Assessment of peritubular capillary rarefaction in kidneys of cats with chronic kidney disease

BACKGROUND: Hypoxia is a key driver of fibrosis and is associated with capillary rarefaction in humans. OBJECTIVES: Characterize capillary rarefaction in cats with chronic kidney disease (CKD). ANIMALS: Archival kidney tissue from 58 cats with CKD, 20 unaffected cats. METHODS: Cross‐sectional study of paraffin‐embedded kidney tissue utilizing CD31 immunohistochemistry to highlight vascular structures. Consecutive high‐power fields from the cortex (10) and corticomedullary junction (5) were digitally photographed. An observer counted and colored the capillary area. Image analysis was used to determine the capillary number, average capillary size, and average percent capillary area in the cortex and corticomedullary junction. Histologic scoring was performed by a pathologist masked to clinical data. RESULTS: Percent capillary area (cortex) was significantly lower in CKD (median 3.2, range, 0.8‐5.6) compared to unaffected cats (4.4, 1.8‐7.0; P = <.001) and was negatively correlated with serum creatinine concentrations (r = −.36, P = .0013), glomerulosclerosis (r = −0.39, P = <.001), inflammation (r = −.30, P = .009), and fibrosis (r = −.30, P = .007). Capillary size (cortex) was significantly lower in CKD cats (2591 pixels, 1184‐7289) compared to unaffected cats (4523 pixels, 1801‐7618; P = <.001) and was negatively correlated with serum creatinine concentrations (r = −.40, P = <.001), glomerulosclerosis (r = −.44, P < .001), inflammation (r = −.42, P = <.001), and fibrosis (r = −.38, P = <.001). CONCLUSIONS AND CLINICAL IMPORTANCE: Capillary rarefaction (decrease in capillary size and percent capillary area) is present in kidneys of cats with CKD and is positively correlated with renal dysfunction and histopathologic lesions.