The first step in diagnosing the cause of fecal incontinence is a physical examination and medical history that includes a description of symptoms and list of all current medications. A physician will visually inspect the anus and perineum (area between the anus and genitals) for conditions that may cause fecal incontinence.

A physician may also use a prodding device or probe to gently examine the area. This includes touching the external anal sphincter (ring-like muscles at the end of the rectum), which normally causes the sphincter muscles to contract and the anus to pucker. This is called an anocutaneous reflex or anal wink test. Failure of the anus to respond to touch may indicate nerve damage. A physician may also perform a digital rectal examination (DRE), which involves inserting a gloved and lubricated finger into the rectum. This is used to check for any abnormalities and to evaluate sphincter muscle strength.

Other tests that may be performed depending on the suspected cause of the incontinence include:

• Fecal tests. Laboratory examination of a sample of stool. Fecal tests may be used by physicians to identify the cause of fecal incontinence in patients with diarrhea.
  
  
• Anal manometry. A common test of anal pressure, rectal elasticity and sensation. A narrow, flexible tube is inserted into the anus and rectum. A small balloon at the end of the tube may be expanded to measure the tightness, sensitivity and function of the anal sphincters. This test may reveal problems with muscle tone or impairment of rectal reflexes or sensation.
  
  
• Anorectal ultrasonography. The use of sound waves to produce images of anal and rectal areas. A narrow, wand-like instrument that emits sound waves is inserted into the anus and rectum. Images are then produced on a computer screen for examination by a physician. This test can reveal structural abnormalities of the anal sphincters, rectal wall and the pelvic muscles that help maintain continence.
  
  
• Defecography. Determines the ability of the rectum to hold and release stool. In this test, a thick barium paste is inserted through the anus, coating the walls of the rectum. X-rays are taken during rest, straining and release of the paste from the bowels. This test may also reveal structural abnormalities in the rectal and anal areas.
  
  
• Sigmoidoscopy. A flexible tube with camera and light attached are inserted in the anus and through the rectum and sigmoid colon (the last two feet of the colon). Tissue samples may be removed for analysis under a microscope. This test is used to detect signs of inflammation, tumors or scar tissue that may cause fecal incontinence.
  
  
• Colonoscopy. Similar to a sigmoidoscopy, except that the tube is inserted farther up the digestive tract, allowing examination of the entire large intestine.
  
  
• Anoscopy. A direct visual examine of the anus, anal canal and lower rectum by a physician using a special instrument (anoscope) that is lubricated and inserted a few inches into the rectum. The test is used to identify hemorrhoids, polyps, inflammation or other possible causes of fecal incontinence.
  
  
• Anal electromyography. Detects nerve damage or muscular causes of poor anal sphincter function by inserting tiny needle electrodes into muscles around the anus.
  
  
• Nerve conduction tests. Electrical impulses are delivered into the rectum to trigger muscle contraction. Physicians may use a glove containing a stimulating electrode in the finger, which is then inserted into the anus. These tests measure the speed between the stimulating impulse and the muscle contraction. Delay may indicate damage to the pudendal nerve that controls anal sphincter muscles, causing fecal incontinence.