A researcher demonstrating the app. Credit: Sonu Gandhi

An inexpensive device can rapidly detect the presence of SARS-CoV-2 antigen in clinical samples1. The antigen generates a colour band in the device and its intensity is measured using a smartphone app.

This detection method does not require any training and can be used by people at home and in remote rural areas, says a team at the DBT-National Institute of Animal Biotechnology in Hyderabad.

The device, which allows lateral flow of fluid, consists of a sample pad, conjugate pad, specific membrane with test and control lines, and an adsorbent pad on a backing card.

The researchers, led by Sonu Gandhi, used the receptor-binding domain (RBD) of the viral spike protein as the antigen. They generated RBD-specific antibodies which were attached to gold nanoparticles and used as a capture probe.

When a drop from a clinical sample containing RBD antigen was added to the sample pad it formed a complex with the antibody-attached nanoparticles. The complex then migrated towards the membrane and created colour bands on the test and control lines indicating the presence of the viral antigen. A band on the control line alone indicated a negative result.

The colour intensity obtained from the smartphone app was in inverse proportion to the concentration of the antigen – a lower intensity indicated a severe infection, while a higher intensity represented a milder infection.

The device can complete the test within 10 minutes, and in 100 clinical samples from COVID-19 patients it detected the viral antigen with 94.3% sensitivity and 90.9% specificity.