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Tiny gold specks may cut false positives in medical tests

Researchers have created a new biomedical assay that eliminates the readout of “false positive” results in medical tests for pregnancy, allergies, infectious disease, and more. It could lead to far fewer worrisome faulty test results and wasted money on additional, unnecessary tests.

“…we looked at how we can make what’s already there work better, by reducing erroneous results.”

“There are these horror stories,” says Tracy Chuong, a researcher from University of California, Santa Barbara, talking about false positives in medical screening. “Usually they go in for second opinions, get another test and it turns out to be a big scare. But it does cost the healthcare system quite a bit of money.”

Not only does the new assay provide greater accuracy, it reduces the wait time for results. It is an improvement on the popular enzyme-linked immunosorbent assay (ELISA), which detects concentrations of proteins that correlate with various conditions.

“We’re not trying to be the best assay,” says Chuong, referring to tests that can get results out of ever-more miniscule sample amounts. “Instead we looked at how we can make what’s already there work better, by reducing erroneous results.”

How it works

The mechanism of an ELISA basically works like this: Blood or other biological fluid is dropped onto plates with little wells whose surfaces bind antibodies and proteins. A second binder is added to these wells, which are tagged with “reporter molecules” that will activate (usually change color) if a target protein is detected.

The test can vary in the number of steps and intermediate steps and their sequences, or in the types of detection molecules or enzymes, depending on the information being sought.

“What we realized was that in the process of doing this assay everything comes down to that one binder and the reporter,” Chuong says. If the reporter happens to bind to other surfaces, or other proteins, she says, it can indicate an abnormal concentration of target proteins that correlate with a disease. Other substances in the test sample may also prompt the reporter to activate.

“Then you suddenly have a false positive,” she says.

The procedure developed by the UCSB researchers looks more closely at the binding, and in effect labels all the parties involved in that step so that any erroneous activations of the reporter can be removed or disregarded, leaving only the true positives to be assessed.

“That’s the unique bit of our assay,” Chuong says.

Why gold nanoparticles?

Key to this technology are gold nanoparticles, infinitesimally tiny bits of gold whose electromagnetic properties enhance the chemical signature of whatever molecules happen to be close by.

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“When we have these gold nanoparticles come together there are these interesting little electromagnetic fields that are generated within these gold surfaces when light hits,” Chuong says. The chemical signal of the labels next to the binder gets amplified by these nanoparticles, she explains.

“You can see everything within that binding spot,” she says, and the signals can be compared from one area of binding to another. Reactions that are not the ones being sought will not have the same signals as the true positives and can be weeded out in the analysis.

The assay—when compared against the performance of the conventional ELISA—has proved to be “up to clinical standards,” according to the researchers, with the added ability of removing the false positives, thereby eliminating the need for repeated testing.

Additionally, this assay can cut out several intermediate steps requiring washing and adding more reagents. The gold nanoparticles can make it possible to bind and label all the necessary proteins in about an hour.

“Theoretically a patient can just hang out for an hour and get their results, instead of waiting for the next day,” says Chuong.

Further research on this project aims to assess multiple targets.

Sticky strands of DNA assemble gold nanoparticles

“Doctors don’t rely on just one protein to make a conclusion, they look at a panel of proteins and their abundances to complete the picture,” Chuong says. “First, we’d like to show that multiple targets can be assessed, and the next step would be to look at them all at the same time.”

A paper describing the research appears in the Proceedings of the National Academy of Sciences.

Source: UC Santa Barbara

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