ConditionBlood & immune system
Sickle Cell Trait
Sickle cell trait means a person has inherited one normal hemoglobin gene and one sickle hemoglobin gene (hemoglobin AS). It is not sickle cell disease and does not cause anemia or pain crises. Most people with the trait live normal, healthy lives. It matters for two reasons: two parents with the trait have a 1 in 4 chance with each pregnancy of having a child with sickle cell disease, and under extreme conditions such as intense exercise, dehydration or very high altitude, rare complications can occur.
Also called: SCT, sickle trait, hemoglobin AS, sickle cell carrier
At a glance
- What it is
- Carrying one sickle hemoglobin gene and one normal gene.
- How common
- About 1 in 13 Black or African American babies in the US. Also common in people with Hispanic, Middle Eastern, South Asian and Mediterranean ancestry.
- Health
- Usually no symptoms. Not a mild form of sickle cell disease.
- Rare risks
- Exertional collapse and muscle breakdown, blood in the urine, kidney concentrating problems, splenic problems at high altitude.
- Key action
- Know your status for family planning, stay hydrated, and train gradually.
When to get help
Call 911 or go to the ER
- During intense exercise: sudden muscle weakness, cramping or collapse, especially without the usual muscle "lock-up" of heat cramps (possible exertional sickling): stop exercising, cool down, call 911
- Severe left upper belly pain at high altitude, during flights in unpressurized aircraft, or mountain climbing (possible splenic infarction)
See a doctor within a day or two
- Dark brown, tea-colored urine after exercise, with muscle pain or weakness (possible rhabdomyolysis)
- Visible blood in the urine
Book a routine appointment
- Planning a pregnancy, to test your partner's status
- Unsure whether you have the trait: a simple blood test can tell you
How sickle cell trait is inherited
Each person inherits one hemoglobin gene from each parent.
| Parent 1 | Parent 2 | Chance for each child |
|---|---|---|
| Trait (AS) | Normal (AA) | 1 in 2 trait, 1 in 2 normal |
| Trait (AS) | Trait (AS) | 1 in 4 normal, 1 in 2 trait, 1 in 4 sickle cell disease (SS) |
| Trait (AS) | Sickle cell disease (SS) | 1 in 2 trait, 1 in 2 sickle cell disease |
| Trait (AS) | Beta thalassemia trait | 1 in 4 chance of sickle beta thalassemia, a form of sickle cell disease |
Other hemoglobin variants, such as hemoglobin C, combined with sickle trait can also cause sickle cell disease. Partners should be tested for all relevant variants.
Why the trait is common
The sickle gene protects carriers against severe malaria. Over thousands of years, people with the trait in malaria-affected regions survived and passed it on, which is why it is common in people with ancestry from sub-Saharan Africa, the Middle East, India and the Mediterranean.
Sickle cell trait vs sickle cell disease
| Sickle cell trait | Sickle cell disease | |
|---|---|---|
| Genes | One sickle, one normal | Two sickle genes, or sickle plus another variant |
| Hemoglobin S in blood | Less than half | Most |
| Anemia | No | Yes, usually |
| Pain crises | No | Yes |
| Life expectancy | Normal | Reduced, though improving with care |
| Treatment needed | None routinely | Lifelong care |
Possible health effects
Most people with sickle cell trait never have problems. Rare complications occur under extreme stress:
Exertional collapse
During very intense exercise, especially with dehydration, heat, altitude or sudden all-out effort early in training, red cells can sickle in working muscles. This can cause muscle breakdown (rhabdomyolysis), dangerously high potassium and collapse. Deaths have occurred in athletes and military recruits. Unlike heat stroke, the person is often alert and the body temperature is not very high at first.
Kidney effects
- reduced ability to concentrate urine (more frequent urination, easier dehydration)
- episodes of blood in the urine, usually from the left kidney
- renal medullary carcinoma: a very rare but aggressive kidney cancer seen almost only in young people with sickle cell trait or disease. Blood in the urine, flank pain or a mass needs prompt investigation
- possibly a higher risk of chronic kidney disease
Other effects
- splenic infarction at high altitude
- a modestly higher risk of blood clots in the veins
- an eye complication (hyphema) can be more serious after eye injury
- may affect the accuracy of HbA1c tests for diabetes in some laboratory methods
Testing
- newborn screening: all US states screen newborns, so many people have a result from birth, though families are not always told
- hemoglobin electrophoresis or HPLC: a blood test that identifies the trait and other variants
- the sickle solubility test detects hemoglobin S but cannot tell trait from disease, so it is not adequate for family planning
- partners should be tested before or early in pregnancy, with genetic counseling if both carry a variant
Living with sickle cell trait
- stay well hydrated, especially in heat, at altitude and during illness
- build up exercise gradually and avoid sudden maximal efforts, timed sprints or "conditioning tests" without acclimatization
- rest and drink fluids if you feel unusually weak, cramped or short of breath during exercise, and tell your coach or trainer
- take care at high altitude, including mountain climbing
- tell your doctor you have the trait, especially if you have blood in your urine
- many athletic organizations screen athletes and use universal precautions in training
Frequently asked questions
What is sickle cell trait?
It means you carry one sickle hemoglobin gene and one normal gene. It is not a disease. Most people with the trait have no symptoms, but they can pass the gene to their children.
Is sickle cell trait the same as sickle cell disease?
No. Sickle cell disease requires two abnormal genes and causes anemia, pain crises and organ damage. Sickle cell trait involves one sickle gene, does not cause anemia or crises, and rarely causes problems.
Can sickle cell trait cause health problems?
Usually not. Rare problems can occur under extreme conditions: collapse during intense exercise, muscle breakdown, blood in the urine, and spleen problems at very high altitude. Staying hydrated and training gradually reduce the risk.
If both parents have sickle cell trait, what are the chances their child will have sickle cell disease?
With each pregnancy, there is a 1 in 4 chance the child will have sickle cell disease, a 1 in 2 chance the child will have the trait, and a 1 in 4 chance the child will have neither.
How do I know if I have sickle cell trait?
A blood test called hemoglobin electrophoresis can tell you. If you were born in the US, your newborn screening result may already show it. Ask your doctor if you are unsure, especially before having children.
Sources
This guide was written from current patient and clinical guidance, including:
- CDC: What you should know about sickle cell trait
- National Heart, Lung, and Blood Institute (NHLBI): Sickle cell disease and sickle cell trait
- American Society of Hematology: Sickle cell trait
- National Collegiate Athletic Association (NCAA) and NATA: Sickle cell trait and the student-athlete
- MedlinePlus Genetics (US National Library of Medicine): Sickle cell disease