What blood test would reveal small bacteria overgrowh in the intestines? What are the symptoms and causes?

Folate (folic acid) Agree with dr. Larsh. Bacterial overgrowth can also cause abnormally high serum folate (folic acid) levels in the absence of supplement use, along with low vitamin B12 levels. Bacterial fermentation increases the folate (folic acid), and they compete for the b12. Bacterial overgrowth can be confirmed with breath hydrogen testing after lactulose (poorly digestible carbohydrate) load.
Breath test/culture. Sibo is diagnosed by a hydrogen breath test or by directly culturing bacteria from a small intestine sample. The most common symptoms are excess gas, bloating, abdominal pain, diarrhea (though sometimes constipation). It is caused by a change in the bacteria in the small intestine becoming more like the bacteria that lives in the colon. Anything that slows down transit time can do this.