Target Cell – Definition, Causes and Diagnosis
Target cells are abnormal red blood cells with a distinctive bullseye appearance. They are an important diagnostic marker for various blood and liver disorders.
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Target cells are abnormal red blood cells with a distinctive bullseye appearance. They are an important diagnostic marker for various blood and liver disorders.
What Is a Target Cell?
A target cell (also called a codocyte or bullseye cell) is a morphologically abnormal red blood cell (erythrocyte) that, when viewed under a microscope, resembles a shooting target or bullseye. The cell displays a dense central area of hemoglobin surrounded by a pale ring, which is in turn bordered by a darker outer rim – giving it the characteristic target-like appearance.
In the field of hematology, target cells are recognized as an important diagnostic marker, as their presence in a blood sample can point to a variety of underlying medical conditions.
Causes and Formation
Target cells form when the ratio of cell membrane surface area to cell volume is altered. If the membrane is relatively too large for the cell content – or the cell content is reduced – the excess membrane folds inward, creating the bullseye pattern seen in a blood smear.
Common causes for the appearance of target cells include:
- Hemoglobinopathies: Inherited disorders of hemoglobin such as sickle cell disease or thalassemia, where abnormal hemoglobin variants are produced.
- Iron deficiency anemia: Low iron levels lead to smaller, paler red blood cells with an altered membrane structure.
- Liver disease: Conditions such as liver cirrhosis or hepatitis alter blood lipid composition, which affects the erythrocyte membrane.
- Splenectomy: After surgical removal of the spleen, target cells may accumulate in the blood since the spleen normally filters out abnormal red blood cells.
- Hemoglobin C disease: A specific hemoglobinopathy in which hemoglobin C is present; this condition is associated with a particularly high number of target cells.
- Obstructive jaundice: Bile duct obstruction can also lead to target cell formation due to changes in cholesterol levels in the red blood cell membrane.
Symptoms and Clinical Significance
Target cells themselves do not cause direct symptoms. Their clinical significance lies in their role as a diagnostic indicator. The symptoms a patient may experience are caused by the underlying condition and can include:
- Fatigue and general exhaustion (associated with anemia)
- Pale skin and mucous membranes
- Shortness of breath on exertion
- Yellowing of the skin and eyes (jaundice) in liver disease
- Episodes of pain in sickle cell disease (vaso-occlusive crises)
Diagnosis
Target cells are identified through microscopic examination of a peripheral blood smear. A drop of blood is spread thinly on a glass slide, stained (e.g., with Giemsa stain or Wright stain), and examined under a microscope.
An experienced hematologist or laboratory specialist can not only detect target cells but also identify other changes in red blood cell morphology that together help indicate the underlying cause.
Additional diagnostic tests may include:
- Complete blood count (CBC) with red cell indices (MCV, MCH, MCHC)
- Serum ferritin and transferrin saturation (iron metabolism)
- Hemoglobin electrophoresis to detect hemoglobin variants
- Liver enzymes and bilirubin levels
Treatment
Since target cells are a manifestation of an underlying condition, treatment is always directed at the root cause:
- Iron deficiency anemia: Iron supplementation (oral or intravenous) and identification and treatment of any bleeding source.
- Thalassemia: Depending on severity, regular blood transfusions, chelation therapy to remove excess iron, or stem cell transplantation.
- Sickle cell disease: Treatment with hydroxyurea, pain management, transfusions, and potentially stem cell transplantation.
- Liver disease: Treatment of the underlying liver condition, such as alcohol abstinence, antiviral therapy, or liver transplantation.
- Post-splenectomy: Generally no specific treatment is needed for the target cells; vaccination against encapsulated bacteria is recommended.
References
- Hoffbrand AV, Moss PAH. Hoffbrand's Essential Haematology. 7th edition. Wiley-Blackwell, 2016.
- Longo DL et al. Harrison's Principles of Internal Medicine. 21st edition. McGraw-Hill, 2022.
- World Health Organization (WHO). Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. WHO/NMH/NHD/MNM/11.1. Geneva, 2011. Available at: https://www.who.int/publications/i/item/WHO-NMH-NHD-MNM-11.1
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