Donated blood is not tested for cancer. Blood banks screen every donation for a specific set of infectious diseases, including HIV, hepatitis B, hepatitis C, syphilis, West Nile virus, and a handful of others, but cancer is not on that list. The reason comes down to both biology and technology: cancer cells are extraordinarily rare in blood, fragile, and nearly impossible to detect with any routine screening method.
What Donated Blood Is Actually Tested For
Every unit of donated blood in the United States goes through a panel of infectious disease tests mandated by the FDA. The CDC lists the standard screenings as hepatitis B, hepatitis C, HIV types 1 and 2, human T-lymphotropic virus (HTLV), syphilis, West Nile virus, and Chagas disease. For some of these, multiple testing methods are used on the same sample to catch infections at different stages. Chagas disease testing, for example, is required for all first-time donors.
These tests are designed to catch pathogens that can survive in stored blood and infect a recipient. Cancer doesn’t fit that profile, which is why it isn’t part of the panel.
Why There’s No Blood Test for Cancer in Donations
Even in a person with metastatic cancer, circulating tumor cells are vanishingly rare. A typical blood sample from someone with advanced disease contains roughly 1 to 10 tumor cells per milliliter, compared to billions of red blood cells, white blood cells, and platelets in the same volume. Picking out those few cancer cells is like finding a handful of specific grains of sand on a beach.
Those cells are also fragile. Less than 1% of tumor cells shed into the bloodstream survive beyond 24 hours. Their half-life is estimated at just 1 to 2.4 hours. Shear stress from blood flow and the immune system destroy most of them almost immediately. In lab studies, when patient blood samples were stored for six days at room temperature without special preservation chemicals, researchers couldn’t find any viable tumor cells on the filters at all.
From a technical standpoint, current methods for detecting circulating tumor cells require specialized equipment and still have low sensitivity because of how few cells are present. Tumor cells are also heterogeneous, meaning they don’t all express the same surface markers, which makes a single catch-all test impractical. There is no quick, reliable, cost-effective screening method that could be applied to the millions of blood donations processed every year.
How Cancer Is Kept Out of the Blood Supply
Instead of lab testing, blood banks rely on donor screening before collection. Every potential donor fills out a health history questionnaire that directly asks: “Have you ever had cancer, blood disease, or a bleeding problem?” People who answer yes are evaluated further. A survey of blood collection centers found that 76% permanently defer donors with a history of leukemia or lymphoma, and 86% permanently defer those with myelodysplastic or myeloproliferative disorders. Policies for solid tumors (breast, colon, prostate, and similar cancers) vary by center, but most require that the donor be cancer-free for a defined period before they can give blood.
An additional layer of protection comes from a processing step called leukoreduction, which filters white blood cells out of donated blood before transfusion. This step exists primarily to reduce immune reactions in the recipient, but research shows that leukocyte depletion filters also significantly reduce the number of tumor-marker-positive cells in blood products. It’s not designed as a cancer filter, but it functions as an incidental safety net.
Can You Get Cancer From a Blood Transfusion?
This is the concern behind the question, and the evidence is reassuring. A large U.S. population study examining the relationship between blood transfusion history and cancer risk found no concrete association between receiving a transfusion and developing cancer afterward. Some researchers have explored a concept called transfusion-related immune modulation, where transfused blood may temporarily suppress certain immune functions like natural killer cell activity. In theory, this could create a brief window of reduced immune surveillance. But the clinical data hasn’t shown this translates into meaningful cancer risk for transfusion recipients.
Where studies have found small upticks in specific cancers after transfusion, such as lymphoma or liver cancer, the increased risk was attributed to carcinogenic infectious agents like hepatitis C or HIV that may have been present in older blood supplies before modern screening existed. Today’s testing catches HIV and hepatitis C at a rate of roughly one missed infection per two million units transfused.
When Donations Reveal Health Problems
While blood banks don’t test for cancer, the routine processing of donations occasionally flags abnormalities that lead to a diagnosis. The American Red Cross has documented cases where donors were notified after their blood showed an unusually high white blood cell count. In one well-known case, a Michigan donor named Hoffman received a letter from the Red Cross within days of her donation, recommending she see her doctor. The abnormality turned out to be leukemia. As one Red Cross medical director put it, blood donation serves as a form of health screening, even though that isn’t its primary purpose. If any test result comes back abnormal or positive for an infectious disease, the donation is discarded and the donor is notified.
This kind of incidental detection is uncommon, but it highlights an important point: the system is built to protect recipients from infectious disease, not to screen donors for cancer. The biological reality that cancer cells are rare, short-lived, and difficult to detect in blood means that the pre-donation questionnaire and processing filters are the main safeguards, and for the vast majority of transfusions, they work.