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Although colon cancer
may also present with blood in stool
and alternate diarrhoea and constipation
but there is nothing which can label these symptoms to be due to cancer.it can be due to haemorrhoids as well as several other causes.
Colonoscopy that is direct visualisation of colon is the investigation of choice to rule out cancer.Tumor marker
in a simple blood test for CEA ,CA 19-9 can be done but a visit to gastroenterologist is a must!
hope this will help you!