“The findings of the lactulose breath test in irritable bowel syndrome patients have been used to suggest that most patients have small intestinal bacterial overgrowth (SIBO), and this association has spawned the widespread use of antibiotics in IBS. The study by Bratten and colleagues demonstrates that this test does not discriminate between IBS patients and healthy controls when criteria from recent clinical IBS studies are applied. ” 
I recently added some more studies and found that the list of bacteria that will consume lactulose is quite large — with both good and bad listed.
Recently I posted about another probiotic that gave positive lactulose breath test when given to healthy individuals. I look at the list above and see many bacteria that are often in yogurt or available as probiotics.
In short, not only have studies on human discredited this test — but logic also discredit it. It is totally unclear which bacteria is overgrown!