Why do we fast before tests or surgeries?

Fasting is most important before any surgery that requires anesthesia, such as joint replacements, heart surgeries, and more.

That's because "anesthetic drugs can impair your protective airway reflexes," says Jeremy Dennis, MD, a board-certified anesthesiologist and assistant professor of clinical anesthesiology at Yale-New Haven Hospital in New Haven, Connecticut. If you have food or liquid in your stomach during surgery, there's a small risk that some of it could get into your lungs while you're under anesthesia.
Aspiration risk

"Normally, your body is able to prevent your stomach contents from entering your lungs," says John Wang, MD, an attending surgeon at the Hospital for Special Surgery in West Palm Beach, Florida. "When under anesthesia, your body is less effective at preventing that. When food, liquids, or stomach acid enter the lungs, doctors call it 'aspiration.' This is rare, but it can be very dangerous and lead to a serious infection and lung damage."

So the main reason for fasting before certain procedures is to protect your lungs from potential damage. It is also helpful if you tend to feel nauseous from anesthesia or certain medications used during the procedure.

As for fasting before blood tests—like, for instance, a glucose tolerance test—it's more about accuracy.

"If you drink a can of Coke before a glucose tolerance test, the results will be artificially inflated," says Leonard J. Pianko, MD, a board-certified cardiologist in Aventura, Florida. "While thyroid and hemoglobin tests do not require fasting, we often include those two tests as part of a panel of testing for which we ask our patients to fast as of midnight the night before."

What type of tests or surgeries require fasting?

Any procedure that requires anesthesia, including local anesthesia (say, wisdom teeth removal) and general anesthesia (for something like knee replacement) requires fasting.

There are often pre-surgery tests, Dr. Wang says, which may also require fasting. Your doctor will advise you about all procedures and tests that require a certain window of avoiding food and most drinks. Beyond that, "it is best to just follow a balanced, nutritious diet prior to surgery and follow fasting instructions," Dr. Wang says.

Certain procedures or tests have their own set of specific fasting instructions or prep, such as for a colonoscopy. You will usually be asked to fast before general blood tests because lipid tests, which measure triglycerides in the blood require fasting. They're part of a normal cholesterol screening. Blood glucose tests also require fasting.

Again, your surgeon or doctor will provide situation-specific guidance.

How long do you have to fast?

For blood tests, you'll often be asked to fast for eight to 12 hours prior to your appointment. Unless you're told otherwise by your doctor, avoid food and all drinks besides plain tap or bottled water, as caffeine, alcohol, and even carbonation can potentially impact certain blood test results.

The general rule for anesthesia used to be to avoid food or drink after midnight. But that's an arbitrary deadline, some doctors say, and guidelines have since changed.

"Since there may be a change in plans, we often recommend a patient fast as of midnight the night before surgery," Dr. Pianko says. "Technically, it only takes eight hours for the stomach to digest a full meal and much less for a light meal. But for the sake of consistency and to prevent confusion or misinterpretation as to what is a light meal as opposed to a full meal, we often recommend midnight as the cutoff point."
New fasting guidelines

The American Society of Anesthesiologists has since published new guidelines (most recently updated in 2017) that permit clear liquids up to two hours before surgery for healthy patients undergoing elective procedures using general anesthesia, regional anesthesia, or sedation.

A study published in 2017 in BJA Education also suggests that patients fast before surgery for a much shorter time and have an oral carbohydrate drink to help them better recover after surgery.

Despite these newer recommendations, some anesthesiologists still stick with older rules for their patients. Consult with your doctor before any procedure to see what's best for you.
Are any foods or drinks allowed when you fast?

If you're told to fast, you shouldn't eat solid foods during that window of time. But can you drink water while fasting?

Because water and clear liquids are quickly absorbed by the stomach in a relatively short amount of time, patients are often allowed to drink clear liquids up to two hours before surgery, Dr. Wang says. Alcohol does not fall into this category and is off-limits while fasting. But sparkling water, mineral water, fruit juices without pulp, clear carbonated drinks (like, say, 7-Up, but not Pepsi), and clear tea are OK.

"The line's drawn when you cannot see through the liquid," says Dr. Wang. "Milk products have protein in them and that increases the time that the liquid spends in your stomach. The goal is to have an empty stomach immediately before surgery."

Although it's not clear, black coffee (without cream or sugar) is fine, too.
What to do if you forget to fast

Mistakes happen, and doctors understand that. Now is not the time to fib.

"Be honest," Dr. Pianko says. "It's important for the doctor and the lab technician to be advised of any deviations from the original instructions. The worst that can happen is that the physician decides to reschedule the test or the procedure."

If you do break your fast before a surgery, let your surgeon and anesthesiologist know ASAP, Dr. Wang says.

"These fasting guidelines were designed with patient safety in mind," adds Dr. Wang. "Violation of the recommended fasting windows add risk of stomach contents entering the lung when a patient undergoes anesthesia. When these fasting guidelines are not followed, the anesthesiologist will compare the risks and benefits of proceeding, with consideration given to the amount and type of liquids or solids ingested."

The surgeon may even be able to rearrange your procedure time to fit you in during your new fasting window.
When can you eat and drink again?

As with many medical questions, the answer to this is: "It depends"—on the type of anesthesia, sedative, procedure, or blood test.

"Most patients undergoing elective orthopedic surgery are able to tolerate light meals right after surgery," Dr. Wang says. "When the anesthesia type is regional anesthesia or sedation, patients often resume eating and drinking normally in the recovery room."

For certain surgical procedures—like abdominal surgery—the surgeon may have very specific instructions, Dr. Dennis says. Again, your doctor will advise you about this. But you should be able to grub on whatever sounds good after you're awake and ready to refuel. Drink plenty of plain water and eat a well-balanced meal to kickstart the post-procedure recovery.

After blood tests, you're also able to begin eating immediately—unless your doctor dictates otherwise.