The test could help people determine what kind of precautions they should take against Covid-19, such as getting a booster shot, the researchers said.
"Among the general population, many people probably want to know how well protected they are," said Hojun Li from Massachusetts Institute of Technology (MIT)'s Koch Institute for Integrative Cancer Research, US.
"But I think where this test might make the biggest difference is for anybody who is receiving chemotherapy, anybody who's on immunosuppressive drugs for rheumatologic disorders or autoimmune diseases, and for anybody who's elderly or doesn't mount good immune responses in general," Li said.
Such people might need to be boosted sooner or receive more doses to achieve adequate protection, the researchers said.
The test is designed so that different viral spike proteins -- that help the virus to enter and infect the cells -- can be swapped in.
The researchers have filed for a patent on the technology and are now hoping to partner with a diagnostic company that could manufacture the devices and seeks approval from the US Food and Drug Administration (FDA).
The researchers drew inspiration from at-home pregnancy tests, which are based on a type of test called a lateral flow assay.
Lateral flow assays generally consist of paper strips embedded with test lines that bind to a particular target molecule if it is present in a sample. This technology is also the basis of most at-home rapid tests for coronavirus.
What are the other options for measuring immunity?
At present, the gold standard approach to measuring immunity involves mixing a blood sample with a live virus and measuring how many cells in the sample are killed by the virus.
However, the procedure is considered hazardous for health. Therefore, another more commonly used approaches involve noninfectious modified "pseudo viral" particles.
But such procedures still require trained personnel working in a lab with specialised equipment.
How will the new test work?
The researchers have developed a device that can detect the presence of antibodies that block the SARS-CoV-2 receptor binding domain (RBD) from binding to ACE2, the human receptor that the virus uses to infect cells.
The first step in the test is to mix human blood samples with viral RBD protein that has been labelled with tiny gold particles which can be visualised when bound to a paper strip.
After allowing time for antibodies in the sample to interact with the viral protein, a few drops of the sample are placed on a test strip embedded with two test lines.
One of these lines attracts free viral RBD proteins, while the other attracts any RBD that has been captured by neutralising antibodies, the researchers said.
A strong signal from the second line indicates a high level of neutralising antibodies in the sample.
