Patient information · Diagnostics
GI diagnostic tests explained: which one do you need?
Quick answer
Gut symptoms are investigated with endoscopy (colonoscopy and gastroscopy), stool and blood tests (such as FIT and ferritin), and scans (ultrasound, CT and MRI).
The right test depends on your symptoms, age and history – the aim is the least invasive test that gives a confident answer.
Many gut symptoms – a change in bowel habit, bleeding, pain, reflux, unexplained weight loss or anaemia – can only be explained properly with the right test. This guide explains the main gastrointestinal (GI) diagnostic tests, what each one looks for, and how to decide which you might need. As a JAG-accredited endoscopist who performs colonoscopy and gastroscopy routinely, my aim is always to use the least invasive test that will give a confident answer.
Tests that look directly inside the gut (endoscopy)
Colonoscopy
A colonoscopy uses a thin, flexible camera to examine the whole large bowel (colon and rectum). It is the gold-standard test for investigating rectal bleeding, persistent changes in bowel habit, iron-deficiency anaemia and a positive FIT test. Its great advantage is that polyps can be removed during the same procedure, which prevents many bowel cancers from ever developing. It is usually done as a day case with sedation, and requires bowel-cleansing preparation the day before.
Gastroscopy (upper GI endoscopy / OGD)
A gastroscopy examines the oesophagus (gullet), stomach and the first part of the small bowel. It is the key test for persistent reflux, indigestion that does not settle, difficulty swallowing, upper abdominal pain and suspected ulcers. Small biopsies can be taken to test for inflammation, coeliac disease or Helicobacter pylori infection.
Flexible sigmoidoscopy
This is a shorter examination of the lower bowel only. It needs less preparation and is sometimes appropriate for fresh rectal bleeding when the rest of the colon does not need to be assessed.
Stool and blood tests
FIT (faecal immunochemical test)
FIT detects tiny, invisible traces of blood in a single stool sample. It is a useful triage test: a negative result is reassuring in the right context, while a positive result is an important signal that a colonoscopy is needed. It does not replace colonoscopy when symptoms are significant.
Blood tests
A full blood count can reveal anaemia; iron studies (including ferritin), B12, folate, inflammatory markers, coeliac antibodies and liver and pancreatic blood tests all help build the picture before deciding on further investigation.
Scans (imaging)
- Abdominal ultrasound – first-line for suspected gallstones and to assess the liver and bile ducts.
- CT scan – used for acute abdominal pain, staging and assessing complex disease.
- CT colonography (virtual colonoscopy) – a CT-based map of the colon, useful when a standard colonoscopy is not suitable or could not be completed.
- MRI – valuable for the rectum, pelvic floor and complex perianal conditions such as fistulas.
How is the right test chosen?
The right test depends on your symptoms, your age, your personal and family history, and any earlier results. The principle is straightforward: match the test to the question being asked, start with the least invasive option that can answer it confidently, and avoid unnecessary repeat investigations. A consultation allows all of this to be weighed up properly rather than guessed from an online checklist.
Frequently asked questions
Is a colonoscopy painful?
Most people have a colonoscopy with sedation and pain-relief, and describe it as uncomfortable rather than painful. The bowel preparation beforehand is the part most people find least pleasant. It is usually a day-case procedure, so you go home the same day.
Do I need a GP referral for a private GI test?
No. Self-referrals are welcome without a GP letter. A consultation first ensures the most appropriate test is arranged and that the results are interpreted in the context of your wider health.
How quickly can a private colonoscopy or gastroscopy be arranged?
Privately, investigations can often be arranged within days when symptoms warrant it, rather than waiting weeks or months.
Can polyps be removed during a colonoscopy?
Yes. One of the main advantages of colonoscopy is that polyps can be removed during the same procedure, which can prevent a bowel cancer from developing.