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Blood Test Tracker

Sickle Cell Screen (Hemoglobin S Solubility)

A quick check for hemoglobin S, the type of hemoglobin linked to sickle cell trait and sickle cell disease. It can't tell the two apart.

AI-verified against cited sources. Awaiting expert reviewAlso called SCST, HbS screen, Hgb S solubilityLOINC 6864-3

What it is

Hemoglobin is the protein in red blood cells that carries oxygen. Most adults make mainly hemoglobin A. Some people inherit hemoglobin S, which can bend red cells into a sickle shape when oxygen is low. For this test, a drop of your blood is mixed with a chemical solution. Hemoglobin S makes the solution cloudy, while other hemoglobins leave it clear. The result is reported as negative or positive. You may also see it called a sickle prep, a sickle solubility test or Sickledex, which is a brand name.

Why it matters

A positive result tells you that hemoglobin S is present. It does not tell you whether you have sickle cell trait (one copy of the sickle gene, which causes no health problems for most people) or sickle cell disease (two copies, or one with another hemoglobin variant, which is managed by a clinician). A follow-up test, such as hemoglobin electrophoresis or HPLC, tells the two apart. The screen is quick and low cost, so it is used when a fast answer is needed, before surgery, in some sports and military programs, and when checking blood donors. A negative result means the test found no hemoglobin S, although it can miss small amounts.

If it’s high

A positive result means the test found something that behaves like hemoglobin S. That can be sickle cell trait or sickle cell disease. Most people with sickle cell trait have no health problems, but they can pass the sickle gene to their children. A confirmatory test is needed to know which one it is, and to sort out the less common reasons for a false positive. Your clinician can explain what it means for you.

Common causes

  • Sickle cell trait (one copy of the sickle hemoglobin gene)
  • Sickle cell disease, including combinations of hemoglobin S with another variant
  • Other hemoglobin variants or lab conditions that can give a false positive, which is why labs confirm positive results

What can help

  • Ask about a confirmatory test, such as hemoglobin electrophoresis or HPLC
  • Ask whether genetic counseling would be useful, especially if you are planning a pregnancy
  • Share the result with your care team, including anyone planning surgery or anesthesia for you
  • If it turns out you have sickle cell trait, ask your clinician what to know about staying hydrated and about intense exertion, extreme heat or high altitude

What can affect your result

  • Age: babies under about 6 months have a high share of fetal hemoglobin, which can hide hemoglobin S
  • Recent blood transfusion or red cell exchange
  • How much hemoglobin S your blood contains: the test can miss small amounts
  • Severe anemia
  • The kit and method your lab uses
  • Other hemoglobin variants, which can cause false positives
  • A recent blood transfusion or red cell exchange adds donor red cells that do not contain hemoglobin S, and can make a positive result read negative. In one adult clinic study, the false-negative tests came from people with sickle cell disease who had recently had red cell exchange or heavy transfusion.
  • Babies under about 6 months have a lot of fetal hemoglobin, which dilutes hemoglobin S and can give a false negative. For this reason the test is not used for newborn screening.
  • Severe anemia, or a low share of hemoglobin S in the blood, can give a false negative. In the same clinic study, every false negative had hemoglobin S below 15%.
  • Other hemoglobin variants and some conditions can give a false positive, which is why labs confirm positive results. One study in young children suggested very high red or white cell counts or high blood fats may play a part, but this was not confirmed.
  • Different kits and methods are in use, so check which one your lab used. MedlinePlus lists 'sickle cell screen' as another name for hemoglobin electrophoresis, which is a different test.

The result reflects the hemoglobin types you inherited, so it does not change over time. A recent transfusion can temporarily hide hemoglobin S.

When to talk to a clinician

A positive screen is not an emergency by itself, but it needs a confirmatory test, so contact the clinician who ordered it. If you already know you have sickle cell disease, follow your care team's plan for pain or other symptoms. Call 911 or your local emergency number for chest pain, trouble breathing, sudden weakness on one side of the body, severe pain, or confusion.

Questions you could ask

  • Is this the solubility test or hemoglobin electrophoresis?
  • Could a recent transfusion or my age affect this result?
  • Which follow-up test will show whether this is sickle cell trait or sickle cell disease?
  • Should I see a genetic counselor, and should my family members be told?
  • Is there anything I should know before surgery, anesthesia or intense exercise?

Questions people ask

Does a positive sickle cell screen mean I have sickle cell disease?

Not necessarily. The screen only shows that hemoglobin S is present. It can't tell sickle cell trait from sickle cell disease. A follow-up test, usually hemoglobin electrophoresis or HPLC, does that. MedlinePlus notes that most people with sickle cell trait do not have health issues, but can pass the gene to their children.

Can the screen miss sickle cell trait?

It can. A negative result means the test found no hemoglobin S, but it can read negative when the amount is low. This can happen in babies under about 6 months, in severe anemia, and after a recent blood transfusion. Tell your clinician if you have had a transfusion recently.

Is the sickle cell screen the same as hemoglobin electrophoresis?

No. The screen checks whether hemoglobin S is present. Hemoglobin electrophoresis separates and measures each type of hemoglobin. Some sources use 'sickle cell screen' for either test, so check which method your report lists.

Show the science and sources

For clinicians and researchers

The sickle solubility test (SCST, 'sickle prep', Sickledex) relies on deoxygenated hemoglobin S being insoluble in a concentrated phosphate buffer that contains a reducing agent, most often sodium dithionite (hydrosulfite). Hemoglobin S precipitates and makes the solution turbid, while hemoglobin A and most other hemoglobins stay in solution (J Hematol 2024; CRMC, which describes a modified Nalbanian kit method). The test is qualitative: it cannot quantify hemoglobin S or separate sickle cell trait (hemoglobin S commonly about 20–40%) from sickle cell disease (J Hematol 2024). One evaluation reported sensitivity of 98.9% and specificity of 100%, while a study of 200 children aged 6 months to 5 years in Africa reported 45% sensitivity and 90% specificity against electrophoresis. False negatives are described with severe anemia or high fetal hemoglobin, and the review's authors suggest they occur when hemoglobin S is below the assay's detection limit. False positives were attributed, without confirmation, to erythrocytosis, leukocytosis or hyperlipidemia. In an adult clinic series of 220 samples, all 207 with hemoglobin S of 15% or more were positive, and the five false negatives, from patients with sickle cell disease after red cell exchange or heavy transfusion, all had hemoglobin S below 15% (J Hematol 2024). Fetal hemoglobin dilution can persist up to 6 months, so the test is not used for newborn screening. The British Society for Haematology guideline accepts a solubility test for emergency use, for example before anesthesia, and states that any emergency result must be followed by definitive analysis (BSH 2023); positive or equivocal results need confirmatory testing. Labcorp's report carries the comment that a variety of conditions and other abnormal hemoglobins in addition to hemoglobin S may give false-positive results, and that positive results should be confirmed by hemoglobin fractionation. LOINC 6864-3 is the solubility test code; 801-1 covers a microscopy sickling test.

LOINC 6864-3 (primary), 4621-9, 801-1. Specimen: whole-blood.

Sources

  1. Sickle Cell Screening in Adults: A Current Review of Point-of-Care Testing, Journal of Hematology (2024)
  2. Significant haemoglobinopathies: A guideline for screening and diagnosis (British Society for Haematology), British Journal of Haematology (2023)
  3. Hemoglobin (Hgb) Solubility (test 005223), sample report, Labcorp (2016)
  4. Sickle Cell Screen (test code Sickle Cell), CRMC Laboratory Test Catalog
  5. Hemoglobin Electrophoresis, MedlinePlus (NIH National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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