Q: When well, my oral temperature is almost always about a degree lower than 98.6. When I’m sick I can have what constitutes a “fever” (that is, higher than 99 degrees), but sometimes I feel feverish at 98.6 or even lower. Is the latter really a fever?

A: Doctors are often quick to dismiss the concerns of patients who feel feverish but have “normal” temperatures, saying something like, “You only have a temperature of 99 point something. That’s not a fever.”

But if you feel as if you have a fever, you probably do. According to the Centers for Disease Control and Prevention, a fever is present when the patient “feels warm to the touch or gives a history of feeling feverish.” The CDC, however, also offers an alternative temperature-based definition of fever, with a threshold of 100.4 degrees Fahrenheit.

The Infectious Diseases Society of America acknowledges that “the definition of fever is arbitrary” but likewise goes on to cite various definitions, including: an oral temperature of 100 or greater; repeated oral temperatures of 99 degrees or greater; and an increase in temperature of more than 2 degrees over the patient’s baseline.

Much of what we know about fever and “normal” temperatures comes from research in the 1860s by Dr. Carl Reinhold August Wunderlich, the German physician who recorded more than 1 million temperature readings in 25,000 patients. His landmark work led to the adoption of 98.6 as the definition of the average normal temperature.

But pay attention to the word “average.” Normal body temperature varies from person to person, with baseline temperatures generally ranging from 1 degree above 98.6 to 1 degree below.

Furthermore, our body temperature is not a constant. Rather, it fluctuates predictably over the course of the day. Body temperature peaks in the late afternoon and hits its nadir in the early morning hours. This circadian rhythm can lead to variations of as much as 1 degree higher or lower.

Choice of thermometers can also add to the confusion. A rectal thermometer is the gold standard, as it comes closest to approximating the body’s core temperature. Other types, such as oral thermometers, tympanic or ear thermometers or forehead thermometers, are more convenient but may yield lower readings.

In addition, normal temperatures tend to vary among certain groups. Women tend to have a slightly higher basal temperature than men. Older adults tend to have lower temperatures than younger people. And other people may just be outliers.

Your best bet is to determine your baseline temperature. To do this, measure your temperature when you are feeling well. Do this using the same thermometer at the same time of day for several days, and record the results to get an average temperature. Repeating your measurements helps maximize accuracy.

Remind your doctor of your baseline reading. If your doctor says you don’t have a fever, remind him or her of the CDC’s definition that fever is present when the patient “feels warm to the touch or gives a history of feeling feverish.”