People need blood transfusions when their body can’t make enough blood on its own, loses too much too quickly, or is missing a specific component that keeps blood functioning properly. The reasons range from emergency trauma and major surgery to chronic conditions like sickle cell disease and thalassemia that require regular transfusions for years or even a lifetime.
Severe Bleeding From Trauma or Injury
The most urgent reason for a blood transfusion is rapid, uncontrolled blood loss. Car accidents, falls, stabbings, gunshot wounds, and other major injuries can cause someone to lose liters of blood in minutes. The body holds roughly 5 liters total, and losing more than about 40% without replacement is life-threatening. In emergency departments, roughly 5% of trauma patients receive blood before doctors even have time to check their blood type. Of those, about 17% need what’s called a massive transfusion, meaning they receive 10 or more units of red blood cells within 24 hours.
Speed matters more than precision in these situations. Hospitals stock universal-donor blood (type O negative) so transfusions can start immediately while lab work catches up. The goal is simple: keep enough oxygen-carrying red blood cells circulating to prevent organ failure.
Blood Loss During Surgery
Planned surgeries are a common reason people receive blood. Hip fracture repairs have some of the highest transfusion rates in orthopedic surgery, with about 25% of patients needing red blood cells. Amputations are even higher, at roughly 33%. Elective procedures like knee and hip replacements carry much lower risk, with transfusion rates of about 2.8% and 4.5% respectively. When patients do need blood during these surgeries, they typically receive around 3 units.
Heart surgery, liver transplants, and complex cancer operations also carry significant transfusion risk because they involve large blood vessels or organs with rich blood supply. Surgeons plan for this ahead of time, and patients are sometimes asked to donate their own blood in the weeks before a scheduled procedure so it’s available if needed.
Childbirth Complications
Postpartum hemorrhage, defined as losing 1,000 milliliters or more of blood within 24 hours of delivery, is one of the leading reasons new mothers need transfusions. That threshold applies to both vaginal and cesarean deliveries. Some bleeding during childbirth is normal, but when the uterus doesn’t contract properly afterward, or if the placenta tears blood vessels, the loss can escalate quickly. Signs include a rapid heart rate, dropping blood pressure, and dizziness. When fluid replacement alone can’t stabilize the mother, red blood cells are transfused to restore oxygen delivery.
Cancer and Chemotherapy
Cancer patients often need blood for reasons that have nothing to do with bleeding. Chemotherapy kills fast-dividing cells, and bone marrow cells, the ones responsible for making new blood, are among the fastest dividers in the body. This means chemotherapy frequently causes anemia (too few red blood cells) and thrombocytopenia (too few platelets). The resulting fatigue can be severe enough to halt treatment.
For most hospitalized patients, doctors transfuse red blood cells when hemoglobin drops below 7 g/dL, a measure of the blood’s oxygen-carrying capacity. Normal hemoglobin runs between 12 and 17 g/dL depending on age and sex. Cancer patients may receive transfusions at slightly higher levels because the combined burden of disease and treatment makes even mild anemia debilitating. Platelet transfusions follow their own thresholds: patients with certain leukemias may need platelets when counts fall below 30,000 cells per microliter, compared to a normal range of 150,000 to 400,000.
Sickle Cell Disease
Sickle cell disease causes red blood cells to become rigid and crescent-shaped, blocking small blood vessels and starving tissues of oxygen. People with this condition may need transfusions to treat acute crises like stroke, acute chest syndrome (a dangerous lung complication), and severe anemia. Some patients also receive regular transfusions to prevent strokes from recurring.
A specialized procedure called red cell exchange goes a step further. Instead of simply adding healthy blood, it removes the patient’s sickle-shaped cells and replaces them with normal donor cells. This is used before surgeries, during complicated pregnancies, and for serious organ complications. Notably, pain crises alone, without other symptoms, are generally not treated with transfusion.
Thalassemia
Thalassemia is an inherited condition where the body produces abnormal hemoglobin, leading to chronic, severe anemia. People with the most serious form, beta thalassemia major, depend on transfusions for survival. According to CDC guidelines, older children and adults typically need blood every 3 weeks, while younger children and those with milder forms may go every 4 weeks. Adults commonly receive 2 to 4 units per visit, adjusted based on their hemoglobin levels before each session.
This lifelong transfusion schedule creates its own problem: iron overload. Each unit of blood carries iron that the body has no natural way to excrete, so patients also take medication to remove excess iron and protect the heart and liver from damage.
Internal Bleeding From the Digestive Tract
Stomach ulcers, torn blood vessels in the esophagus (common in liver disease), and colon conditions like diverticulosis can all cause significant internal bleeding. Sometimes the bleeding is obvious, showing up as blood in vomit or dark, tarry stools. Other times it’s slow and invisible, gradually draining hemoglobin over days or weeks until the person becomes dangerously anemic.
For patients with gastrointestinal bleeding, doctors use a restrictive transfusion approach. Those with heart disease receive blood when hemoglobin drops below 8 g/dL, while otherwise healthy patients are transfused at 7 g/dL. The key exception is active, unstable bleeding: if blood pressure drops below 100 mm Hg and the heart rate climbs above 100 beats per minute, transfusion may be needed regardless of the hemoglobin reading because lab values haven’t yet caught up with the actual blood loss.
Clotting Problems and Plasma Transfusions
Not every transfusion involves red blood cells. Some people need plasma, the liquid portion of blood that contains clotting proteins. Liver disease is one of the most common reasons because the liver manufactures most of the body’s clotting factors. When the liver fails, even minor cuts or procedures can cause dangerous, prolonged bleeding.
Plasma transfusions are also used for disseminated intravascular coagulation, a condition where the clotting system goes haywire and simultaneously creates tiny clots throughout the body while depleting the clotting factors needed to stop actual bleeding. This can happen during severe infections, certain pregnancy complications, and some cancers. In a rare blood disorder called thrombotic thrombocytopenic purpura, plasma provides a specific enzyme the patient’s body is missing, one that prevents dangerous clot formation in small blood vessels.
How Doctors Decide When to Transfuse
Modern guidelines favor a “restrictive” approach, meaning doctors wait until blood levels drop to specific thresholds rather than transfusing early. For most hospitalized patients, that threshold is a hemoglobin of 7 to 8 g/dL. This approach, endorsed by the AABB’s 2023 international guidelines, reflects decades of research showing that patients do just as well, and sometimes better, with fewer transfusions. Transfusions carry small but real risks including allergic reactions, fever, and in rare cases, infections or immune complications.
For patients with active heart disease, the threshold is slightly more generous, around 8 g/dL, since the heart depends heavily on adequate oxygen delivery. A landmark trial of over 3,500 patients with heart attacks compared restrictive and liberal transfusion strategies, helping refine these cutoffs. The takeaway across all settings is the same: blood is given when the benefit of restoring oxygen delivery clearly outweighs the risks of the transfusion itself.