What counts as a fever during chemotherapy?

During chemotherapy, a fever is a temperature of about 38 degrees Celsius—100.4 to 100.5 degrees Fahrenheit—or higher. The exact number differs slightly from one source to the next, because they are rounding the same body temperature. Call your health care team if you reach it.

Here is what each source says, in its own words:

• National Cancer Institute: a "fever of 100.5 °F (38 °C) or higher" — its printed 100.5 °F is a conversion slip, since 38 °C is 100.4 °F; act at 100.4 °F (38 °C) — listed as a sign of infection to report to your health care team.
• American Cancer Society: "For people with cancer, a fever is defined as a temperature of 100.4°F (38°C) or higher for at least one hour."
• CDC, writing for people getting chemotherapy: "Call your doctor immediately if you have a temperature of 100.4ºF (38ºC) or higher."
• The clinical definition of febrile neutropenia, the one used inside hospitals: "a single oral temperature greater than or equal to 101 F (38.3 C) or a temperature greater than or equal to 100.4 F (38 C) for at least an hour."

None of these is wrong. 38.0 degrees Celsius converts exactly to 100.4 degrees Fahrenheit, and NCI's 100.5°F is a rounding of that same Celsius reading.

The number that matters most is your own team's. The American Cancer Society says so directly: "Ask your cancer care team what temperature they consider to be a fever. It might be different depending on your situation, but 100.4°F (38°C) is often used." Ask at your next visit, write the number somewhere you can find it at 2am, and use theirs.

If your temperature lands between these numbers and you are not sure what to do, call. No oncology service minds being called about a fever, and they would far rather hear from you early.

This matters because some cancers and treatments like chemotherapy lower your white blood cells—the cells that fight infection—so an infection can become serious quickly. Other signs of infection include chills, cough or sore throat, diarrhea, and redness or swelling where a catheter enters the body.

Infections during cancer treatment can be life threatening and need urgent attention. Ask your care team what number to call and when, so you are ready if a fever happens.