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Faecal testing for the detection of colorectal cancer

Colorectal cancer is one of the most common malignancies and one of the main causes of death from cancer. Survival is much improved when a diagnosis is made at an early stage, hence the importance of screening tests to enable a diagnosis as early as possible.

 

The incidence of this type of cancer increases with age, with the majority of cases occurring in those over the age of 50. For this reason, the majority of professional bodies and guidelines recommend starting screening tests at the age of 50, bearing in mind that each person’s case must be assessed individually to more accurately estimate that person’s risk.

 

Amongst the most straight-forward tests, due to their ease and affordability, are those performed on samples of faeces, these are basically of three types: those which detect occult blood in stools, and “multitarget” tests which detect not only blood but also genetic mutations associated with this type of cancer. Below we explain these tests in more detail.

 

Test para detectar sangre oculta en heces.

 

Test for detecting occult blood in faeces:
 

1. Immunohistochemical test: directly measures the haemoglobin (a blood component) in faeces. Only a small sample of faeces is required, this must be taken to the laboratory within 24 hours so that the sensitivity of the test is not diminished. Testing once a year, the sensitivity for detecting colorectal cancer is almost 74% and the sensitivity for detecting polyps of 10 mm or more is 24%. For smaller polyps measuring 6 to 9 mm or less than 5 mm, sensitivity is lower, at a little more than 7%.

 

2. The guaiac test: detects the presence of haemoglobin by changing the colour of paper impregnated with a reagent (alpha-guaiaconic acid). When performed yearly it shows a sensitivity for detecting colorectal cancer of 70%, a sensitivity for detecting polyps, of at least 10 mm, of 24%, a sensitivity for detecting polyps of 6 to 9 mm of 12.4% and a sensitivity for polyps of less than 5 mm of 7.5%. .

 

Multitarget stool DNA test.

 

This test combines the detection of haemoglobin using immunohistochemical techniques, with the detection of genetic mutations associated with colorectal cancer in the remains of DNA eliminated in the faeces. Their sensitivity when performed yearly is of 92.3% for the detection of colorectal cancer, 42.4% for the detection of polyps of at least 10 mm and 17.2% for polyps of 6 to 9 mm or less than 5 mm.

 

The following table summarises these tests:

 
 

 

A positive result from any of these tests, requires more specific testing, in other words colonoscopy. This facilitates the detection of the cause of the positive result, as in many cases it will not be due to a malignant tumour. It also enables the removal of small lesions such as polyps.

 
 

The study of each person’s individual case is still recommended to ascertain their potential risk and select the most appropriate method of testing in each case.

 

José Miguel Rosales Zábal

Gastroenterology Unit .

 

 

May 29, 2020

 

 

 

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