FIT looks for blood. Stool DNA looks for the tumour.

What FIT does

The faecal immunochemical test measures human haemoglobin in your stool. Haemoglobin is the protein in red blood cells, so FIT is a test for hidden bleeding. Bowel tumours and large polyps have fragile surfaces that tend to bleed, which is why it works.

FIT is quantitative: it returns a number in micrograms of haemoglobin per gram of faeces, and a threshold decides who gets referred. Those thresholds differ across the UK. Scotland and Wales refer at 80µg/g. England and Northern Ireland have used 120µg/g, and England has been moving to the more sensitive 80µg/g threshold. A lower threshold catches more cancers and also sends more people for colonoscopy who turn out not to have one.

FIT is a good test, it is free on the NHS, and if you are invited you should do it.

The limitation nobody mentions

FIT can only find a tumour on a day that tumour is bleeding. Bleeding is intermittent. A tumour may bleed one week and not the next, and a single sample captures the day it was taken and nothing else. Lesions high in the bowel are also harder to catch, because haemoglobin is partly broken down on the way through.

This is not a flaw in the test so much as a consequence of what it measures. If you are looking for blood, you need there to be blood.

What stool DNA adds

Cells are shed from the bowel wall continuously, and cells shed from a tumour carry that tumour's genetic changes with them. Testing the stool for those changes gives a second route to detection, one that does not depend on bleeding.

ColoAlert, the test we supply, measures five things from a single sample:

• Haemoglobin and haptoglobin, by ELISA. Haptoglobin survives further along the bowel than haemoglobin, so blood originating higher up is less likely to be missed.
• KRAS and BRAF, by quantitative PCR. Genes frequently mutated in colorectal tumours.
• hDNA, the overall quantity of human DNA in the sample, which tends to rise when bowel cells are being shed abnormally fast.

How to think about the two together

They are not rivals. FIT is a population screening tool, free, well evidenced and backed by the referral machinery of the NHS. A stool DNA test is something you buy, and what you are buying is a second angle on the same question, taken on a day of your choosing.

The sensible position for most people: take part in NHS screening when you are invited, and consider a private test if you are outside the invited age range, if you want to test between NHS rounds, or if you have a family history you want to be more careful about.

What neither of them is

Neither test is a diagnosis. An abnormal result from either means further investigation, usually colonoscopy. A normal result from either lowers the probability of disease without eliminating it, and neither result should override a symptom. Colonoscopy remains the reference standard because it sees the bowel directly and can remove a polyp at the same time.

If you take one thing from this: a test that finds nothing is not the same as a bowel with nothing in it. It is a reason to keep going with regular screening, not a reason to stop paying attention.