Most of the time, an achy stomach isn’t anything serious. But when your pain persists, migrates, or is accompanied by other serious symptoms, a trip to the emergency room could be in order.

For more than 600,000 people each year, stomach pain is their first clue that their appendix has torn and they’re suffering appendicitis.

How do you sort out when it’s a stomach ache and when it’s appendicitis? Dr. Rodrigo F. Alban, associate director of the General Surgery Residency Program at Cedars-Sinai, offers this explanation of appendicitis and the symptoms to watch out for.

The appendix is part of the large intestine—a small, finger-shaped organ about 4 inches long, whose function is still largely a mystery, though it might offer a breeding ground for healthy gut bacteria after an infection. Or it may do nothing at all. Experts are still undecided. But when it tears, it has to be removed.

Sometimes stool can get stuck in the appendix, which is shaped like a tube with one closed end. Like a balloon that’s been tied off, there’s no way for what’s trapped inside to escape. The pressure builds as the appendix continues producing its normal secretions. The organ swells, and once this process starts, the appendix can tear or rupture.

In children, appendicitis can start when the certain tissues in the appendix that are rich in immune system cells get swollen. When kids get appendicitis, it’s usually this and not a fecal obstruction that causes it.

Migrating pain: Your appendix troubles might start several days before the organ becomes torn or ruptures. During this time, you might have pain in the middle of your abdomen that feels like an upset stomach. If your appendix is the issue, this pain tends to move to the right or lower right side of the abdomen.

Nausea and vomiting: Nausea and vomiting are associated with appendicitis, but diarrhea is more consistent with a gastrointestinal infection.

Loss of appetite: Dr. Alban will ask a patient, “What is your favorite food?” And then, “Would you want to have something like that right now?” If a patient says they’re not interested in their favorite dish, it tips him off to pain so bad that it has caused loss of appetite.

Fever: A small tear might cause pain contained to the lower right side of the body. If untreated, the infection can spread through the abdomen causing fever and other symptoms.

Doctors will often administer a blood test to check for a high white blood cell count, which would clue them in to an infection. You might also have an imaging test, like a CT scan.

Ruling out other conditions is also part of the process. Women can sometimes be more difficult to diagnose, because appendicitis has similar symptoms to ovarian cysts and other gynecologic conditions. Doctors will also try to rule out gastroenteritis and bowel diseases, as well as kidney stones in older people.

Read: Heartburn and Acid Reflux: What You Need to Know

Once you’ve been diagnosed with appendicitis, there are several options.

If the appendix is swollen and infected, but has not torn, it’s usually removed with a minimally invasive procedure requiring a few small incisions. These patients often go home on the same day.

If the appendix is already torn or ruptured, the surgery is more complex. Any abscess has to be drained and then the patient is treated with antibiotics. Once the swelling goes down, the appendix is removed.

More severe cases might require an open surgery, with a larger incision.