, however, it is not known if the differences are significant. A comparison with data from a study by Shi et al. showed similar values of kidney volume to this study [1]. On the other hand, a comparison of data collected by Tsehay et al. [35] and Ezeofor et al. [16] showed that the study population presented in this article had smaller kidney volumes in relation to BSA and height. In comparison to a study by Tsehay et al. [35], the difference was greater for higher percentiles. For BSA equal to 1.0 m2, the kidney volume values reported by Tsehay et al. [35] were 56 cm3 (2.5th percentile), 98.6 cm3 (50th percentile) and 167.9 cm3 (97.5th percentile). Values reported in the current study for the same BSA were 47.4 cm3, 69.7 cm3 and 98 cm3, respectively. The same dependency in relation to height was observed after comparison to a study by Ezeofor et al. [16], although they did not provide the corresponding percentile values. In their study, mean values for heights 106–110 cm, 126–130 cm and 146–150 cm for females were 66.4 cm3, 91.9 cm3 and 117.3 cm3 respectively, and for males 66.5 cm3, 97.9 cm3 and 119.3 cm3, respectively. In the present study, values of the 50th percentile for heights 105–109 cm, 125–129 cm and 145–149 cm were 45.7 cm3, 62.8 cm3 and 82.2 cm3, respectively. In a study by Tsehay et al. [35], however, the mean body length of the participants was greater and a different BSA calculation method was used; in the study by Ezeofor et al. [16], in turn, anthropometric parameters were not published.