Total serum sialic acid is a general disease marker rather than a specific tumour marker in dogs.

The aim of the present study was to examine the levels of total sialic acid (TSA) in serum of clinically healthy dogs and dogs with various diseases to evaluate the usefulness of TSA as a tumour marker. TSA levels in clinically healthy dogs were not different between sexes, but pregnant and lactating dogs had higher mean (+/- standard deviation (SD)) TSA levels than clinically healthy female dogs (642 +/- 78 vs. 495 +/- 73 mg/l, P < 0.001). Eighty-eight dogs with different tumours (54 malignant and 34 benign tumours of different tissues) had higher mean TSA levels than 148 clinically healthy dogs (675 +/- 143 vs. 498 +/- 75 mg/l, P < 0.01). Fifty dogs with other diseases excluding tumours (skin, urinary system, and gastrointestinal diseases, pyometra, other inflammatory diseases, and Cushing's syndrome) had slightly higher TSA levels than the tumour-bearing dogs (730 +/- 159 mg/l, P = 0.02). TSA levels in dogs with malignant tumours did not differ from dogs with benign tumours (682 +/- 144 vs. 664 +/- 142 mg/l, P = 0.73). A receiver-operating characteristic (ROC) plot revealed a maximum sensitivity and specificity combination of 69% and 91% (TSA cut-off concentration 595 mg/l) in distinguishing between healthy dogs and dogs with tumours. When evaluating TSA measurements to distinguish dogs with other diseases from dogs with tumours, a maximum sensitivity and specificity combination of 50% and 75% was found (cut-off concentration 761 mg/l). WHO staging of mammary tumours revealed an increase in TSA levels with increasing stage (P < 0.0001, rs, = 0.62). In conclusion, the nonspecificity of increases makes TSA determinations unsuitable as a tumour marker. TSA levels seem instead to be a general disease marker. Whether serial TSA measurements could be used in the follow-up of dogs operated for malignant tumours should be further investigated.