To diagnose an anal fistula, your GP will look at your medical history and carry out a physical examination. If there are several fistula tracts (channels), further tests may be needed to determine their position.

Your GP will also pay particular attention to any history of anal abscesses or conditions that affect your bowels, such as Crohn's disease. These conditions can lead to an anal fistula developing.

You will usually be referred to a specialist in bowel conditions, known as a colorectal surgeon, for further investigation.

Physical examination

The specialist will examine your anus (the opening where waste leaves the body) and the surrounding area for any physical signs of a fistula. The opening of a fistula usually appears as a red, inflamed (swollen) spot, which often oozes pus.

If the opening of the fistula is found, the specialist may be able to work out where the path of the fistula lies. The path of the fistula can sometimes be felt as a hard, cord-like structure beneath the skin.

Digital rectal examination

Your specialist will also perform a digital rectal examination (DRE). This involves placing a finger into your anus. During the DRE, the finger will be covered with a glove and lubricated with gel.

The DRE allows your doctor to find out where the internal opening of the fistula is, and if there are any secondary tracts branching off it.

During the DRE, your doctor may ask you to squeeze your sphincter muscles (the rings of muscles that open and close the anus) around their finger, to assess how well they are working.

The DRE will help determine what kind of treatment you need, or whether further tests are necessary.

Fistula probe

Your specialist may also need to use a proctoscope (special telescope with a light on the end) to see inside your rectum. They may also use a fistula probe, which is a tiny instrument inserted through the fistula.

These examinations may be performed under general anaesthetic, where you are asleep.

Further tests

If you have a complicated fistula with several branches, you may need further tests to determine the exact position of the fistula tracts. This will help determine your treatment.

Some further tests that may be recommended include:

• Anal endosonography (ultrasound) - this test uses high-frequency sound waves to create an image of the inside of your body. This is an accurate and frequently used way of locating the internal opening of a fistula.
• Magnetic resonance imaging (MRI) scan - an MRI scan uses strong magnetic fields and radio waves to produce a detailed image of the inside of your body. This type of scan is often used in cases of complex or reoccurring fistulae.
• Computerised tomography (CT) scan - a CT scan uses X-rays and a computer to create detailed images of the inside of your body. This may be used if you have an inflammatory bowel disease, such as Crohn's disease, as it can be used to assess the extent of the inflammation.