High CA 15 3
A raised CA 15 3 level can occur in breast cancer, particularly in advanced or metastatic disease. Results have limited specificity, meaning elevated levels may also be seen in benign conditions and cannot diagnose cancer on their own. High CA 15 3 is therefore most useful for monitoring known breast cancer—for example, assessing treatment response or detecting recurrence.

Low or Normal CA 15 3
A low or normal CA 15 3 level does not exclude breast cancer, as tumour markers have limited sensitivity and may remain normal in early stage disease or even in some advanced cases. Normal results must always be interpreted alongside imaging and clinical findings.