Sickle cell test results look different depending on which test was performed, but they all aim to answer the same basic question: which types of hemoglobin are in your blood, and how much of each? A simple screening test gives you a positive or negative result, while a confirmatory lab test prints out a detailed breakdown showing each hemoglobin type as a percentage of the total. Understanding what those numbers and labels mean is the key to reading your result, whether it shows normal hemoglobin, sickle cell trait, or sickle cell disease.
The Quick Screening Test
The most common first step is the sickle cell solubility test, sometimes called the “Sickledex” test. This is a pass-or-fail type of result. A blood sample is mixed with a chemical solution that removes oxygen. If sickle hemoglobin (HbS) is present, it clumps together and makes the solution cloudy or turbid. If no HbS is present, the solution stays clear. Your result will read something like “positive” or “negative” for the presence of HbS.
The important limitation here is that a positive solubility test does not tell you whether you have sickle cell trait or sickle cell disease. It only confirms that HbS is somewhere in your blood. The test cannot measure how much HbS you have, and it cannot distinguish between someone who carries one copy of the sickle gene and someone who carries two. That is why a positive screening result always requires a follow-up confirmatory test.
What an HPLC Confirmatory Result Looks Like
The gold standard for confirming sickle cell results in most labs is high-performance liquid chromatography, or HPLC. If you have ever seen one of these reports, it looks nothing like a typical blood test. Instead of a simple list of numbers, the printout includes a graph called a chromatogram, which shows a series of peaks. Each peak represents a different type of hemoglobin found in your blood, and the position and size of each peak tells the lab what type it is and how much is present.
Each hemoglobin variant passes through the machine at a slightly different speed, and the time it takes to appear is called the retention time. Labs have established windows for where common variants show up. If a peak lands inside the known window for HbS, it gets labeled as HbS. If a peak falls outside any recognized window, it gets flagged as “unknown” and requires further investigation.
Alongside the chromatogram, the report lists each hemoglobin fraction as a percentage of the total. That percentage breakdown is the most useful part of the result for you and your doctor. A normal result, a sickle cell trait result, and a sickle cell disease result each produce a distinctly different pattern of percentages.
Reading the Hemoglobin Percentages
For adults, here is what the main hemoglobin patterns look like on a confirmatory report:
- Normal (HbAA): Hemoglobin A makes up roughly 95–98% of the total. Hemoglobin A2 sits around 2–3%. Fetal hemoglobin (HbF) is usually under 1%. No HbS is detected.
- Sickle cell trait (HbAS): Both HbA and HbS appear. HbA is the larger fraction, typically around 55–60%, and HbS is the smaller fraction, usually around 35–40%. HbA2 is still in the normal range, and HbF remains low.
- Sickle cell disease, HbSS type: HbS dominates, often making up 80% or more of the total. There is no HbA present at all. HbF may be elevated, and HbA2 is usually in the 2–4% range.
- Sickle cell disease, HbSC type: Roughly equal amounts of HbS and HbC appear, each around 45–50%. Again, no HbA is detected.
The critical thing to notice is whether HbA is present. In sickle cell trait, HbA is always the majority hemoglobin. In sickle cell disease (HbSS), HbA is completely absent. That single distinction between “HbA present” and “HbA absent” is the clearest dividing line between trait and disease on any lab report.
Rapid Point-of-Care Tests
In settings where a full lab is not available, rapid point-of-care tests offer results in about 10 to 20 minutes. These are lateral flow immunoassays, and they work a lot like a home pregnancy test: you apply a small blood sample, and colored lines appear in labeled windows to indicate which hemoglobin types are present. The test strip typically has three windows labeled A, S, and C, plus a control line.
If only the A line and the control line appear, the result is normal (HbAA). If both A and S lines appear, it suggests sickle cell trait (HbAS). If the S line appears without an A line, it points toward sickle cell disease (HbSS). If S and C lines both appear without A, that suggests HbSC disease. One validated rapid test showed about 99% sensitivity and 99% specificity for correctly diagnosing HbSS, HbAS, HbSC, HbAC, and HbAA using venous blood, with the same level of accuracy when tested at the bedside with a finger prick.
These rapid tests are particularly valuable in low-resource settings for newborn screening. In a large study of cord blood samples from 365 newborns, one such rapid test correctly identified hemoglobin phenotypes in about 97% of cases, and produced zero false positives and zero false negatives for HbSS disease specifically.
Newborn Screening Results
In the United States and many other countries, every newborn is tested for sickle cell disease as part of a routine heel-prick blood test done in the first few days of life. The result format varies by state, but the report typically lists hemoglobin types in order from the largest amount to the smallest. The way these are written can be confusing if you have never seen them before.
A normal newborn result is often reported as “FA,” meaning fetal hemoglobin was the dominant type and adult hemoglobin A was also detected. This is completely normal because babies have high levels of fetal hemoglobin at birth. A result of “FAS” means fetal hemoglobin was dominant, adult hemoglobin A was present, and a small amount of HbS was also detected. FAS indicates sickle cell trait. A result of “FS” means fetal hemoglobin was dominant and HbS was present but no HbA was found, which suggests sickle cell disease (HbSS type). “FSC” indicates the baby has both HbS and HbC, suggesting HbSC disease.
One automated HPLC system designed for newborn screening could process one sample per minute and detect HbS and HbC at levels as low as 0.5% of total hemoglobin.
When Numbers Shift From the Typical Ranges
The percentages on a sickle cell report are not always textbook. Several conditions can push the numbers in unexpected directions, and knowing about them helps you avoid misreading a result.
One common scenario involves co-inherited alpha thalassemia. People with sickle cell trait normally have about 35–40% HbS. But in a study of individuals who had both sickle cell trait and alpha thalassemia, HbS levels dropped below 35%, averaging around 31.8%, while their red blood cells were also smaller than usual. The alpha thalassemia was reducing overall hemoglobin chain production in a way that shifted the balance between HbA and HbS. If your report shows sickle cell trait with an unusually low HbS percentage and smaller-than-normal red blood cells, your doctor may want to investigate whether alpha thalassemia is also present.
Iron deficiency can have a similar effect, temporarily lowering HbS percentages in someone with sickle cell trait. Recent blood transfusions are another common cause of shifted numbers: a person with sickle cell disease who has received donor blood will temporarily show HbA on their report, because the donor’s normal hemoglobin is now circulating alongside their own HbS. If the test is done shortly after a transfusion, the result can look misleadingly like sickle cell trait rather than disease.
How Treatment Changes What You See on the Report
If you or someone you know is being treated for sickle cell disease, the lab reports will change over time. The most commonly prescribed medication, hydroxyurea, works by boosting fetal hemoglobin production. Fetal hemoglobin interferes with the sickling process, so raising its level reduces symptoms.
On a lab report, this shows up as a rising HbF percentage. In an early study of patients treated with hydroxyurea, seven out of ten responded with a two- to tenfold increase in fetal hemoglobin, going from an average of about 1.6% up to about 6.8%, with three patients reaching 10 to 15% HbF. In some cases, the response can be even more dramatic. Two patients in a separate report saw their HbF climb from 5% to 25% and from 3% to 31% on hydroxyurea.
Doctors monitor these percentage shifts regularly to gauge whether treatment is working. A steadily rising HbF is a good sign. The HbS percentage does not disappear, but it shrinks as a share of the total because fetal hemoglobin is taking up a larger proportion. Understanding this shift prevents confusion when comparing a patient’s current lab report to one from before treatment began.
False Positives and Unusual Variants
No test is perfect, and sickle cell testing is no exception. The solubility screening test is the most vulnerable to errors. Some rare hemoglobin variants can cause a false positive on the solubility test. In one documented case, a blood donor with a rare variant called Hemoglobin Geldrop St. Anna tested positive on a standard HbS screen even though they did not actually carry sickle hemoglobin. This is part of why a positive screen always needs a confirmatory test.
HPLC can also be tripped up by unusual hemoglobin variants that happen to elute at the same retention time as HbS, producing a peak in the S window when the hemoglobin is actually something else entirely. When the HPLC pattern looks unusual or does not match the clinical picture, labs may move to additional methods like capillary electrophoresis or genetic testing to sort out exactly which variant is present. Genetic testing directly examines the DNA sequence of the beta-globin gene and can pinpoint the exact mutation, removing any ambiguity from the result. This level of testing is more expensive and is typically reserved for cases where the standard methods leave questions unanswered.
Sickle Cell Disease Versus Sickle Cell Trait on Paper
The difference between sickle cell trait and sickle cell disease is the single most important distinction on any test result, and it is worth understanding clearly because the two conditions have very different implications for your health.
Sickle cell trait means you inherited one sickle gene and one normal gene. Your body makes both HbA and HbS, with HbA being the majority. Most people with sickle cell trait live entirely normal lives and experience no symptoms. Sickle cell disease means you inherited two abnormal hemoglobin genes, whether two sickle genes (HbSS), or one sickle gene combined with another variant like HbC (producing HbSC disease). On your lab report, the absence of HbA is the hallmark of disease rather than trait.
The clinical differences between subtypes of sickle cell disease also show up on lab work. Patients with HbSS tend to have more severe anemia, more hemolysis (destruction of red blood cells), and higher white blood cell counts compared to those with HbSC disease. HbSC patients, on the other hand, tend to show changes in their lipid profiles and kidney markers. These patterns help doctors calibrate treatment even beyond the initial diagnosis.
What to Do After Getting Your Result
If your result shows sickle cell trait (HbAS), the main practical consideration involves family planning. If both you and your partner carry sickle cell trait, each pregnancy has a one-in-four chance of producing a child with sickle cell disease. Genetic counseling can help couples understand their specific risk and the options available to them. Beyond family planning, sickle cell trait generally does not require ongoing medical management, though your doctor may note it in your chart for situations like surgery or high-altitude activities where extreme oxygen changes can rarely trigger complications.
If the result shows sickle cell disease, the report becomes a baseline that your medical team will compare against over time. They will track your HbS and HbF percentages, your hemoglobin levels, and markers of organ function. Changes in these numbers help guide decisions about when to start or adjust treatment. If you are on hydroxyurea, your HbF trend line is one of the most closely watched numbers at every visit.
If the result shows an unfamiliar pattern, such as an unknown peak on HPLC or an ambiguous band on electrophoresis, further testing is the appropriate next step. Rare hemoglobin variants number in the hundreds, and some of them mimic sickle hemoglobin on routine tests while behaving very differently in the body. Labs that specialize in hemoglobinopathy testing can usually resolve these cases with a combination of advanced electrophoresis and molecular genetic analysis.
Why the Same Person Can Get Different Numbers at Different Labs
If you have had sickle cell testing done at more than one facility, you may have noticed that the percentages are not identical. This is normal and does not mean one lab made an error. Different HPLC instruments, reagent lots, and calibration methods produce slight variations in the measured percentages. One large quality-control evaluation of automated HPLC newborn screening found that hemoglobin concentrations varied by about 14–18% between laboratories, while retention times were far more consistent, matching within fractions of a minute across labs and reagent batches. The hemoglobin type identification is highly reliable; it is the exact percentage that wobbles a bit.
This is why clinicians focus more on the overall pattern than on any single number. A report showing 38% HbS and another showing 35% HbS for the same person at two different labs both point clearly to sickle cell trait. The diagnostic conclusion does not change with a few percentage points of variation. Where precision really matters, such as monitoring HbF trends during hydroxyurea therapy, doctors prefer to use the same lab each time so that any shift in the numbers reflects a genuine change in the patient’s hemoglobin rather than instrument-to-instrument variation.
Hemoglobin Electrophoresis and Other Confirmatory Methods
HPLC is not the only confirmatory method. Hemoglobin electrophoresis, which separates hemoglobin types based on their electrical charge, is still widely used. On an electrophoresis result, you will see bands at different positions on a gel or readout, each band corresponding to a hemoglobin type. The report lists the identified hemoglobins and their approximate percentages, similar to what HPLC produces but derived through a different physical principle.
Capillary electrophoresis is a newer variation that automates the process and provides cleaner separation of variants that can overlap on older gel-based methods. It is increasingly used as a second-line confirmatory test when HPLC produces an ambiguous result, and it served as the reference standard in the neonatal rapid-test validation study mentioned earlier. In practice, many diagnostic labs run both HPLC and electrophoresis on the same sample to cross-check results, especially when the pattern is not straightforwardly HbAA, HbAS, or HbSS.
Molecular genetic testing sits at the top of the diagnostic ladder. Rather than measuring the hemoglobin protein itself, it reads the DNA sequence of the beta-globin gene directly. This can confirm the specific mutation responsible for sickle hemoglobin and detect compound heterozygous states where sickle hemoglobin is paired with another rare variant. Genetic testing is more expensive and requires specialized laboratory staff, so it is typically reserved for unclear cases rather than routine screening.