Batten disease is a rare inherited lysosomal storage disorder. Lysosomes are compartments within a cell that degrade substances that are no longer needed. Lysosomes that are unable to degrade wastes as intended, as in Batten disease, allow lipofuscins, fatty granular substances, to accumulate inside cells. This can lead to the formation of abnormal and distinctive vacuoles, or holes or cavities inside cells.
The presence of these vacuoles can easily be detected with a blood test by analyzing white blood cells, or lymphocytes. Blood tests are a common diagnostic tool for identifying vacuolated lymphocytes in juvenile Batten disease, also known as CLN3 disease.
How is a blood test for Batten disease done?
The first step in a blood test is to make a blood film — a drop of blood that is spread in a thin layer across a glass slide and air-dried. The slide is then viewed under a light microscope. Large vacuoles can easily be detected with this method.
For a more detailed analysis, or in cases where the presence of abnormally shaped lymphocytes is not certain, the blood sample can be further analyzed using an electron microscope. An electron microscopy offers a very high-resolution and allows the identification of structures down to a size of 1 nm. (The diameter of a hair follicle is about 100,000 times this size.)
Can a blood test identify Batten disease by type?
Different types of Batten disease are associated with a specific lymphocyte morphology.
For example, juvenile Batten disease is marked by multiple, large, and bold vacuoles seen in a high percentage of lymphocytes. Under an electron microscope, fingerprint-like structures can also be identified. These are also seen in late infantile Batten disease or CLN2 disease. The lymphocytes in late infantile Batten disease, however, do not contain vacuoles.
Curvilinear bodies have been reported in patients with late infantile Batten disease. Patients with this Batten type and mutations in the CLN7 gene are also identified through distinct and compact lipofuscins (fatty granular substances) that form fingerprint-like structures.
Usefulness of blood tests
A blood test is a minimally invasive, low cost, and rapid way to screen for Batten disease. False positive results — positive results in the absence of Batten disease — are also very rare with a blood test.
In early stages of Batten disease, the number of abnormally shaped lymphocytes can be small. To reduce the likelihood of a false negative diagnosis — a negative result for a patient who actually has Batten disease — multiple blood films should be analyzed, for which only one blood sample is needed.
Although a blood test is a reliable tool to detect certain types of Batten disease, abnormally shaped lymphocytes have not been reported for all forms. The results of blood tests should, therefore, be confirmed with a genetic test, which will also give certainty about the underlying genetic cause of the disease.
Genetic analysis can be performed with the same blood sample used for blood tests.
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