Diagnostic Accuracy of Renal Cytology and Ultrasound in the Cat

Cytology is often used as a relatively noninvasive method for diagnosing renal abnormalities. Ultrasound guidance aids sample collection and can also help identify and localize disease.

Researchers at the University of Minnesota (UMN) recently investigated the diagnostic utility of ultrasound-guided fine-needle aspirate (FNA) cytology for neoplastic and non-neoplastic renal abnormalities in feline patients. The investigators also studied whether common anatomic features noted during ultrasound examination correlated with patient diagnoses.

Study Design

Feline patient records from the UMN Veterinary Medical Center were examined retrospectively to identify cases in which renal FNA cytology was performed.

The investigators, who were blinded to histopathology results, interpreted the Wright-Giemsa—stained cytologic samples. Features including cellularity, blood contamination, and numbers of different cell types were all scored from rare or mild to severe. Background material, hemorrhage, mitotic figures, and infectious agents were also noted. Each cytologic sample was categorized as diagnostic or nondiagnostic and was assigned one of the following diagnostic categories: inflammation, neoplasia, hyperplasia, necrosis, cyst, hemorrhage, or normal.

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The investigators also reviewed archived renal ultrasound images while blinded to the patients’ cytologic and histopathologic findings. Renal features and abnormalities, including masses, a dilated pelvis, or stones, were recorded. When available, histologic samples collected via necropsy, ureteronephrectomy, or tumor biopsy were used as the gold standard method for calculating diagnostic accuracy of cytology. Finally, ultrasound features and histology were compared to determine diagnostic trends.

Results

From 2005 to 2014, 96 cytologic samples were obtained from 58 neutered male cats, 1 intact male cat, and 37 spayed female cats, altogether representing 13 feline breeds. Sixty-eight percent of the cytologic samples were diagnostic, and the remaining nondiagnostic cases were all noted to be acellular. Some degree of blood contamination was noted in 98% (94/96) of all samples.

Of the 64 diagnostic samples obtained via ultrasound-guided FNA, diagnosis was categorized as follows:

• Neoplasia (33%)
• Renal tubular hyperplasia (21%)
• Inflammation (6%)
• Necrosis (1%)
• No abnormalities noted (39%)

Analysis of 12 cases compared cytologic and histologic diagnoses. Sensitivity, specificity, positive predictive value, and negative predictive value for diagnosis of neoplasia via cytology were all 100%. For non-neoplastic diagnoses, sensitivity and negative predictive value dropped to 16.7% and 54.5%, respectively.

The highest diagnostic yields were associated with certain ultrasonographic findings, including subcapsular infiltrate, diffuse renal enlargement without pelvic dilation, and normal or enlarged infiltrative/nodular kidneys. Interestingly, the presence of a mass on ultrasound was associated with only a 50% diagnostic yield. All cases of renal carcinoma that were confirmed via histology had neoplastic characteristics (anisocytosis and anisokaryosis) in the corresponding FNA cytologic sample.

Take-home Message

Results suggest that renal FNA offers the highest diagnostic accuracy for neoplasia, but is less reliable at ruling out non-neoplastic abnormalities. Overall diagnostic accuracy of renal FNA was comparable to that in other sites, including cutaneous masses and lymph nodes, determined from earlier studies.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.