Sickle cell trait means you carry one copy of the sickle cell gene and one normal copy. It is not the same as sickle cell disease. Most people with sickle cell trait live completely normal lives and never develop symptoms, but the trait does carry a few specific health risks worth knowing about, especially during intense physical activity or when planning a family.
How Sickle Cell Trait Differs From Sickle Cell Disease
Your red blood cells contain hemoglobin, a protein that carries oxygen throughout your body. People with sickle cell trait produce both normal hemoglobin (called hemoglobin A) and a variant form (hemoglobin S). Because you still make plenty of normal hemoglobin, your red blood cells function well under everyday conditions.
Sickle cell disease is a different situation entirely. People with sickle cell disease inherited two copies of the sickle gene, one from each parent, so their red blood cells produce mostly hemoglobin S. This causes the cells to become hard, sticky, and curved into a C-shape (the “sickle” shape). Those misshapen cells can block blood flow and cause severe pain episodes, organ damage, and other serious complications. Sickle cell trait does not cause these pain crises.
Who Carries It
About 1 in 13 Black or African American babies in the United States is born with sickle cell trait. The trait is also common among people whose ancestors come from sub-Saharan Africa, the Mediterranean, the Middle East, and parts of India. This geographic pattern exists because carrying one copy of the sickle gene offers some protection against malaria, so the gene persisted in populations where malaria was historically widespread.
How It’s Diagnosed
A simple blood test can detect sickle cell trait. The test checks whether your body produces hemoglobin S and how much. The most common laboratory methods are high-performance liquid chromatography, capillary electrophoresis, and isoelectric focusing. In the United States, newborn screening programs test all babies at birth, so most people find out early in life. If you were born before universal screening or in a country without it, you can request the blood test at any time. Genetic testing can also confirm whether you carry one or two copies of the sickle gene.
The NCAA requires all student-athletes to either provide documented results from a previous sickle cell test or undergo testing during their preparticipation medical exam.
Health Risks to Know About
For most daily activities, sickle cell trait causes no problems. But under extreme physical stress, the hemoglobin S in your blood can cause some red blood cells to sickle, and that creates real risks in specific situations.
Intense Exertion
The most well-documented danger is exertional rhabdomyolysis, a condition where muscle tissue breaks down rapidly during extreme exercise. This has been reported during high-intensity military training and elite college-level football training, often in combination with severe heat and dehydration. In rare cases, the muscle breakdown triggers a cascade of kidney failure and dangerous heart rhythms that can be fatal. These deaths are preventable with proper hydration, gradual conditioning, and rest breaks during intense activity.
High Altitude
At high elevations, lower oxygen levels can trigger sickling of red blood cells even in people who only carry the trait. This can cause splenic infarction, where blood flow to the spleen gets blocked. It typically causes sudden, severe pain in the left upper abdomen. Unpressurized airplane flights and rapid ascents above 8,000 to 10,000 feet carry the highest risk.
Kidney Health
Sickle cell trait has a specific connection to kidney problems. About 2.5% of hospitalized patients with the trait experience blood in the urine (hematuria), caused by sickling in the small blood vessels of the inner kidney where oxygen levels are naturally low. In most cases this resolves on its own, but it should never be ignored. Blood in the urine can occasionally be the first sign of renal medullary carcinoma, a rare kidney cancer that occurs almost exclusively in people with sickle cell trait or disease. This cancer is estimated to affect roughly 1 in 20,000 people with the trait, presents at a median age of 22, and is more common in males. Typical symptoms include blood in the urine, flank pain, and unexplained weight loss or fatigue.
Blood Clots
Sickle cell trait is also associated with a higher risk of pulmonary emboli, which are blood clots that travel to the lungs. This risk is worth mentioning to your healthcare provider before surgeries or long periods of immobility.
Exercise and Athletics
Having sickle cell trait does not mean you need to avoid exercise or sports. Many elite athletes carry the trait and compete at the highest levels. The key is understanding your limits and taking precautions during extreme exertion. Staying well hydrated, building up training intensity gradually, taking rest breaks, and avoiding training through warning signs like unusual muscle pain, weakness, or cramping can prevent the rare but serious complications associated with the trait. Exercising in extreme heat while dehydrated is the combination most likely to cause problems.
What It Means for Having Children
This is where sickle cell trait matters most for many people. The inheritance math is straightforward:
- One parent has the trait, the other doesn’t carry the gene at all: None of their children will have sickle cell disease. Each child has a 50% chance of inheriting the trait.
- Both parents carry the trait: Each pregnancy has a 25% chance of producing a child with sickle cell disease, a 50% chance of the trait, and a 25% chance of neither.
- One parent has the trait, the other has sickle cell disease: Each child has a 50% chance of sickle cell disease and a 50% chance of the trait.
If you carry sickle cell trait and are planning a family, knowing your partner’s status changes the picture significantly. A simple blood test for both partners gives you the information you need to understand the possibilities. Genetic counselors can walk you through the specifics for your situation, including how other hemoglobin variants like hemoglobin C or beta-thalassemia interact with the sickle gene.