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Chronic kidney disease (CKD) is a relevant disease in feline clinic. The tubulointerstitial damage, with collagen deposition and fibrosis, is an important result of this process. The aim of this study was to quantify and correlate the deposition of collagen and severity of interstitial fibrosis (IF) in the kidney from cats in different stages of CKD. Kidney fragments from 10 adult cats with CKD were analyzed and stained by Masson's trichrome (MT) and Picrosirius red (PSR) for circular polarized microscopy. Random quantitative analysis was performed on MT sections to classify the degree of IF, per field area, with and without circular polarization. Statistics correlations were performed by Spearman's ( ρ; p < .05). There was a significant correlation of IF quantification with the area of interstitial collagen deposition by polarized PSR (PSRp) ( r = .7939, p = .0098) and nonpolarized PSR (PSR n) ( r = .7781, p = .0080). There was a positive correlation of serum creatinine (sCr) at different stages of CKD with PSRp ( r = .7939, p = .0098), PSRn ( r = .8667, p = .0027) and MT ( r = .7818, p = .0117). Correlations between the percentage of quantified area was also positive from PSRp to PSRn ( r = .9030, p = .0009) and PSRp to MT ( r = .7939, p = .0098). The PSRN was also correlated with MT ( r = .9273, p = .0001). The correlation with IF and sCr follows the disease evolution and the quantification of collagen by PSR is an excellent tool for analyzing the disease severity at different stages. [ABSTRACT FROM AUTHOR]