Rapid diagnostic test for antibody deficiencies ‘tool to help’ in underserved areas
A prototype point-of-care rapid screening device differentiated between low immunoglobulin G antibody levels associated with agammaglobulinemia and normal levels, according to a study published in Journal of Clinical Immunology.
The Rapid IgG Screen, or RIgGS, test could be used in underserved areas with limited resources to help identify children with primary antibody deficiencies (PADs) who may be at risk for acquiring and spreading polioviruses through polio vaccination, according to the researchers.
The need for testing
Children with low immunoglobulin G (IgG) levels are at a 3,000-fold increased risk for shedding immunodeficiency-related vaccine-derived polioviruses, potentially initiating vaccine-derived poliovirus (VDPV) outbreaks, the researchers wrote.
But currently, the researchers added, there are no rapid diagnostic tests available that can screen individuals for PADs at the point of care in low-income and middle-income countries.
“People with a rare class of hereditary diseases called primary immunodeficiency diseases (PID) are often not able to mount an adequate immune response to clear the weakened polio virus from oral polio vaccine,” Roger B. Peck, BSc, a research scientist and senior program officer at PATH, a global nonprofit based in Seattle, told Healio.
“This means they are far more likely to shed the virus for a prolonged period of time, increasing the risk of mutation into a vaccine-derived poliovirus that can cause paralysis,” Peck said.
Global polio eradication efforts require effective tools to identify people with PID, Peck said, which is why PATH is pushing forward development of a rapid diagnostic test that helps detect PID by identifying low antibody levels that indicate these diseases.
“A rapid diagnostic test will equip communities with an important tool to help identify undiagnosed cases of PID and limit prolonged shedding of vaccine polio virus,” Peck said.
The availability of a rapid diagnostic test that could be used anywhere would be particularly important in rural and low-resource settings, Peck continued, as testing currently is only available in reference centers located in major cities, making them difficult for many people to access.
“This test could become even more important when wild polio virus is eradicated,” Peck said. “Individuals with prolonged shedding of vaccine poliovirus could be the final reservoir of the poliovirus at that point.”
Screening and diagnostic tests will be important tools that work in tandem with medications that are being developed and evaluated to clear the virus from those individuals, Peck said.
How well the prototypes performed
The researchers developed three prototypes designed to determine IgG threshold levels that indicate PAD in less than 20 minutes by using fingerstick blood, plasma or sera in a simple workflow that does not require power or instrumentation.
The prototypes target agammaglobulinemia (AG), common variable immunodeficiency (CVID) and hyper-immunoglobulin M syndrome (HIGM), which are major causes of PAD. Designated blue, green and pink, the three prototypes differ in construction and by quantity of protein A and protein L used on their lower membranes.
Based on literature review, the prototypes were designed to detect approximately 3 g/L IgG as the initial threshold for identifying children at risk for PAD. The researchers used commercially available serum specimens (Discovery Life Sciences) with IgG levels ranging from 0.12 g/L to 5.76 g/L to develop the prototypes, although these specimens were not necessarily from individuals with PID.
The 118 samples came from 32 healthy donors and 86 patients with PAD who had been diagnosed with AG, CVID and HIGM, of whom 29 were receiving IV IgG treatment (AG, n = 8; CVID, n = 16; HIGM, n = 5) and 57 were not (AG, n = 19; CVID, n = 24; HIGM, n = 14). Specimens were tested on all three prototypes.
With IgG levels between 4.5 g/L and 15.4 g/L, the blue and green prototypes identified healthy controls in 100% of the 32 tests, and the pink prototype identified them in 29 (90.6%).
When the 19 samples from individuals with AG not yet receiving IV IgG substitutive treatment were tested, with IgG reference results ranging from 0.1 g/L to 3.1 g/L, the blue prototype was nonreactive (indicating low IgG levels) in 17 (89.5%) of the tests. The green and pink prototypes were each nonreactive in 16 (84.2%).
With IgG reference test results of 5.7 g/L to 12.4 g/L, all three prototypes reacted in 100% of the tests of samples from eight patients with AG who had received IV IgG treatment.
The tests also included samples from 24 individuals with CVID who had not been receiving IV IgG substitutive treatment, with IgG reference results ranging between 0.2 g/L and 4.8 g/L. The blue and pink prototypes were each nonreactive in seven (29.2%) of the tests, and the green was nonreactive in nine (37.5%).
With IgG reference test results ranging from 3.8 g/L to 19.4 g/L, the samples included 16 individuals with CVID who had been receiving IV IgG substitutive treatment. The green prototype was reactive in 100%, the pink prototype was reactive in 15 (93.8%) and the blue prototype was reactive in 14 (87.5%).
There also were 14 samples from individuals with HIGM who had not been receiving IV IgG substitutive treatment, with IgG reference test results of 0.1 g/L to 3.5 g/L and IgM ranging from 1 g/L to 20.8 g/L. Each of the three prototypes was nonreactive in five (35.7%) of the tests.
All three prototypes also reacted to 100% in five tests involving individuals with HIGM who were receiving IV IgG treatment, with IgG reference test results between 3.5 g/L and 10.8 g/L.
The researchers said the blue prototype had the preferred form factor, integrating sample collection and its test housing, and usability characteristics, as it did not require sample dilution or a premeasured buffer.
The blue prototype also had the highest agreement with the reference values, with total agreement across all case definitions at 67.4%, meaning it could differentiate between low IgG levels associated with AG and normal IgG antibody levels.
“The most surprising results were when we transitioned from specimens that were used for product development to evaluating the prototypes with archived specimens from PID patients in Tunisia,” Peck said.
The evaluation in Tunisia indicated that the test did not accurately classify HIGM and CVID, which are important for a screening test to identify.
“From our research and development work, and the specimens we had available to guide our progress, we believed that the test prototypes would perform well,” Peck said. “The reality was that these sub-groups of PID expressed more variability than we anticipated, resulting in performance that was not quite what we expected.”
The next steps in development
Next, the researchers said development should focus on honing the assay threshold and the mitigating effect of HIGM specimens to improve the identification of individuals at risk for PAD and prevent or stop administration of the oral poliovirus vaccine that could lead to VDPV outbreaks.
“Our expectation is that this test will be a tool to help identify individuals with PID in resource-limited settings, referring them to appropriate follow-up for confirmatory diagnosis and appropriate care and treatment,” Peck said.
“We feel that information generated from disease surveillance programs that use this test would help to inform appropriate evidence-based community action, including education, prevention and appropriate use of vaccines and treatments,” he continued.
Since the study’s publication, Peck said the researchers have advanced the prototype by further research and development to adjust the assay cut-off to better address performance with HIGM and CVID patients and that it is undergoing another round of independent evaluation in Tunisia.
“If the Tunisia evaluation is favorable, we will continue with a technology transfer of the product to a manufacturer,” Peck said. “PATH will work with the manufacturer and partners to conduct evaluation studies in the intended use settings and can support regulatory submissions. After that, the manufacturer will take on commercialization and sustainable supply of the test.”
For more information:
Roger B. Peck, BSc, can be reached at rpeck@path.org.