Immunocytochemical detection of Ki-67 in Diff-Quik-stained cytological smears of canine mammary gland tumours

Objective: To investigate whether Diff-Quik stained fine needle aspirate smears can be used to evaluate Ki-67 expression by immunocytochemistry. Methods: Both cytological and histological samples were obtained from 24 dogs with spontaneously developed mammary gland tumours. The cytological and histological specimens were examined by Diff-Quik and H&E stains, respectively. After examination, both samples were immunostained using the same Ki-67 antibody. The % Ki-67 values were calculated based on the percentage of positively stained tumour cells per 500 and 1000 tumour cells in cytology and histology specimens, respectively. Results: Ki-67 staining was successful in 17/24 smears (71%) and 19/23 sections (83%). The correlation coefficient between the percentage of Ki-67-positive cells in cytological smears and in the histological sections was 0.677 (P<0.01). These values were significantly different between histologically benign and malignant tumour groups both in cytology and histology samples (P<0.001). The threshold value of the percentage of Ki-67-positive cells for distinguishing benign from malignant tumours was set at 4.85% with 90.9% sensitivity and 92.3% specificity by Receiver Operating Characteristic (ROC) curve using histopathology as the gold standard. Conclusion: Diff-Quik-stained cytology smears can be used to detect the presence of Ki-67 antigen when histology sections are not available.