Interpreting Diagnostic Tests for the Novel Coronavirus

A recent article in the Journal of the American Medical Association (JAMA) helps us interpret the tests done in Covid-19 illness.  

Thus far, the most reliable test for the diagnosis of Covid-19 illness has been the RT-PCR test of the nasal cavity secretions taken with a swab. This test can also be performed on swabs taken from the throat, the saliva, the sputum and even from stools.

However, the results of secretions from the nasal cavity are most accurate and reliable.

RT-PCR (reverse transcriptase polymerase chain reaction) is a genomic test that looks for the membrane or the nuclear proteins of the virus. To identify the presence of the virus from the tiny amount of the viral proteins picked up on the swab, the machine in the laboratory amplifies the genomic content by 20-60 or even 100-fold. Of course, if the viral load (the amount of virus and viral proteins) on the swab is high, then less amplification is needed for the RT-PCR test and vice versa.

The journal article was accompanied with the following drawing that shows when the tests become positive after someone has been exposed to the virus.

The broken vertical line defines the time a person develops the symptoms of Covid-19. As is well known, it takes about 14 days to develop symptoms after being exposed to the virus (shown on the left side of the broken line as week -2 and week -1). Let us focus on the solid blue line which represents the RT-PCR test done from nasal secretions.

The graph shows that the test starts to become positive one week after the exposure. This is the time the individual does not have any symptoms and does not realize that the virus has taken hold and can unknowingly be transmitted to others.

As the curve of the solid blue line shows, the positivity of RT-PCR test increases to a peak when the symptoms appear, remains high for two weeks or so and then declines over the next 3-4 weeks. In rare cases the test has remained positive for up to 6 weeks after symptom onset indicating that the individual is still shedding the virus.

The length of time the RT-PCR test remains positive depends on the amount of virus in the secretions also called the viral load. The positivity of the RT-PCR test also depends on other factors such as accuracy of obtaining the nasal swab and if the swab was inserted deep in the nasal cavity, the way the specimen was handled and the reagents  and quality control of the laboratory that processes the specimen.

Whereas the RT-PCR test detects active viral infection, the antibody tests are designed to detect if someone has already had the infection and may have developed immunity.

The broken green and purple lines in the graph represent the antibody production by the immune system. In people with normal immune system, the immune response (antibody production) starts by week 2 of the illness.

Many laboratories are rushing to develop different types of antibody tests. Not all of the antibody tests have been validated by the FDA.

The most accurate tests look for antibodies to the most abundant protein in the virus called the “nucleocapsid” rather than antibodies to different parts of the virus such as the capsule or the spikes on the virus.

Certain types of immune proteins (antibodies) are called the neutralizing antibodies. These can be specifically extracted from the plasma of people who have recovered from Covid-19. The “neutralizing antibodies” can be used to “neutralize the virus” in the early phase of infection, just as the immune protein given to those who may have been stuck with a needle contaminated with the blood or body fluids of hepatitis-B or HIV positive individual.

The neutralizing antibodies are not be confused with the convalescent plasma, that is used as “prophylaxis” to prevent high-risk front-line workers from getting infected in the first place.

Source: Journal of American Medical Association. June 9, 2020. Vol: 323, No: 22, Page:2249