Rule out other cause
Ibs is mostly a diagnosis of exclusion, meaning that a full work up reveals nothing, and the patient is treated symptomatically.
I find that majority of ibs cases are du to overgrowth of yeast
or other opportunistic bugs in the GI tract, in conjunction with food sensitivities. Start with adding a probiotic
and eliminating grains and dairy products, as these are the most common food sensitiviities.