The short answer
Neutropenia is a low neutrophil count, common during chemotherapy. Fever may be the only sign of infection, so know your team's temperature threshold and call at once.
During chemotherapy a temperature of 100.4 F (38 C) or higher is a sign of infection and a reason to call.
Do not take acetaminophen or ibuprofen for the fever first, because it can mask a more serious problem.
With very few neutrophils your body may not make redness, swelling or pus, so fever can be the only sign.
A fever at 2 a.m. on a Sunday means using the after-hours number, not waiting for the clinic to open.
Choose how you want to understand this
The full explanation.
Neutrophils are the white blood cells that do most of the work of fighting infection. The National Cancer Institute defines neutropenia as "a condition in which there is a low number of neutrophils." Chemotherapy and some cancers lower that number, and while it is low, your body cannot mount its usual defense.
The one thing to memorise
A fever of about 100.4–100.5 °F (38 °C) or higher is a sign of infection — call your team.
NCI states: "Call your health care team if you have signs of an infection. Infections during cancer treatment can be life threatening and require urgent medical attention."
The authorities word the threshold slightly differently, and all of them are describing the same body temperature:
- 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."
- The clinical definition of febrile neutropenia: "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, with an absolute neutrophilic count (ANC) of less than 1500 cells/microliter."
38.0 °C converts exactly to 100.4 °F, so NCI's 100.5 °F is a rounding of the same reading. None of them is wrong. ACS gives the instruction to act on: "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."
So: use your own team's number, and read it as written. Not 100 degrees, not "about 101," not "if it stays up." If you are between these numbers and unsure, call anyway — no oncology service minds being called about a fever.
And urgent means urgent. If your temperature reaches that number at 2 a.m. on a Sunday, you call then — not when the clinic opens on Monday. Your oncology team has an after-hours number for exactly this reason. This is one of the few things in cancer care that cannot wait for business hours. Ask your team now, before you need it, which signs mean calling them and which mean going straight to the emergency room.
Do not take anything for the fever first
This is the step people get wrong out of good intentions. NCI warns: "Be sure to talk with your doctor or nurse before taking medicine—even aspirin, acetaminophen (such as Tylenol®), or ibuprofen (such as Advil®) for a fever. These medicines can lower a fever but may also mask or hide signs of a more serious problem."
Bringing the number down does not treat the infection. It hides the signal your team needs.
Other signs of infection to report
Alongside fever, NCI lists:
- 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
The American Cancer Society adds fast heartbeat, breathing problems, dizziness or lightheadedness, and joint or bone pain, and says to "talk with your cancer care team right away if you have any signs or symptoms of an infection."
An important caveat: with very few neutrophils, your body may not produce the usual redness, swelling, or pus. Fever can be the only sign you get.
Why your counts drop, and when
NCI explains that some cancers and treatments such as chemotherapy raise infection risk because they lower white blood cell numbers: "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." NCI adds that stress, poor nutrition, and not enough sleep can also weaken the immune system.
Ask your team when in your cycle your count is expected to be lowest — that is the window to be most careful in.
NCI notes that "medicine may sometimes be given to help prevent infection or to increase the number of white blood cells." ACS describes these as growth factors, "also called granulocyte colony-stimulating factors (G-CSFs)," usually given as a shot.
Lowering your risk day to day
NCI's prevention advice:
Wash your hands often and well. Use soap and warm water, especially before eating. Have people around you wash their hands well too.
Stay extra clean. If you have a catheter, keep the area around it clean and dry. Clean your teeth well and check your mouth for sores each day. Clean any scrape or cut well. Tell your doctor or nurse if your bottom is sore or bleeds, as this could increase your risk of infection.
Avoid germs. Stay away from people who are sick or have a cold. Avoid crowds, and people who have just had a live vaccine such as one for chicken pox, polio, or measles.
Handle food safely. Make sure meat, fish, and eggs are well cooked. You may be advised to eat only fruits and vegetables that can be peeled, or to wash all raw fruits and vegetables very well.
Questions for your team
NCI suggests bringing these to your visit:
- Am I at increased risk of infection during treatment? When am I at increased risk?
- What steps should I take to prevent infection?
- What signs of infection should I look for?
- Which signs signal that I need urgent medical care at the emergency room? Which should I call you about?
Sources
- National Cancer Institute: Infection and Neutropenia during Cancer Treatment. https://www.cancer.gov/about-cancer/treatment/side-effects/infection
- American Cancer Society: What to Do for Low White Blood Cell Counts. https://www.cancer.org/content/dam/cancer-org/cancer-control/en/what-to-do/what-to-do-for-low-white-blood-cell-counts.pdf
- American Cancer Society: Low White Blood Cell Counts (Neutropenia). https://www.cancer.org/cancer/managing-cancer/side-effects/low-blood-counts/neutropenia.html
- American Cancer Society: Fevers. https://www.cancer.org/cancer/managing-cancer/side-effects/infections/fever.html
- MedlinePlus (National Library of Medicine): Cancer treatment - preventing infection. https://medlineplus.gov/ency/patientinstructions/000913.htm
- StatPearls (NCBI Bookshelf): Febrile Neutropenia — the clinical definition. https://www.ncbi.nlm.nih.gov/books/NBK541102/
Words to know
Tap any term to see what it means.
Common questions
What is neutropenia?
Neutrophils are the white blood cells that do most of the work of fighting infection. The National Cancer Institute defines neutropenia as a condition in which there is a low number of neutrophils. Chemotherapy and some cancers lower that number, and while it is low your body cannot mount its usual defense.
Should I take something for a fever before I call?
No, and this is the step people get wrong out of good intentions. NCI warns you to talk with your doctor or nurse before taking medicine for a fever, even aspirin, acetaminophen or ibuprofen, because these can lower a fever but may also mask or hide signs of a more serious problem. Bringing the number down does not treat the infection. It hides the signal your team needs.
When in my treatment cycle is my risk highest?
NCI says that during chemotherapy there will be times in your treatment cycle when the number of white blood cells is particularly low and you are at increased risk of infection. Ask your team when in your own cycle the count is expected to be lowest, because that is the window to be most careful in.
Is there medicine that raises white blood cell counts?
NCI notes that medicine may sometimes be given to help prevent infection or to increase the number of white blood cells. ACS describes these as growth factors, also called granulocyte colony-stimulating factors or G-CSFs, usually given as a shot.
Does anything besides treatment affect my infection risk?
Yes. Alongside the cancer and its treatment, NCI notes that stress, poor nutrition and not enough sleep can also weaken the immune system.
Questions to ask your doctor
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Written by: Cancer ExplainedSources last checked: 2026-08-13 what this meansLast updated: 2026-08-20Next planned review: 2027-01-28
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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