Can distinction between the renal cortex and outer medulla on ultrasonography predict estimated glomerular filtration rate in canine chronic kidney diseases?

Abstract

Background: Quantitative evaluation of renal cortical echogenicity (RCE) has been tried and developed in human and veterinary medicine.

Objectives: The objective of this study was to propose a method for evaluating RCE quantitatively and intuitively, and to determine associations between ultrasonographic renal structural distinction and estimated glomerular filtration rate (eGFR) in canine chronic kidney disease (CKD).

Methods: Data were collected on 63 dogs, including 27 with normal kidney function and 36 CKD patients. Symmetric dimethylarginine and creatinine concentrations were measured for calculating eGFR. RCE was evaluated as 3 grades on ultrasonography images according to the distinction between the renal cortex and outer medulla. The RCE grade of each kidney was measured.

Results: There was a significant difference in eGFR between the group normal and CKD (p < 0.001). As mean of RCE grades (the mean values of each right and left kidney's RCE grade) increases, the proportion of group CKD among the patients in each grade increases (p < 0.001). Also, severity of RCE (classified as "high" if any right or left kidney evaluated as RCE grade 3, "low" otherwise) and eGFR is good indicator for predicting group CKD (p < 0.001).

Conclusions: The degree of distinction between the renal cortex and the outer medulla is closely related to renal function including eGFR and the RCE grade defined in this study can be used as a method of objectively evaluating RCE.

Keywords: Canine chronic kidney disease; cortex; eGFR; outer medulla; renal cortical echogenicity.

Fig. 1. RCE grade classification. The right image of each figure is an enlarged image of the area indicated by the circle in the left image. (A) Grade 1 is defined as a clear distinction of the border of the cortex and outer medulla. In (B), the boundary between the cortex and outer medulla becomes ambiguous. Although unclear, if the boundary between the cortex and outer medulla can be distinguished, as in this example, the authors classified the RCE grade as 2. In (C), the boundary between the cortex and outer medulla cannot be distinguished. In this case, RCE grade was classified as grade 3. The arrow head is the boundary between the cortex and outer medulla. If a patient's left and right kidney were classified as RCE grade 1 and 2, for example, mRCE (1.5) and sRCE (low) were recorded. If a patient's left and right kidney were classified as RCE grade 2 and 3, for example, mRCE (2.5) and sRCE (high) were recorded.

RCE, renal cortical echogenicity; mRCE, mean of renal cortical echogenicity grades; sRCE, severity of renal cortical echogenicity grades.

Fig. 2. Box plots of eGFR in group normal, CKD. The central line in the box represents the median. The horizontal lines at the top and bottom of the box are the upper and lower quartiles. The whiskers on the top and bottom of the box represent the highest and the lowest data within the 1.5 interquartile range. The dots (●) are considered outliers. The significance of the comparisons between the groups (p value) is indicated by the solid line.

eGFR, estimated glomerular filtration rate; CKD, chronic kidney disease.

Fig. 3. The bar graph summarizes the distribution of the mRCE of group normal and CKD.

mRCE, mean of renal cortical echogenicity grades; CKD, chronic kidney disease.

Fig. 4. Predicted curves for probability of each mRCE with various eGFR values. The sum of the probabilities of measuring each 5 mRCE at a particular eGFR value is 1. According to this probability curves, as eGFR increases, the probability of measuring a relatively lower mRCE increases.

eGFR, estimated glomerular filtration rate; mRCE, mean of renal cortical echogenicity grades.

Fig. 5. Predicted curves for probability of each group with sRCE and eGFR. As eGFR increases, the probability of predicting group normal is increasing and the probability of predicting group CKD is decreasing. At the same eGFR level, when the sRCE high, the probability of predicting group CKD is higher and the probability of predicting group normal is lower.

eGFR, estimated glomerular filtration rate; sRCE, severity of renal cortical echogenicity grades; CKD, chronic kidney disease.