The lab diagnosis of sickle cell anemia rests on identifying hemoglobin S and showing that it has replaced normal adult hemoglobin. In practice that means a screening step (newborn screening or a sickle solubility test), followed by a confirmatory test that separates and measures each hemoglobin type, usually high-performance liquid chromatography (HPLC) or hemoglobin electrophoresis, with DNA testing when results are unclear. A complete blood count and blood smear support the picture by showing chronic hemolytic anemia.
This guide walks through each test, what the results look like, and the pitfalls that can lead to a wrong answer.
Why Sickle Cell Anemia Is Diagnosed in the Lab
Sickle cell anemia is caused by a single change in the HBB gene on chromosome 11. It swaps valine for glutamic acid at position 6 of the beta-globin chain, producing hemoglobin S (HbS). When oxygen levels fall, HbS molecules polymerize into long fibers that distort the red cell into its characteristic sickle shape.
The condition is autosomal recessive. A person with one HbS gene has sickle cell trait and is usually healthy; a person who inherits HbS from both parents (HbSS) has sickle cell anemia. Because symptoms may not appear until several months of age, when fetal hemoglobin falls, the laboratory is the only reliable way to diagnose it early. The HbS gene is most common in people with ancestry from sub-Saharan Africa, the Mediterranean, the Middle East, and India, reflecting the origins of sickle cell disease in regions where malaria was widespread.
Screening Tests
Newborn screening
Many countries screen every newborn using a heel-prick blood spot analyzed by HPLC or isoelectric focusing (IEF). At birth, fetal hemoglobin (HbF) dominates, so a baby with sickle cell anemia typically shows an “FS” pattern: HbF and HbS with no HbA. A baby with sickle cell trait shows “FAS”. Abnormal screens are always repeated on a fresh sample to confirm.
Sickle solubility test
The sickle solubility test mixes blood with a reducing agent in a buffer. HbS becomes insoluble and turns the solution cloudy, while normal hemoglobin stays clear. It is quick and cheap, but it has important limits:
- It is positive in both sickle cell trait and sickle cell anemia, so it cannot tell them apart.
- It can be falsely negative in infants under about six months, because their high HbF dilutes the HbS.
- Severe anemia and recent transfusion can also give misleading results.
For these reasons a solubility test should never be used on its own to diagnose sickle cell anemia or to guide genetic counseling.
Confirmatory Testing: Measuring Each Hemoglobin
Hemoglobin electrophoresis separates hemoglobins by their electrical charge. Alkaline electrophoresis (pH around 8.6) is done first; because HbS shares a position with HbD and HbG, and HbC with HbE and HbO, an acid (citrate agar) run is used to separate these look-alikes. Many laboratories now use HPLC as the main confirmatory method, since it separates variants and gives precise percentages of HbA2 and HbF in one run.
The table shows the typical adult patterns. Exact percentages vary between people and with treatment such as hydroxyurea.
| Condition | HbA | HbS | Other key findings |
|---|---|---|---|
| Normal adult | About 95–98% | None | HbA2 about 2–3.5%; HbF under 1–2% |
| Sickle cell trait (HbAS) | Majority | Less than half | Normal blood count |
| Sickle cell anemia (HbSS) | None | Predominant | HbF variable; HbA2 normal or slightly raised |
| Sickle-beta zero thalassemia | None | Predominant | HbA2 raised; small red cells (low MCV) |
| Sickle-beta plus thalassemia | Present but less than HbS | Majority | HbA2 raised; low MCV |
| Hemoglobin SC disease | None | About half | HbC making up the other half |
Distinguishing HbSS from sickle-beta zero thalassemia matters because it changes family counseling. Clues include a low MCV, a raised HbA2, and parental testing, and DNA analysis settles doubtful cases.
DNA analysis
Molecular testing of the HBB gene identifies the exact mutation. It is used when protein results are ambiguous, after a recent transfusion, for rare variants, and for prenatal diagnosis using chorionic villus sampling or amniocentesis.
Supporting Blood Tests
A diagnosis of sickle cell anemia is also reflected in routine tests, which help assess severity and establish a baseline for each patient.
- Complete blood count: hemoglobin is typically low, often in the range of about 6–9 g/dL at steady state, with normal-sized red cells in HbSS.
- Reticulocyte count: raised, as the marrow works hard to replace cells destroyed early.
- Blood smear: sickle cells, target cells, and, once the spleen stops working, Howell-Jolly bodies.
- Hemolysis markers: raised lactate dehydrogenase (LDH) and unconjugated bilirubin, with low haptoglobin.
A normal red cell survives about 120 days, while a sickle red cell survives only a fraction of that. This shortened lifespan explains the chronic anemia and the jaundice many patients notice.
Common Pitfalls in Interpreting Results
- Recent transfusion: donor HbA can mask the pattern for up to about three months. Record transfusion history on every request.
- Hydroxyurea: raises HbF, which changes percentages but not the diagnosis.
- Coexisting iron deficiency: lowers HbA2 and can hide beta thalassemia trait in a parent.
- Co-migrating variants: an alkaline electrophoresis band in the “S position” is not proof of HbS without a second method.
Getting these details right is especially important in children with sickle cell anemia, where an early, correct diagnosis allows penicillin prophylaxis, vaccinations, and parent education to start in infancy. Our sickle cell guide covers the wider picture of care.
Key Takeaways
- Screening (newborn HPLC or IEF, or a solubility test) raises suspicion; HPLC or electrophoresis confirms it.
- Sickle cell anemia shows predominant HbS with no HbA; trait shows more HbA than HbS.
- A solubility test cannot separate trait from disease and can miss HbS in young infants.
- DNA testing resolves unclear cases and enables prenatal diagnosis.
- Always interpret results alongside the blood count, transfusion history, and family studies.
Frequently Asked Questions
What is the best test to diagnose sickle cell anemia?
HPLC or hemoglobin electrophoresis is the standard confirmatory test, because it identifies and measures each type of hemoglobin. DNA testing is added when those results are unclear. A solubility test alone is not enough.
Can a blood test tell sickle cell trait from sickle cell anemia?
Yes. HPLC or electrophoresis shows that a person with trait still makes more normal HbA than HbS, while a person with sickle cell anemia makes no HbA. The solubility test cannot make this distinction.
How early can sickle cell anemia be detected?
It can be detected before birth through DNA testing of fetal cells, and at birth through newborn screening. An abnormal newborn result is confirmed on a repeat sample in the first months of life.
Does a recent blood transfusion affect the test?
Yes. Transfused red cells contain normal HbA, which can make sickle cell anemia look like trait. Hemoglobin testing is best delayed until about three months after transfusion, or DNA testing is used instead.
