Doctors use lab tests, upper GI endoscopy, and imaging tests to rule out other diseases and conditions that cause nausea and vomiting.

Lab tests include:

• Blood tests - to look for signs of anemia, dehydration, inflammation, infection, and liver problems.
• Urine tests-  to look for signs of dehydration, infection, and kidney problems. 
• These tests can also show mitochondrial disease.

Upper GI endoscopy

Upper GI endoscopy is performed to look for problems in your upper digestive tract that may be causing nausea and vomiting.

Imaging tests include

• Ultrasound of abdomen
• Gastric emptying test, also called gastric emptying scintigraphy. This test involves eating a bland meal, such as eggs or an egg substitute, that contains a small amount of radioactive material. An external camera scans the abdomen to show where the radioactive material is located. A radiologist can then measure how quickly the stomach empties after the meal. Health care professionals perform gastric emptying tests only between episodes.
• Upper Gi series
• MRI scan of the brain
• CT scan of the brain

Click here to learn more about the tests used to diagnose CVS: 

https://www.niddk.nih.gov/health-information/digestive-diseases/cyclic-vomiting-syndrome/diagnosis