Fever and Infection During Chemotherapy

A fever during chemotherapy is a medical emergency. That means roughly 100.4–100.5 °F (38 °C) or higher — the exact figure is worded slightly differently by different authorities, and the section below lays out all four. The National Cancer Institute lists a fever of 100.4 °F (38 °C) or higher as a sign of infection during cancer treatment, and states that "infections during cancer treatment can be life threatening and require urgent medical attention."

Call your cancer care team as soon as you see that number — at 3 a.m., on a Sunday, on a holiday. Do not wait until morning. Do not wait to see if it settles. Do not take acetaminophen and go back to bed to see how you feel later.

Chemotherapy lowers your white blood cells, including the neutrophils that do most of the work of fighting bacteria. When there are too few neutrophils, the condition is called neutropenia. NCI explains that "during chemotherapy, there will be times in your treatment cycle when the number of white blood cells (called neutrophils) is particularly low and you are at increased risk of infection."

With too few neutrophils, your body cannot mount the usual response to an infection. Redness, swelling, and pus may be reduced or missing. A fever may be the only warning sign you get, and an infection that would be minor otherwise can become life-threatening in hours. That is why the threshold is low and the response immediate.

NCI notes that you will have blood tests to check for neutropenia, and that medicine may be given "to help prevent infection or to increase the number of white blood cells."

Authoritative sources word the threshold slightly differently. All four are describing about the same body temperature:

Source	What it says
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."
MedlinePlus (NIH)	A "fever of 100.4 °F (38 °C) or higher."
Clinical case definition of febrile neutropenia (used by clinicians, not a home threshold)	"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."

These are four separate published thresholds, and we are not going to reconcile them for you. 38.0 °C converts to 100.4 °F, and NCI publishes 100.5 °F; why NCI settled on that figure is not something the agency explains. The last row is a clinical case definition doctors use, not a number for you to take a decision on at home. Your own team's threshold is the one that governs.

ACS gives the practical answer: "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 your own team for their number, write it on the fridge, and use theirs — their instruction outranks any published figure.

The sources differ by a tenth of a degree. Do not let that become a reason to hesitate. If you are somewhere in that range and unsure, call anyway. No oncology service minds being called about a fever.

ACS advises using an oral thermometer, since those "are usually more accurate than temperatures taken from the armpit, ear, or forehead," and warns: "Never place a thermometer in your rectum if you're neutropenic."

NCI lists these signs of infection alongside fever:

• Chills
• Cough or sore throat
• Diarrhea
• Ear pain, headache or sinus pain, or a stiff or sore neck
• Skin rash
• Sores or white coating in your mouth or on your tongue
• Swelling or redness, especially where a catheter enters your body
• Urine that is bloody or cloudy, or pain when you urinate

NCI's instruction is simple: "Call your health care team if you have signs of an infection."

MedlinePlus adds more to report during cancer treatment: sweats, diarrhea that will not stop or is bloody, severe nausea and vomiting, being unable to eat or drink, extreme weakness, redness or swelling or drainage where an IV goes in, new skin rash or blisters, stomach pain, a severe or lasting headache, a worsening cough, or trouble breathing at rest or with simple activity.

Chills and shaking without a measured fever still count, as does feeling suddenly and badly unwell. Call.

NCI's prevention advice:

• Wash your hands often with soap and warm water, especially before eating.
• Keep catheter areas clean and dry, and clean your teeth daily.
• Stay away from people who are sick, avoid crowds, and avoid people recently vaccinated with live vaccines.
• Follow food safety rules. Cook meat, fish, and eggs thoroughly and keep foods at proper temperatures.
• Eat fruits and vegetables that can be peeled, or wash produce thoroughly.

None of this replaces calling. Prevention lowers the odds; it does not make a fever safe to sit on.

Put your cancer center's after-hours number in your phone now, while nothing is wrong. Keep a working thermometer. Make sure whoever lives with you knows the number and the threshold.

If you cannot reach your team, go to the emergency department and say at the desk that you are on chemotherapy and have a fever. That sentence changes how quickly you are seen.

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• National Cancer Institute: Infection and Neutropenia during Cancer Treatment. https://www.cancer.gov/about-cancer/treatment/side-effects/infection
• MedlinePlus (National Library of Medicine): Low white blood cell count and cancer. https://medlineplus.gov/ency/patientinstructions/000675.htm
• American Cancer Society: Fevers. https://www.cancer.org/cancer/managing-cancer/side-effects/infections/fever.html
• MedlinePlus (National Library of Medicine): patient instructions on fever and infection during cancer treatment. https://medlineplus.gov/ency/patientinstructions/000047.htm
• StatPearls (NCBI Bookshelf): Febrile Neutropenia — clinical definition. https://www.ncbi.nlm.nih.gov/books/NBK541102/