Sickle cell test results tell you which types of hemoglobin are in your blood and in what proportions. A normal adult result shows mostly hemoglobin A (HbA). A result showing both HbA and hemoglobin S (HbS), with more A than S, indicates sickle cell trait. A result showing HbS with no HbA points to sickle cell disease, and the exact pattern, along with other hemoglobins, identifies which form of the disease is present.
In this guide I explain which tests are used, how the letters and percentages on a report should be read, and the common pitfalls that can lead to a misleading result.
Sickle Cell Disease in Brief
Sickle cell disease (SCD) is a group of inherited red blood cell disorders caused by a single change in the HBB gene, which codes for the beta-globin chain of hemoglobin. The altered protein, hemoglobin S, sticks together into long polymers when oxygen levels are low.
These polymers distort red cells into the characteristic sickle shape. Sickled cells are rigid, break down early, and block small vessels, causing anemia, pain crises, infections, stroke, and organ damage.
A person who inherits one HbS gene and one normal gene has sickle cell trait. Trait is usually harmless but can be passed on. SCD requires two abnormal beta-globin genes, at least one of which is HbS.
Which Tests Are Used?
Several laboratory methods can detect HbS. They differ in what they can tell you, which is why a screening result often needs confirmation. A fuller overview for clinicians is in our guide to sickle cell testing.
- Solubility test (sickle screen): a quick test that turns cloudy if HbS is present. It cannot distinguish trait from disease and can give false negatives in infants and in severe anemia.
- Hemoglobin electrophoresis: separates hemoglobin types by electrical charge and shows which are present.
- High-performance liquid chromatography (HPLC): the standard method in most laboratories; it identifies and measures each hemoglobin as a percentage.
- Isoelectric focusing: widely used in newborn screening programs.
- DNA testing: confirms the genotype, identifies beta-thalassemia mutations, and is used in prenatal diagnosis.
A complete blood count and blood smear are always reviewed alongside the hemoglobin analysis. Red cell size (MCV) and the smear appearance help distinguish between look-alike patterns.
How to Read Adult and Child Results
After about the first year of life, fetal hemoglobin falls to low levels and the adult pattern emerges. The table below shows typical patterns. Exact percentages vary between laboratories and individuals.
| Result pattern | Typical hemoglobin findings | Interpretation |
|---|---|---|
| AA | HbA about 95–98%, HbA2 about 2–3.5%, HbF under 2% | Normal |
| AS | HbA greater than HbS; HbS usually around 35–40% | Sickle cell trait |
| SS | HbS the large majority, no HbA, HbF variable | Sickle cell anemia (HbSS) |
| SC | HbS and HbC in roughly equal amounts, no HbA | Hemoglobin SC disease |
| S/beta-zero thalassemia | HbS majority, no HbA, raised HbA2, small red cells | Sickle beta-zero thalassemia, usually severe |
| S/beta-plus thalassemia | HbS greater than HbA, raised HbA2 | Sickle beta-plus thalassemia, often milder |
Two points are worth highlighting. In trait, HbA always exceeds HbS; if HbS exceeds HbA, a sickle thalassemia or recent transfusion must be considered. And SS and S/beta-zero thalassemia can look almost identical on HPLC, so red cell indices, family studies, or DNA testing may be needed to tell them apart.
Newborn Screening Results
At birth, most hemoglobin is fetal hemoglobin (HbF), so newborn results are listed with F first, followed by other hemoglobins in order of quantity.
| Newborn result | Likely meaning | Next step |
|---|---|---|
| FA | Normal | No action |
| FAS | Sickle cell trait | Family counseling; confirm if needed |
| FS | Sickle cell anemia or S/beta-zero thalassemia | Urgent confirmation and referral to a sickle cell clinic |
| FSC | Hemoglobin SC disease | Confirmation and specialist follow-up |
| FSA | Likely S/beta-plus thalassemia (S more than A) | Confirmation and specialist follow-up |
Babies with SCD look well at birth because HbF protects them. Symptoms usually begin once HbF falls, which is why early diagnosis allows penicillin prophylaxis and vaccinations to start before serious infections occur.
Common Pitfalls in Interpretation
- Recent transfusion: donor HbA can make SCD look like trait for up to about three months.
- Hydroxyurea treatment: raises HbF, which changes the percentages in a known patient.
- Iron deficiency: can lower HbA2 and mask beta-thalassemia trait.
- Co-inherited alpha-thalassemia: lowers the HbS percentage in trait.
- Solubility test alone: never sufficient to diagnose disease or to reassure a couple planning a pregnancy.
When to ask for a repeat or confirmatory test
In my practice, I ask for confirmation whenever the result does not fit the clinical picture. Examples include an apparent trait result in someone with frequent pain crises, a sickle screen done during a period of heavy transfusion, or an HbS percentage that sits outside the usual trait range.
Couples planning a pregnancy should both have full hemoglobin analysis, not just a solubility screen. Beta-thalassemia trait and hemoglobin C trait do not show up on a sickle screen, yet either can combine with HbS in a child to cause sickle cell disease.
From Results to Management
Confirmed SCD should be managed by a specialist team. Standard care includes infection prevention, stroke screening with transcranial Doppler in children, hydroxyurea to raise HbF and reduce crises, and transfusions for selected complications. Bone marrow transplantation can be curative, and gene therapies are now approved in some countries for eligible patients. Comprehensive care has steadily improved the life span of sickle cell patients.
People with trait generally need no treatment, but should know their status before starting a family and should stay well hydrated during intense exercise or at high altitude. For more on the condition, see our sickle cell guide.
Frequently Asked Questions
What is a normal sickle cell test result?
A normal result shows only adult hemoglobins: mostly HbA with small amounts of HbA2 and HbF, and no HbS. On a newborn report, a normal result is written as FA.
Can sickle cell trait turn into sickle cell disease?
No. Trait and disease are determined by the genes you inherit, which do not change. A person with trait will always have trait, though they can pass the HbS gene to their children.
If both parents have sickle cell trait, what are the chances for their child?
With each pregnancy there is a 1 in 4 chance of sickle cell disease, a 1 in 2 chance of trait, and a 1 in 4 chance of neither. Genetic counseling and prenatal testing are available for couples who both carry the gene.
Why do I need a repeat test if the newborn screen was positive?
Newborn screens are designed to be sensitive, and some patterns look alike at birth when HbF dominates. A confirmatory test, usually in the first months of life, establishes the exact diagnosis so the right care can begin.
