The short answer
Blood tests help diagnose and monitor cancer throughout treatment. A complete blood count (CBC) checks red cells, white cells, and platelets. A chemistry panel shows how organs like the kidneys and liver are working. Tumor marker tests can help show how well treatment is working. Results are reported as a range, and a single result alone cannot confirm cancer.
Lab tests of blood, urine, and other fluids help diagnose and monitor cancer throughout your care.
A complete blood count (CBC) measures red blood cells, white blood cells, platelets, hemoglobin, and hematocrit.
A blood chemistry test shows how well your kidneys, liver, and other organs are working.
Tumor marker tests can help measure how well treatment worked and watch for cancer's return.
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The full explanation.
The short version
If you are in treatment, blood tests probably feel like a routine part of nearly every visit. Lab tests of blood, urine, and other fluids help diagnose and monitor cancer, and understanding what each one is actually checking can make the results less confusing and less worrying.
The complete blood count (CBC)
A complete blood count, or CBC, measures red blood cells, white blood cells, and platelets, along with hemoglobin and hematocrit. It can help diagnose some cancers, especially leukemias, and it is also used to monitor your health during and after treatment. Because chemotherapy and other treatments can affect the cells your bone marrow makes, your care team may check your CBC often to see how your body is responding.
The blood chemistry test
A blood chemistry test measures substances that are released into your blood by your organs and tissues. It shows how well your kidneys, liver, and other organs are working. High or low levels on this test can signal disease, or they can signal a side effect of treatment, which is one reason your care team may order this test regularly throughout your course of care.
Tumor marker tests
Tumor marker tests measure substances made by cancer cells, or substances the body makes in response to cancer. These tests can help measure how well treatment is working and can help your care team watch for signs that cancer has returned. Not every cancer has a well-established tumor marker, so whether this test applies to you depends on your specific diagnosis.
Why blood tests are checked so often
During chemotherapy, white blood cell counts can drop at certain times, so blood tests are used to check for neutropenia — a drop in white blood cells. Regular blood tests give your care team an ongoing picture of how your body is handling treatment, not just a single snapshot.
Making sense of your results
It is common for results to appear in your patient portal before your doctor has had a chance to discuss them with you. Seeing a number outside the "normal" range can be alarming in the moment, but it helps to remember a few things:
- Normal ranges vary from person to person, and even from day to day for the same person.
- Results are reported as a range, not a single fixed number that everyone should match.
- A single result alone cannot confirm cancer or a specific problem. Your doctor looks at your results together with your exam, history, and other tests before drawing conclusions.
If you see a result that concerns you before your appointment, it is reasonable to write down your questions and bring them up when you talk with your care team, rather than trying to interpret the number entirely on your own.
Questions for your team
Ask your care team which blood tests you can expect to have regularly, what each one is checking for in your specific case, and how they would like you to handle results that show up in the portal before your next conversation. Having this groundwork in place can make routine lab work feel far less unsettling.
Words to know
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Common questions
▸What is a complete blood count (CBC)?
A CBC measures red blood cells, white blood cells, and platelets, along with hemoglobin and hematocrit. It can help diagnose some cancers, especially leukemias, and is used to monitor your health during and after treatment.
▸What does a blood chemistry test check?
A blood chemistry test measures substances released into the blood by your organs and tissues, and shows how well your kidneys, liver, and other organs are working. High or low levels can signal disease or treatment side effects.
▸What are tumor marker tests?
Tumor marker tests measure substances made by cancer cells, or made by the body in response to cancer. They can help measure how well treatment worked and help watch for the cancer's return.
▸Why do my results show up in the patient portal before my doctor calls?
Results often appear in the patient portal before the doctor has a chance to discuss them with you. Normal ranges vary from person to person and even day to day, so it can help to wait and talk through what a result means with your care team rather than interpreting it alone.
▸Can one blood test result confirm or rule out cancer?
No. Results are reported as a range, and a single result alone cannot confirm cancer. Your doctor looks at your results together with your exam, history, and other tests.
Questions to ask your doctor
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