Despite the tendency to order diagnostic tests in the face of irritable bowel syndrome (IBS) symptoms, the diagnostic criteria for IBS, such as those supported by the Rome Committee, encourage clinicians to make a positive diagnosis on the basis of validated symptom criteria, and emphasize that IBS is not a diagnosis of exclusion, despite the extensive list of other conditions that masquerade as IBS. The current Rome guidelines for IBS state that IBS can be diagnosed in the absence of 'alarm features,' and is 'often properly diagnosed without testing.'
Your doctor may suggest a blood test to check for Celiac disease (gluten intolerance) if you are experiencing diarrhea or diarrhea and constipation (a positive test should be confirmed by a biopsy). Unless there are "warning" signs this is generally all that is needed to diagnose IBS.
There currently is no consistent biological marker of IBS that can be tested to make the diagnosis. Research interest is underway to find a biological marker, or set of markers. They include use of a blood test, stool sample, or tissue sample from the colon. These would enhance the diagnostic accuracy of symptom criteria.
What if the test results are negative?
If you have the typical symptoms of IBS, and your doctor says, "Your tests are negative" it means that you don't have some other disease. Tests look for other things, not for IBS. At this point your doctor might also say something like, "Based on your symptoms and exam, you have IBS." Your signs and symptoms fit the standard for diagnosing IBS. The next step is to learn all you can about IBS and work with your doctor on a treatment plan that fits you. IBS is a problem with the way a system works. There is no visible evidence that shows up on a test.
That's not the same thing as "nothing is wrong." People have painful headaches, but nothing shows up on tests. It's the same with IBS. What if you have a test result that is positive? It may be a minor problem. You may have something else along with your IBS, or you may have something else entirely. A doctor will help you sort this out.