What temperature counts as a fever? Medical experts explain when to worry

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Sick adults and parents concerned over their children's health may have wrong ideas about what actually counts as a fever.

The answer isn't as clear as many people might think and depends on a number of factors, medical experts said. Doctors broke down what the numbers on your thermometer actually mean:

What is a normal body temperature?

People grow up hearing that 98.6 degrees Fahrenheit is the gold standard body temperature, but there's a range of body temperatures that are all considered normal, doctors said. For most people, it's between 96 and 99 degrees, Dr. Amy Horwitz, a family medicine physician at the Cleveland Clinic, said.

Body temperatures also fluctuate throughout the day, and it's not necessarily because you have a fever, Horwitz said. They can be impacted by the climate you live in, the time of day or if you've been recently exercising.

Body temperatures tend to be lowest in the morning, according to the Cleveland Clinic.

In women, a normal body temperature can vary based on their menstrual cycle, the medical center added. Body temperature rises slightly after ovulation.

What is a fever?

A fever is when your "body's overall thermal regulation is elevated," Dr. Angela Mattke, a pediatrician with the Mayo Clinic, said. It's usually a response to your body trying to fight off some kind of infection.

"People think fevers are bad, but they're actually not necessarily something we have to eliminate at all times," Mattke said.

In addition to an elevated temperature, people with fevers may experience decreased appetite or a change in mental status, according to Dr. Baruch S. Fertel, MD MPA and emergency physician at NewYork-Presbyterian/Columbia University Irving Medical Center.

Someone with a fever can also have sweats, chills, muscle aches, a headache, fatigue and general weakness, Horwitz said.

What temperature is considered to be a fever in adults?

The Centers for Disease Control and Prevention considers a person to have a fever when they have a measured temperature of 100.4 degrees Fahrenheit or higher.

Some people view a fever as 101 degrees Fahrenheit and up, but most people agree on the 100.4 minimum, Fertel said.

What temperature is considered to be a fever in children?

The same 100.4 degrees Fahrenheit or higher standard applies to children, but parents need to take extra care when it comes to newborns, doctors said.

"We worry most about infants under 3 months as they are so young and their immune systems are not fully developed," Horwitz said.

While fevers in newborns are emergencies, parents of older children shouldn't obsess over numbers, Mattke said.

"It's not the actual number, it's how your child acts," Mattke said. "I think parents get really caught up in numbers. I think we forget to look at the child and see how they're acting."

What's the best way to take a temperature?

Parents should get rectal temperatures for infants, doctors said, as it's the most accurate way of assessing a newborn's temperature.

For older children, readings can be taken orally or by ear or armpit. For parents taking a child's temperature, sticking with one method will help to track variations in temperature, Mattke said.

"I wouldn't switch between methods," Mattke said. "Just pick one method and go with it."

What counts as a low-grade fever?

A low-grade fever is a colloquial term used often by parents, but experts say there's no real number associated with it and most doctors do not use the term.

The phrase tends to refer to a temperature higher than 98.6 degrees Fahrenheit but lower than 100.4.

Does a fever always mean you're sick?

People can have fevers without being sick, doctors said. For example, some people develop a fever after getting an immunization shot, according to Horwitz.

"This is a very normal and expected response to vaccination as it is the body's response to help protect you in the future," Horwitz said. "It is less common, but patients with autoimmune conditions or cancer may also develop fevers. Dehydration can also cause a fever."

What's the best way to treat a fever at home?

Doctors advise treating a fever at home with rest, hydration and baths. People can also take acetaminophen or ibuprofen. Parents should be mindful to follow appropriate weight-based dosing for children. It's always best to contact a doctor if you have any questions.

When should you go to the doctor?

For a baby less than 3 months old, it's important to seek medical care right away if the infant has a temperature higher than 100.4 degrees, Horwitz said. For children older than 3 months, seek medical care if the child has a fever and seems irritable, sluggish or very uncomfortable.

"In older kids that are immunized, fever is not necessarily 'you have to run to the emergency department.' But if your child's not acting well, we definitely want to see them," Mattke said.

Adults should get medical assistance if, along with a fever, they're dealing with a severe headache, stiff neck, light sensitivity, confusion, chest pain, shortness of breath or severe abdominal pain, according to Horwitz.

People who are immunosuppressed should call a doctor if they have a fever, Fertel said. Anyone whose fever persists for three days should also call a doctor.

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