What to Expect from Blood Tests During Treatment

If you are in treatment, blood tests probably feel like part of nearly every visit. Lab tests of blood, urine, and other fluids help diagnose and monitor cancer. Knowing what each one checks can make the results less confusing and less worrying.

A complete blood count, or CBC, measures red blood cells, white blood cells, and platelets. It also reports hemoglobin and hematocrit. A CBC can help diagnose some cancers, especially leukemias. It is also used to monitor your health during and after treatment. Chemotherapy and other treatments affect the cells your bone marrow makes, so your care team may check your CBC often.

A blood chemistry test measures substances that your organs and tissues release into the blood. It shows how well your kidneys, liver, and other organs are working. A high or low level can point to disease. It can also point to a side effect of treatment. That is one reason your care team may order this test throughout your course of care.

Tumor marker tests measure substances made by cancer cells. They also measure substances the body makes in response to cancer. These tests can show how well treatment is working. They can also help your care team watch for signs that a cancer has returned. Not every cancer has a well-established tumor marker, so whether this test applies to you depends on your diagnosis.

During chemotherapy, white blood cell counts can drop at certain points. Blood tests check for neutropenia, a drop in one kind of white blood cell. Regular testing gives your care team a moving picture of how your body is handling treatment, not a single snapshot.

Results often appear in your patient portal before your doctor has had a chance to talk them through with you. Seeing a number outside the "normal" range can be alarming in the moment. A few things help:

• Normal ranges vary from person to person, and even from day to day in the same person.
• Results are reported as a range, not one fixed number everyone should match.
• One result on its own cannot confirm cancer or any other problem.

Your doctor reads your results alongside your exam, your history, and your other tests. If a result worries you before your appointment, write down your questions. Bring them to your care team rather than trying to read the number on your own.

Ask which blood tests you can expect to have regularly. Ask what each one is checking for in your case. Ask how your team wants you to handle results that appear in the portal before your next conversation. Having that groundwork in place makes routine lab work feel far less unsettling.

A low count only matters through how you feel. Do not wait for the next draw.

• Call 911 or go to an emergency department if a fever comes with confusion, a racing pulse, breathlessness, or bleeding you cannot stop with pressure.
• Phone your treatment team the moment it happens, day or night, if your temperature reaches 100.4°F (38°C) or higher, or you get hard chills, while you are having treatment. With a low white count a fever is a medical emergency on its own, even if nothing else feels wrong. Go to an emergency department if the team cannot be reached quickly.
• Call your care team the same day if you get a sore throat or stinging when you pass urine.
• Call your care team within a day or two if bruises appear without knocks, your gums ooze when you brush, or you are newly breathless climbing stairs.

Sources: National Cancer Institute — Infection and Neutropenia During Cancer Treatment; CDC — Fever During Chemotherapy.