⚕️ Educational content only. This article explains sickle cell disease from a haematology and molecular science perspective. It is not medical advice. Consult a healthcare professional for personal health concerns.
Sickle cell disease (SCD) is a group of inherited haemoglobin disorders caused by a mutation in the HBB gene encoding the beta-globin chain of haemoglobin. It is one of the most common serious monogenic diseases worldwide and a key topic in haematology, molecular genetics, and global health. Understanding its molecular basis, laboratory features, and diagnostic approach is central to biomedical science education.
Key Takeaways
- SCD is caused by a point mutation in HBB (Glu6Val), substituting glutamic acid for valine at position 6 of the beta-globin chain.
- HbS polymerises under low oxygen tension, causing red cells to sickle, increasing haemolysis and vaso-occlusion.
- Laboratory diagnosis uses HPLC and haemoglobin electrophoresis to identify HbS and other variant haemoglobins.
- The blood film shows sickle cells, target cells, Howell-Jolly bodies (functional asplenia), and polychromasia.
The Molecular Basis of Sickle Cell Disease
The cause of SCD is a single nucleotide substitution in codon 6 of the HBB gene: adenine is replaced by thymine (GAG → GTG), resulting in valine replacing glutamic acid at position 6 of the beta-globin chain (Glu6Val). The resulting abnormal haemoglobin is called HbS. Under deoxygenated conditions, HbS molecules polymerise into long fibrous chains that distort red blood cells into a characteristic sickle or crescent shape. These rigid, deformed cells cause vascular occlusion, haemolysis, and end-organ damage.
Inheritance and Genotypes
SCD follows autosomal recessive inheritance. The most common and severe form is HbSS (homozygous — both beta-globin alleles are HbS). Other clinically significant compound heterozygous forms include HbSC (one HbS allele, one HbC allele — HbC results from Glu6Lys mutation) and HbS/β-thalassaemia (one HbS allele, one thalassaemia allele). Sickle cell trait (HbAS) — one normal HbA allele, one HbS allele — is generally asymptomatic under normal conditions, though there are clinical considerations under extreme physiological stress.
Laboratory Diagnosis
HPLC (High-Performance Liquid Chromatography) is the first-line method for neonatal screening and diagnostic haemoglobinopathy testing in many countries. It separates haemoglobin variants by charge and retention time, identifying HbA, HbS, HbC, HbF, and other variants quantitatively. Haemoglobin electrophoresis (cellulose acetate at alkaline pH or isoelectric focusing) separates variants by charge under an electric field. Abnormal variants must be confirmed with a second method at different pH because some variants co-migrate. Sickling test (sodium metabisulphite) causes deoxygenation — sickle cells observed under microscopy confirm the presence of HbS, though this does not distinguish HbSS from HbAS or HbSC.
Blood Film Findings
The peripheral blood film in HbSS shows: sickle cells (elongated, crescent-shaped cells with pointed ends); target cells (bull’s-eye appearance, due to excess membrane); polychromasia (blue-tinged reticulocytes, reflecting increased erythropoiesis); Howell-Jolly bodies (nuclear remnants in red cells, a sign of functional hyposplenism/asplenia caused by repeated splenic infarction); and nucleated red blood cells in severe disease. The reticulocyte count is typically elevated (5–15%) reflecting compensatory erythropoiesis.
Full Blood Count Findings
Typical FBC in HbSS: haemoglobin 60–90 g/L (normochromic normocytic anaemia); MCV usually normal unless co-existing thalassaemia or iron deficiency; elevated WCC (neutrophilia, often baseline due to chronic inflammation); thrombocytosis (reactive, from functional asplenia); elevated reticulocytes. HbSC tends to have higher haemoglobin (90–110 g/L) and a milder clinical course than HbSS.
References
- WHO. Sickle-cell disease and other haemoglobin disorders. who.int
- NHS. Sickle cell disease. nhs.uk
- Rees DC, Williams TN, Gladwin MT. Sickle-cell disease. Lancet. 2010;376(9757):2018-2031.
More questions answered
Which blood tests can be used when sickle cell disease is being investigated?
There is usually no single blood test that answers every question about sickle cell disease. Depending on the clinical situation, testing may include full blood count, blood film, reticulocyte count and haemoglobin analysis together with more general laboratory measurements. The purpose of each test is different, so a result that supports one part of the assessment may not establish the diagnosis by itself. A healthcare professional chooses and interprets tests according to symptoms, examination findings and other evidence.
Can blood tests confirm sickle cell disease on their own?
Blood tests can provide important evidence, but they are not automatically diagnostic of sickle cell disease by themselves. Results are interpreted with clinical findings, medical history and sometimes imaging, pathology or genetic information. Some abnormal laboratory patterns can also occur in other haemoglobin disorders or causes of anaemia, which is why context matters. Personal results should be interpreted by an appropriately qualified healthcare professional.
Why can a full blood count be useful in sickle cell disease?
A full blood count measures red cells, white cells and platelets, so it can reveal changes that may accompany sickle cell disease or its complications. The exact pattern depends on the condition and on other factors affecting blood-cell production or survival. Similar changes can occur for unrelated reasons, so the count is not specific. Clinicians interpret it alongside full blood count, blood film, reticulocyte count and haemoglobin analysis and the wider clinical picture.
Why might laboratory results change over time in sickle cell disease?
Laboratory values can change as sickle cell disease changes, as treatment affects the body or as unrelated illnesses occur. Looking at a trend over time can therefore be more informative than comparing one result with a single cut-off. Laboratories also consider differences in sample timing, method and biological variation. A healthcare professional decides whether a change is clinically meaningful.
Can another condition cause blood-test changes that look similar to sickle cell disease?
Yes. Laboratory findings associated with sickle cell disease are often shared with other haemoglobin disorders or causes of anaemia or with temporary physiological changes. This overlap is why clinicians avoid diagnosing from one marker or one abnormal result. They combine multiple sources of evidence to decide which explanation fits best. If an individual result is concerning, it should be discussed with a healthcare professional.
Why might several different tests be ordered for sickle cell disease?
Different tests answer different questions about sickle cell disease, such as whether inflammation is present, whether red blood cells is affected or whether a characteristic marker can be detected. A panel may therefore include full blood count, blood film, reticulocyte count and haemoglobin analysis rather than relying on one value. Results can complement each other even when none is specific enough to stand alone. The combination is interpreted in the context of the person’s clinical assessment.
What laboratory tests may be used to monitor sickle cell disease?
Monitoring may involve full blood count, reticulocyte count and haemoglobin-related testing, although the exact plan depends on the condition and treatment. Some tests track disease activity, while others look for treatment effects or organ changes. A result outside its usual range does not automatically mean the condition has worsened because other factors may influence it. The monitoring schedule should come from the treating healthcare team.