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Sickle Cell Trait (Sickle Cell Carrier) and Sickle Cell Screening Tests

Sickle Cell Trait (Sickle Cell Carrier) and Sickle Cell Screening Tests

Sickle cell trait means that you carry a gene for a serious condition called sickle cell disease (SCD). People with sickle cell trait are well, and will usually only know about their trait if they are tested for it. Pregnant women and couples planning children may want to know whether they have sickle cell trait, because if both parents have it, their child might inherit SCD. In England, tests for sickle cell trait and SCD are offered to pregnant women and newborn babies.

Sickle cell trait means having one gene for a condition called sickle cell disease (SCD). This in itself does not normally cause problems and sickle cell trait is not considered as a disease. It is extremely rare for it to cause problems or complications, which mainly occur under conditions of severe physical stress (explained below).

Sickle cell trait is important because your children can inherit the sickle cell gene. If BOTH parents have sickle cell trait, their children could get a double dose of the sickle cell gene, which would give them the serious condition called SCD. This is relevant if you are pregnant or wanting to start a family.

What are sickle cell anaemia and SCD?

Sickle cell anaemia (SCA) is a serious condition which is inherited (genetic). It affects the red blood cells in the blood. With SCA, the red blood cells have a tendency to go out of shape and become sickle-shaped (like a crescent moon) - instead of their normal doughnut/disc shape. This can cause various problems such as episodes of pain, infections and various complications. In between the episodes of illness, people with SCA feel well.

SCD is a group of conditions that cause red cells to become sickle-shaped. By far the most common is SCA. Indeed, the terms sickle cell anaemia and sickle cell disease are often used interchangeably. However, there are various other disorders that are classified as subtypes of SCD with each having a slightly different genetic makeup. Examples are sickle cell/beta thalassaemia and sickle cell/HbC. The symptoms, diagnosis and treatment of these conditions are similar to SCA. See separate leaflet called Sickle Cell Disease and Sickle Cell Anaemia for more details.

Anyone can have sickle cell trait, but it is most common in people whose family origin is Black African, Black Caribbean or Black British. It also occurs in people who originate from the Middle East, India and Eastern Mediterranean areas. In other populations, sickle cell trait is unusual but can occur.

In England, about 240,000 people carry a sickle cell gene. About 1 in 2,400 babies born have SCD, but this is an average figure. Some urban areas such as London have much higher rates - for example:

  • 6 in 100 pregnant women carrying a gene for a haemoglobin disorder (mostly sickle cell trait).
  • 1 in 300 newborn babies having SCD.

You inherit haemoglobin genes from both parents. One sickle cell gene gives you sickle cell trait; two sickle cell genes give you SCD.

sickle cell inheritance

This means that: if both parents have sickle cell trait, their children have: a 1 in 2 chance of having sickle cell trait, a 1 in 4 chance of having SCD, and a 1 in 4 chance of having no sickle genes (as shown in the diagram above).

If one parent has SCD and the other has sickle cell trait, their children have: a 1 in 2 chance of having sickle cell trait, and a 1 in 2 chance of having SCD.

If one parent has sickle cell trait and the other has no sickle genes, their children have: a 1 in 2 chance of having sickle cell trait, and a 1 in 2 chance of having normal haemoglobin genes.

Actually, the inheritance of SCD is slightly more complicated than that. This is because there are other haemoglobin genes which can interact with sickle cell trait. If you have one of these genes plus a sickle gene, you also get SCD (the combination behaves like two sickle cell genes). Examples of these interacting genes are HbC, beta thalassaemia, Hb Lepore and HbO Arab.

The result of these genes is that an important body chemical called haemoglobin is altered. This is what affects the red blood cells and makes them change into a sickle shape. With sickle cell trait, you have only small amounts of altered haemoglobin, so the blood cells are not much affected (unless conditions are very extreme). With sickle cell disease, there is more altered haemoglobin and the red blood cells easily change into sickle shape, which causes various problems (as mentioned above).

The diagnosis is made from a blood test. The blood sample is analysed to see what type of haemoglobin is present in the blood. (This is done using tests such as electrophoresis or other methods.) This can diagnose most cases of sickle cell trait and SCD. Sometimes, the result is unclear and extra tests such as DNA (genetic) tests are needed.

Policies for screening pregnant women and babies vary throughout the UK - see the UK Screening Portal link below.

Tests for pregnant women

If the result shows that you carry a sickle cell gene then a test is also offered to the baby's father (if possible). The results of both parents' tests will help to decide whether your baby could be affected by sickle cell disease (SCD). The results will be explained to you.

If there is a chance that the baby could inherit SCD, you will be offered counselling to discuss whether you would like to have a further test for the unborn baby (a prenatal test). A prenatal test finds out whether the unborn baby actually has SCD. If so, you will be offered counselling to discuss how this could affect the baby, and whether you want to continue with the pregnancy.

Would SCD make my baby ill during pregnancy?
No; it does not affect the baby while in the womb. Symptoms start from around age 3 months, and treatment for SCD should begin by this age.

When is the best time to have a test?
If you are pregnant, the ideal time to have a sickle cell blood test for yourself is before you are 10 weeks pregnant. This allows more time to test your partner or your baby, if needed. You can ask your doctor for a test early in pregnancy if it is not already offered to you at that time. However, tests can still be done at a later stage.

A prenatal test (on the unborn baby) can be done from 10 weeks of pregnancy onwards, depending on the type of test used. The usual tests offered are chorionic villus sampling (CVS) or amniocentesis.

Should I and my partner have tests before starting a family?

Women or couples may want to be tested for sickle cell trait before starting a family, especially if their family origins make sickle cell trait more likely. The UK's Sickle Cell Society and many health professionals encourage awareness of sickle cell trait and early testing. You can ask your doctor for a sickle cell test.

The advantage of having tests before you become pregnant is that you will know whether or not there is a possibility that your baby could inherit SCD. This may be helpful when making decisions about pregnancy. For example, you may want to have a prenatal test during pregnancy if there is a risk of SCD for the baby.

The current recommendations are that women who are undergoing tests for infertility and women who are receiving infertility treatment should be tested for sickle cell trait.

Tests for newborn babies

In the UK, all newborn babies are offered a bloodspot test at 5-8 days after birth. This tests for a number of medical conditions which are considered important because early treatment makes a difference. The test is done by taking a small spot of blood from the baby's heel. The bloodspot test now includes testing for SCD throughout the UK. You will be given the results about six weeks later.

If the baby has sickle cell trait, no action or treatment is needed. If the baby has SCD, the result will be explained. You will be given a clinic appointment to check the diagnosis and to start treatment. Treatment should begin by the time the baby is 3 months old.

It is extremely uncommon for people with sickle cell trait to have any symptoms. Complications of sickle cell trait are not common. Most of the complications which do occur are due to extreme physical conditions, and so are usually preventable. The possible complications are:

Complications of severe low oxygen conditions or severe dehydration

These can occur in conditions where oxygen is extremely low - such as flight in an unpressurised aircraft, very high altitude or problems during a general anaesthetic. Also, if you become severely lacking in fluid in the body (dehydrated).

In these situations, sickle cells can form and can block small blood vessels. This may cause episodes of pain in bones, muscles or the spleen. If in the spleen, it is called splenic infarction and may cause pain in the tummy (abdomen) or chest. These problems may be severe enough to make you very unwell and to need treatment in hospital. They will usually clear up with treatment and when oxygen and hydration levels are back to normal.

Complications of severe physical exercise

Sometimes people can become seriously ill as a result of extreme physical training. This can particularly occur if it is a forced pace, under very hot conditions, if they are unused to the training, or if they do not drink enough fluid. Anyone can become ill under these conditions, but people with sickle cell trait are probably at higher risk.

In these conditions, people with sickle cell trait may get pain episodes or pain in the spleen (as explained above).

How can complications of severe exercise be prevented?
The following are a few recommendations which may be useful to prevent complications occurring:

  • Tell your trainer, sports association, doctor and nurse that you have sickle cell trait, and make sure they are aware of the guidelines.
  • Avoid extremes such as heat, very high altitude, exertion to the point of collapse, or sudden increase in exercise without proper training. You can still take part in sports if you take precautions. There are professional athletes who have sickle cell trait.
  • When exercising, drink enough fluid. Have breaks for drinks, rest and cooling.
  • Sports where you can pace yourself are safer than a forced pace.
  • If you have symptoms such as pain or difficulty breathing, stop, report your symptoms, rest and rehydrate. Get medical advice if symptoms don't improve quickly.
  • Build up your training gradually. If going to a high altitude, take time to acclimatise.
  • If you also have asthma, make sure it is treated and well controlled.
  • Extreme activities (such as flying in unpressurised aircraft or visiting very high altitudes) may not be suitable - you will need medical advice.
  • As with any condition affecting the blood, it is healthier not to smoke.

Other complications

There are some other conditions which seem slightly more common in people with sickle cell trait. However, there is still uncertainty as to how far these conditions are linked to sickle cell trait. Possibly, you may be more prone (compared with people without the trait) to:

  • Blood in the urine (haematuria) - this may be noticeable, or in a tiny quantity found only on urine tests. It may affect about 1 in 50 people with sickle cell trait. If you have blood in the urine, it should be investigated (tests done) to see if there is any particular cause. Treatment is needed in some cases.
  • Bladder or kidney infections (urinary tract infections).
  • A blood clot in the leg or lung (deep vein thrombosis (DVT) or pulmonary embolus).

To help prevent these problems, advice is to avoid dehydration (drink enough fluid). To prevent a DVT or a pulmonary embolus, keep mobile and move your legs when on a long journey or flight. See separate leaflets called Deep Vein Thrombosis, Preventing DVT When You Travel and Pulmonary Embolism for more details.

Further reading & references

  • Sickle cell disease; NICE CKS, November 2010 (UK access only)
  • Tsaras G, Owusu-Ansah A, Boateng FO, et al; Complications associated with sickle cell trait: a brief narrative review. Am J Med. 2009 Jun;122(6):507-12. Epub 2009 Apr 24.
  • Scheinin L, Wetli CV; Sudden death and sickle cell trait: medicolegal considerations and implications. Am J Forensic Med Pathol. 2009 Jun;30(2):204-8.
  • Eichner ER; Sickle cell trait in sports. Curr Sports Med Rep. 2010 Nov-Dec;9(6):347-51.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Naomi Hartree
Current Version:
Dr Colin Tidy
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
9211 (v5)
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