We need rapid, accessible, affordable and ubiquitous tests to detect those who are actively infected and can infect others. The good news is that they are coming soon.
SARS-CoV-2 genome tests
The first type of test to detect active infection measures the presence of the virus genome. These tests measure the presence of the virus in nasal and oral secretions or in sputum samples taken from the lung. The genome of SARS-CoV-2 is RNA which can be detected using several techniques. The advantage of genome tests is that can be very sensitive. Genome tests are also specific, that is they detect only SARS-CoV-2. The disadvantage of the first generation genome tests is that the samples must be sent to a laboratory for testing and that the materials and equipment for such tests are in short supply. This means there aren’t enough tests to go around and test results are too long in coming.
The speed of viral genome tests can be improved to yield results in 5 to 15 minutes but at the sacrifice of sensitivity. The rapid genome tests now deployed may miss up to 25% of those actually infected. New CRISPR based tests to detect viral RNA may be fast but they are also less sensitive than the early laboratory based genome tests.
SARS-CoV-2 antibody tests
A second type of test measures antibodies to SARS-CoV-2. Antibodies are made by our body in reaction to the infection. Anti SARS-CoV-2 antibodies appear in the blood beginning one to two weeks after infection and may persist many months thereafter. Antibody tests are rapid, with results known within 5-20 minutes, and can be made inexpensively in large quantities. They are also easy to use, with no complex machinery needed (think over-the-counter pregnancy tests). Such tests are now available. The US Food and Drug Administration is now assuring that only tests that meet their criteria for accuracy and reliability can be sold, resolving some the early issues with these tests.
The disadvantage is that antibody tests do not measure the presence of the virus itself. They only measure the body’s reaction to the virus. Such tests will miss the early stages of infection, a time before symptoms appear when people are most likely to transmit the virus.
What is needed is a test that combines the detection of the virus itself but is cheap to make and easy to use.
SARS-CoV-2 viral protein tests (antigen tests)
Tests for the presence of viral proteins satisfy the need to detect infection before the body has a chance to make antibodies and the need for easy to make, inexpensive rapid tests, tests that can even be self-administered. SARS-CoV-12 is made of proteins that protect the genome, provide the machinery for virus reproduction, and allow the virus to attach to and enter our cells. These proteins can be detected in nasal and oral secretions and in the sputum of the infected person.
Typically, the protein detection tests employ antibodies that attach to the proteins. Such tests can be very accurate and rapid. They are also inexpensive to make and easy to use. The virus proteins are also called antigens because they cause the body to make antibodies. Such tests are often called antigen tests— not to be confused with antibody tests.
Such protein based tests are currently used for the detection in blood (usually by finger stick) of the HIV, influenza, and dengue viruses and malaria.
A potential disadvantage is the sensitivity of the tests. They may miss people who are actually infected but produce only a low level of virus. The sensitivity of such tests can be improved over time.
I am pleased that these tests are now being developed by some the biggest diagnostic companies with years of experience in manufacturing and distribution, even in low resource settings, including Roche, Quidel, and Becton Dickenson. Several smaller companies are actively developing such tests as well.
In an earlier story I estimated that the United States needs up to one billion virus detection tests per year. Others estimate the need at ten billion tests per year. SARS-CoV-2 protein detection tests are the only practical way to meet this demand. Thank goodness they are on the way.