Most people with sickle cell trait have no symptoms and live normal, full-length lives (NHLBI). The trait means you carry one sickle cell gene and one normal gene; it is not a mild form of sickle cell disease and never turns into it (NHLBI). About 1 in 13 Black babies born in the United States has the trait (NHLBI). The rare problems that do happen show up under strain: very hard exercise, dehydration, extreme heat or high altitude. A few others are worth knowing by name: blood in the urine, a higher risk of kidney disease and of blood clots in the lungs, and an A1C diabetes test that can read lower than your true blood sugar.
What sickle cell trait is, and how you get it
Sickle cell trait means one copy of the sickle cell gene from one parent and a normal hemoglobin gene from the other. People with it are sometimes called carriers. More than 2 million people in the United States have it (NHLBI). In 2010 newborn screening data, it occurred in 73.1 per 1,000 Black newborns, and every state and every racial and ethnic group had babies with the trait (Ojodu et al., 2014).
Inheritance is the part that matters most for families. If both parents have the trait, each pregnancy has a 1 in 4 chance of a child with sickle cell disease, a 1 in 2 chance of a child with the trait, and a 1 in 4 chance of neither (CDC). A simple blood test tells you your status (CDC). Almost all US newborns are screened, so a child's results are usually on record; adults can ask their provider for a test, especially before or during a pregnancy (NHLBI). See our guides to newborn screening and sickle cell trait, pregnancy and family planning.
Symptoms: what carriers can actually feel
Usually nothing. The National Heart, Lung, and Blood Institute says most people with the trait have no symptoms and that complications are rare (NHLBI). The CDC adds that some people with the trait do experience complications of sickle cell disease, such as pain crises, and that the conditions most likely to cause trouble, in extreme forms and rare cases, are (CDC):
- Higher air pressure, as in scuba diving.
- Low oxygen, as in mountain climbing or exercising extremely hard in military boot camp or athletic training.
- Dehydration.
- High altitude, including flying or visiting a city at high elevation.
NHLBI adds extreme heat or cold that pushes body temperature abnormally high or low (NHLBI).
The rare complications worth knowing by name
Blood in the urine (hematuria). The CDC says some people with the trait have blood in their urine, that it can be a sign of a serious condition and always needs a full medical evaluation, and that dehydration or intense exercise may play a part (CDC). In very rare cases it is linked to renal medullary carcinoma, an aggressive kidney cancer. The National Cancer Institute says it is most common in males, in African Americans and in teens and young adults, that people with the trait or the disease are at higher risk, and that blood in the urine is its most common sign, along with a lump in the abdomen, fever and night sweats, flank pain, fatigue and weight loss (NCI). It is very rare. Blood in the urine is still never something to ignore.
Kidney disease. A 2018 systematic review in the Annals of Internal Medicine found high-strength evidence that the trait raises the risk of chronic kidney disease and protein in the urine (Naik et al., 2018). In a JAMA analysis of five large US studies of 15,975 African Americans, chronic kidney disease was found in 19.2% of carriers against 13.5% of noncarriers (Naik et al., 2014). Ask your doctor whether your kidney checkups should include a urine albumin test along with eGFR. Our guide to kidney disease and APOL1 in Black adults covers the other inherited kidney risk many Black families carry.
Community health centers run A1C and blood sugar panels on a sliding scale that is often $0, and screening for prediabetes is a routine part of an annual physical if you have insurance. Our directory of free and charitable clinics lists centers by state. An at-home A1C kit is self-pay and is not billed to Medicaid, so the clinic route usually costs less. If you would rather test at home:
Everlywell
Check your A1C at home.
$49, FSA/HSA eligible
Order the A1C testBlood clots in the lungs. The same review found high-strength evidence of a higher risk of pulmonary embolism, while noting that the absolute risk is small and that the trait was not linked to deep vein thrombosis alone, heart failure or stroke (Naik et al.). In a 22-year study of middle-aged African Americans, carriers had about twice the risk of pulmonary embolism (Folsom et al., 2015). Know the warning signs in our pulmonary embolism guide, and mention the trait before surgery or long periods of immobility.
Spleen damage. A splenic infarct, death of tissue in the spleen, is rare but more likely at high altitude, with severe dehydration or very intense activity. The CDC's advice is to drink plenty of water, build up exercise slowly and take frequent rest breaks, especially at altitude (CDC).
Eye injuries. After an eye injury, people with the trait are more likely to have bleeding inside the eye followed by a rise in eye pressure, which can blur or damage vision. The CDC says anyone with the trait who injures an eye should see an eye specialist right away (CDC).
Exercise, sports and the military: what the evidence says
Here the official sources do not say exactly the same thing, so it is worth being precise. The CDC says some people with the trait are more likely to have heat stroke and muscle breakdown during intense exercise in harsh conditions (CDC). NHLBI says more recent studies in athletes and military recruits have not linked the trait to injuries in extreme heat or heavy exercise (NHLBI).
The largest study helps square them. Among 47,944 Black US Army soldiers on active duty, all under the Army's exertional-injury precautions, the trait was not linked to a higher risk of death, but it was linked to a 54% higher risk of exertional rhabdomyolysis, a breakdown of muscle during exertion, an effect similar in size to smoking and smaller than that of statins or antipsychotic medicines (Nelson et al., 2016). The systematic review rated the evidence for this risk as moderate and the absolute risk as small (Naik et al.).
The practical advice is the same either way. The CDC's tips for athletes with the trait (CDC):
- Set your own pace and build intensity slowly.
- Rest often between repeated sets and drills.
- Drink plenty of water before, during and after training.
- Stay cool in heat and humidity, with misting or air-conditioned breaks.
- Get medical care right away if you feel ill.
Know the signs of rhabdomyolysis that MedlinePlus lists: dark, red or cola-colored urine, passing less urine, general weakness, and muscle stiffness, aching or tenderness (MedlinePlus). Any of these after a workout is a reason to stop and get checked. Our guide to sickle cell trait screening for young athletes covers school and college sports.
Your A1C may read low
The A1C test is the standard check for diabetes. In a JAMA study of 4,620 African Americans, people with the trait had A1C values about 0.3 percentage points lower than people without it at the same blood sugar level, and fewer were classed as having prediabetes or diabetes when A1C was used (Lacy et al., 2017). The authors' point is that A1C may underestimate blood sugar in people with the trait. If you have the trait, tell your doctor, and ask whether fasting glucose or another test should be used alongside A1C. See our A1C chart for Black adults.
Living with the trait
Most of daily life is unaffected. The trait does not shorten life, and it never turns into sickle cell disease (NHLBI). People with the trait can donate blood, organs and tissue (CDC). The habits that matter are simple: stay hydrated, respect heat and altitude, build up exercise gradually, and tell doctors you have the trait so they can factor it into tests like A1C, blood in the urine and surgery planning.
If you are not sure of your status, ask for a hemoglobin test. If both you and your partner have the trait, a genetic counselor can walk through the 1-in-4 odds before or during a pregnancy. For the disease itself, see sickle cell disease, and for a quick definition, our glossary entry.
When to see a doctor
See a doctor promptly for blood in your urine, even once (CDC). Get emergency care for the rhabdomyolysis signs above, for collapse or confusion in the heat, and for sudden chest pain or shortness of breath, which can be a pulmonary embolism (see our pulmonary embolism guide). After any eye injury, see an eye specialist the same day (CDC).
Frequently asked questions
What are the symptoms of sickle cell trait? ▼
Usually none. Most people with the trait never have symptoms. Rare problems, such as muscle breakdown, spleen damage or blood in the urine, tend to happen with extreme exertion, dehydration, heat or high altitude.
Can sickle cell trait turn into sickle cell disease? ▼
No. The trait means one sickle cell gene and one normal gene, and it never becomes the disease. If both parents have the trait, each child has a 1 in 4 chance of sickle cell disease.
How common is sickle cell trait in Black people? ▼
About 1 in 13 Black babies born in the United States has the trait, and more than 2 million people in the US carry it.
Does sickle cell trait affect A1C? ▼
It can. In a large study, A1C values were about 0.3 points lower in people with the trait at the same blood sugar, so A1C may underestimate blood sugar. Tell your doctor you have the trait.
Can I play sports with sickle cell trait? ▼
Yes. Build intensity slowly, rest between drills, drink plenty of water, stay cool in heat and humidity, and stop and get care if you feel ill or have severe muscle pain or dark urine.
