Home / Chroniques / Covid, pregnancy, heart attack: how to improve tests
Mass testing for Covid-19 / SARS-CoV‑2 infection concept: Several rapid antigen test kits. One kit in the middle showing a positive result (two lines visible)
π Health and biotech π Science and technology

Covid, pregnancy, heart attack: how to improve tests

Fanny Mousseau
Fanny Mousseau
postdoctoral researcher at LOB* at École Polytechnique (IP Paris)
Key takeaways
  • Lateral flow tests (LFA) are faster, cheaper, and less demanding than clinical laboratory tests.
  • But they only provide a qualitative response such as “yes” or “no”, where a quantitative response would be valuable as well.
  • The use of luminescent nanoparticles in LFA tests could improve their sensitivity by 40-50 times, while reducing their cost.
  • This method is being developed with the aim of providing easy access to test results in any location and at any time.
  • A small device has been developed to illuminate nanoparticles, facilitating the reading of the result obtained.

Since the Covid-19 pan­dem­ic, we have all become famil­iar with lat­er­al flow tests (LFA), com­mon­ly referred to as “anti­gen tests”. This type of test can, in prin­ci­ple, detect the pres­ence of any pro­tein anti­gen in body flu­ids (urine, blood, sali­va etc.) – not only SARS-CoV­‑2. Their advan­tage over con­ven­tion­al tests per­formed in med­ical lab­o­ra­to­ries (such as ELISA) is that they are fast (15 min­utes instead of sev­er­al hours), inex­pen­sive and do not require heavy lab­o­ra­to­ry equipment. 

They are there­fore use­ful for detect­ing many dis­eases, but also changes in the body such as preg­nan­cy, or the pres­ence of a pro­tein food that could cause food poi­son­ing. How­ev­er, unlike ELISAs, one of the lim­i­ta­tions of LFA tests is that they only give a qual­i­ta­tive answer. For a preg­nan­cy test, for exam­ple, the results are still lim­it­ed to “yes/no” – either you are preg­nant or you are not. How­ev­er, there are many cas­es where it would be use­ful to deter­mine the amount of the pro­tein being test­ed for. For exam­ple: “yes” you are preg­nant, but for how many weeks? Or, “yes” you have Covid, but with what viral load? 

Our method can be con­sid­ered 40 to 50 times more sen­si­tive than tra­di­tion­al ones.

Anoth­er lim­i­ta­tion of LFA tests is that they are not par­tic­u­lar­ly sen­si­tive. Devel­op­ing quan­ti­ta­tive and sen­si­tive LFA tests would push the bound­aries of what these tests can be used for. They could even indi­cate how long ago a preg­nan­cy start­ed. This sen­si­tiv­i­ty and accu­ra­cy would also allow the test to detect the amount of cTnI pro­tein nat­u­ral­ly secret­ed before car­diac arrest, for exam­ple. Cou­pled with its new acces­si­bil­i­ty, it could be per­formed direct­ly in the ambu­lance, facil­i­tat­ing patient management.

120x less antibody, 40x more sensitive

Fan­ny Mousseau, a researcher at the Optics and Bio­sciences Lab­o­ra­to­ry at Insti­tut Poly­tech­nique de Paris, is work­ing on the devel­op­ment of LFA tests based on a new tech­nol­o­gy. LFA tests are based on the use of detec­tion anti­bod­ies that attach to the tar­get­ed pro­teins. “The anti­body (~10 nm) in this type of test must be cou­pled to some­thing per­cep­ti­ble to the naked eye,” explains the researcher, “some­thing larg­er and coloured, hence the use of gold nanopar­ti­cles (~50–100 nm) in com­mer­cial tests. The first step in our work was there­fore to find a sub­sti­tute with a more intense and quan­tifi­able opti­cal signal.”

The inno­va­tion: replac­ing gold nanopar­ti­cles with lumi­nes­cent nanopar­ti­cles. “By using lumi­nes­cent nanopar­ti­cles, we were able to improve the sen­si­tiv­i­ty of the test and make it quan­ti­ta­tive and more robust.” And after some research into opti­mis­ing the use of these nanopar­ti­cles, the Laboratory’s team man­aged to use 4x few­er of them than gold nanopar­ti­cles, which amounts to using 120x less detec­tion anti­body and thus con­sid­er­ably reduc­ing the cost of a test. “When analysing LFAs made with our nanopar­ti­cles with the naked eye, the low­est detectable pro­tein con­cen­tra­tion is 5x to 10x low­er,” says the researcher. “Using our opti­cal analy­sis method, made with the help of the appli­ca­tion we devel­oped for this pur­pose, we can detect con­cen­tra­tions that are 2–4 times low­er. With the two com­bined, our method can be con­sid­ered 40–50 times more sen­si­tive than the tra­di­tion­al one.”

In addi­tion to this increase in sen­si­tiv­i­ty, using lumi­nes­cent nanopar­ti­cles allows this test to become quan­ti­ta­tive, instead of qual­i­ta­tive as with gold nanopar­ti­cles. “A test is con­sid­ered accu­rate if after three rep­e­ti­tions we get the same mea­sure­ment three times,” says Fan­ny Mousseau. “How­ev­er, by accu­rate­ly deter­min­ing the quan­ti­ty of tar­get­ed pro­teins present in the body, we can, for exam­ple, deter­mine the stage of a dis­ease and fol­low its evolution.”

Two tests in one, multiplexing 

The Laboratory’s team is also work­ing on the poten­tial that this method has with regard to mul­ti­plex­ing – the pos­si­bil­i­ty of detect­ing sev­er­al pro­teins in a sin­gle test, and there­fore in a sin­gle pro­ce­dure. “In the case of cer­tain patholo­gies, it is not the evo­lu­tion of a sin­gle pro­tein that is inter­est­ing, but that of sev­er­al,” explains the researcher. For exam­ple, endo­can is a bio­mark­er of inflam­ma­tion. This pro­tein exists in two forms, ‘native’ and ‘cleaved’, and know­ing the respec­tive quan­ti­ties of these two forms may make it pos­si­ble to bet­ter define the treat­ment of cer­tain pul­monary dis­eases. To decide on the most suit­able treat­ment for the patient accord­ing to his oth­er con­di­tions, it is nec­es­sary to observe the quan­ti­ty of “native” endo­can and that of “trans­formed” endocan.

We are able to deter­mine the pres­ence of three dif­fer­ent pro­teins in one mul­ti­plexed test with an accu­ra­cy rate of 30%.

Devel­op­ing this poten­tial is still a work in progress: at the moment the research team does not con­sid­er this method to be reli­able enough for mul­ti­plex­ing. The rea­sons for this lack of reli­a­bil­i­ty are known, how­ev­er, and are due to the phe­nom­e­non of cross-reac­tiv­i­ty. “For the detec­tion of two pro­teins, two sep­a­rate detec­tion sys­tems are need­ed [one for the anti­body, anoth­er for the nanopar­ti­cles], which gives rise to the pos­si­bil­i­ty of so-called cross-reac­tiv­i­ty between the dif­fer­ent systems.”

“So far, mul­ti­plexed tests are still qual­i­ta­tive. This year, I devel­oped an inno­v­a­tive cal­i­bra­tion curve, which takes this cross-reac­tiv­i­ty into account, to try to read the results of a mul­ti­plex more accu­rate­ly. As a result, we can deter­mine the pres­ence of three dif­fer­ent pro­teins in one mul­ti­plexed test with an accu­ra­cy rate of 30%. This is a promis­ing start,” she says. The researcher is opti­mistic that clin­i­cal tri­als will begin soon. 

Virtually automated analysis

Once the tech­nique for improv­ing the sen­si­tiv­i­ty of LFA test strips had been iden­ti­fied, all the researchers had to do was devel­op an analy­sis method for these tests. “The main appli­ca­tion of our research was to replace blood sam­pling,” says Fan­ny Mousseau. With our method, the results are quick­er, but just as accu­rate. This method was devel­oped with a view to facil­i­tat­ing these analy­ses in places where access to blood tests is lim­it­ed [in war zones, in devel­op­ing coun­tries, etc.], or when they are urgent­ly need­ed [in an ambu­lance in case of a heart attack, for example].

Tool for illu­mi­nat­ing LFA tests with ultra­vi­o­let radi­a­tion to excite nanopar­ti­cles, with the mobile phone (placed on top) show­ing the inter­face of the appli­ca­tion in ques­tion. – Fan­ny Mousseau

As a result, the ongo­ing research required a tool to illu­mi­nate the nanopar­ti­cles to make it eas­i­er to read the results. “The analy­sis meth­ods required the use of huge equip­ment, which was con­tra­dic­to­ry to the portable advan­tage of this type of test,” explains the researcher. “In our lab­o­ra­to­ry, we invent­ed a small device capa­ble of per­form­ing the test. This small device, which is thick­er but not larg­er than a mobile phone (~10 cm x 5 cm x 5 cm), comes with an appli­ca­tion with which the user pho­tographs the test.”

“We have inte­grat­ed the analy­sis pro­gram. The user sim­ply takes the pic­ture and press­es the con­trol strip on the LFA strip (the strip that ver­i­fies that the test has worked). The algo­rithm, know­ing the exact dis­tance between the con­trol strip and the test strip (the strip on which our pro­tein of inter­est is detect­ed) will analyse, pix­el by pix­el, the colour inten­si­ty of the test strip.

The result is then dis­played, sim­ply indi­cat­ing the amount of tar­get pro­tein present in the test. “In the long term, the goal is to sim­pli­fy the process as much as pos­si­ble so that every­one can have access to it,” she concludes. 

Pablo Andres

Our world explained with science. Every week, in your inbox.

Get the newsletter