Most episodes of acute diarrhea resolve quickly without antibiotic therapy and with simple dietary modifications. See a doctor if you feel ill, have bloody diarrhea, severe abdominal pain or diarrhea lasting more than 48 hours. In patients with mild acute diarrhea, no laboratory evaluation is needed because the illness generally resolves quickly. Your doctor may perform stool tests for bacteria and parasites if your diarrhea is severe or bloody or if you traveled to an area where infections are common. If you have severe diarrhea, blood tests will be helpful to guide replacement of fluid and electrolytes and minerals such as magnesium, potassium and zinc that can become depleted.