Testing dried blood spots in NMOSD diagnosis may be ‘viable alternative’
Method could be used in countries that lack access to specialized laboratories
Testing for the self-reactive antibodies that drive most cases of neuromyelitis optica spectrum disorder (NMOSD) in dried blood spots (DBS) is as accurate as testing blood samples, a study shows.
The findings suggest the dried blood spot technique could be a viable alternative to conventional blood sample testing for antibodies against the AQP4 protein (AQP4-IgG), whose presence is a main diagnostic criterion in NMOSD. This may be especially important in low-income countries that lack access to the specialized laboratories needed for gold-standard blood diagnostic tests.
“The DBS represents a viable alternative for detecting AQP4-IgG in resource-limited settings to diagnose NMOSD, offering high sensitivity and specificity comparable to [blood] testing,” the researchers wrote.
The study, “Dried blood spot improves global access to aquaporin-4-IgG testing for neuromyelitis optica,” was published in the Annals of Clinical and Translational Neurology.
NMOSD is a rare autoimmune disease marked by inflammation of the optic nerve, which relays signals between the eye and the brain, and the spinal cord. People with the disease have NMOSD symptoms like eye pain, vision loss, muscle weakness, and involuntary muscle contractions.
The exact cause of NMSOD isn’t completely understood, but most cases are associated with self-reactive antibodies that target AQP4, a protein on the surface of nervous system cells called astrocytes, and trigger an immune attack that damages and kills the cells, disrupting their neuron-supporting roles.
Detecting NMOSD-related antibodies in DBS
Detecting anti-AQP4 antibodies in the blood is one of the main criteria to confirm an NMOSD diagnosis and helps clinicians differentiate between the disease and other neurological conditions. But the blood tests require specialized equipment and pose logistical challenges in the matter of collection, storage, and transportation, making them inaccessible in resource-limited settings.
“Only half of African and Eastern Mediterranean countries have access to AQP4-IgG testing, with only 15% of World Bank-defined low-income countries having access to these antibody tests,” the researchers wrote.
DBS has emerged as a potentially more accessible diagnostic tool. It’s a form of sampling that starts with a finger or heel prick, followed by spotting and drying blood samples on filter paper. These dried samples can then be shipped to a laboratory to be analyzed.
“DBS collection is simpler, minimally invasive, and cost-effective, requiring a smaller volume of blood compared with traditional [blood sample collection],” wrote the researchers, who said it also minimizes “the risk of bacterial contamination and [red blood cell destruction], and [allows] preservation for extended periods with minimal degradation of the [samples] at [room temperature].”
DBS comparable to blood sample testing
To get a better idea about the diagnostic utility and accuracy of DBS anti-AQP4 antibody testing relative to conventional blood testing, researchers in the U.S. and Uganda compared results between both methods using samples from 47 adults with NMOSD and 103 adults with other neurological diseases who tested negative for anti-AQP4 antibodies. All were seen in clinics in the U.S., Uganda, and the Republic of Guinea.
All but one NMOSD patient had previously tested positive for anti-AQP4 antibodies. The NMOSD group had a higher proportion of women (87% vs. 68.5%), an older age at disease onset (mean, 55.9 vs. 47.3 years), and a longer period between onset and blood collection (mean, 10.3 vs. 6.4 years) relative to the control group.
Utilizing a technique known as cell-based assay to detect anti-AQP4 antibodies, the results showed that using DBS as samples allowed the discrimination between people positive for these antibodies and those testing negative with an accuracy of 97%. When using a cutoff of 2 or higher for the cell-based assay, detection of antibodies against AQP4 in DBS showed a sensitivity of 87% and a specificity of 100%. A test’s sensitivity is its ability to correctly identify those with a given feature or disease, while specificity refers to correctly identifying those without it.
Researchers also turned to a technique called enzyme-linked immunosorbent assay (ELISA) to detect antibdies. ELISA testing of the DBS had a sensitivity of 65.2% and specificity of 95.2%, which compared to the 69.6% sensitivity and 98.4% specificity of ELISA testing of conventional blood samples.
Researchers also assessed the long-term stability of DBS and tested the same sample from antibody-positive NMOSD patients at six, 12, and 24 months after collection. They found consistently positive results for the two-year period, except for one patient who had low antibody levels at the first assessment and eventually got a negative result.
“The findings demonstrated that AQP4-IgG detection using DBS exhibits both high sensitivity and high specificity, almost comparable to the current gold standard [blood] testing,” the researchers wrote. “This method proved feasibility in resource-limited regions and within the United States, performed by patients themselves.”
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