Parvovirus B19

Parvovirus B19 testing is not generally used to test those with mild and uncomplicated infections. It is normally used to detect a current infection or check the immunity in those who are at an increased risk of complications, such as those with sickle cell anaemia. There are several methods of detecting a parvovirus B19 infection:

Antibody testing
Two types of parvovirus B19 antibodies may be produced in response to an infection: IgM and IgG. IgM antibodies are the first to be produced by the body in response to a parvovirus infection. They are present in most individuals within a week or two after the initial exposure. IgM antibody production rises for a short time period and declines. Eventually, the level of parvovirus IgM antibody usually falls below detectable levels. IgG antibodies are produced by the body a few weeks after the initial infection to provide long-term protection. Levels of IgG rise during the active infection, then stabilise as the parvovirus B19 infection disappears. Once a person has been exposed to parvovirus B19, they will have some measurable amount of IgG antibody in their blood for the rest of their life.

Parvovirus B19 antibody testing may be done to check immunity to parvovirus in pregnant women who have been exposed to someone with parvovirus B19 or who have symptoms suggestive of parvovirus infection. It may also be requested on people who have acute or chronic anaemia or persistent joint pain that may be due to a parvovirus B19 infection. By comparing the absence or presence of both IgG and IgM in the same sample, the doctor can tell the difference between current, recent, and past infections. Antibody testing is usually not done on children who have the characteristic fifth disease rashes and, since parvovirus B19 infection is widespread and causes few problems to those with healthy immune systems, general population screening is rarely done.

Viral detection
Viral detection involves finding parvovirus B19 genetic material (DNA) in a blood sample or, less commonly, in a  bone marrow sample. Parvovirus B19 DNA testing is performed primarily to detect active parvovirus infection in immune-compromised people who have acute or persistent anaemia. These people will frequently not produce a sufficient amount of antibodies to detect or to resolve the infection.

Parvovirus B19 testing is usually not required when a child has the characteristic "slapped cheek" and lacy rashes. The distinctive rashes are sufficient evidence for the doctor to be able to diagnose the infection.

Parvovirus B19  IgG and IgM  antibody tests may be requested when a pregnant female has flu-like symptoms and/or has been exposed to someone with a parvovirus B19 infection to determine if she has an active infection, had a recent infection, or has been exposed in the past. Antibody testing and/or DNA testing may be requested when someone has acute or persistent anaemia or joint pain that the doctor suspects may be due to a parvovirus B19 infection. Parvovirus B19 DNA testing is usually requested when the affected person is immune-compromised .

One or more parvovirus B19 tests may be repeated if they are initially negative, but the doctor still suspects that a parvovirus infection is present, or to evaluate changes in concentrations of antibody over time.

Antibody testing
If both parvovirus B19 IgG and IgM are present, then it is likely that the person tested has an active, or had a recent, parvovirus infection. This can be confirmed by measuring IgG levels again 2 or 3 weeks later. A high level of IgG is not as important as an increasing concentration of antibody. If there is a 4-fold increase in IgG between the first and second sample, then the patient has an active, or had a recent, infection.

If only IgM is present, then the patient may have been infected very recently. If only parvovirus B19 IgG is present, then the person had a parvovirus infection at some time in the past and has protection against the virus.

If antibody tests are negative, then the person tested has not had the infection and is not immune. If a pregnant woman is not immune and has been exposed to someone with the disease, she will typically be closely monitored by her doctor.

If a person is symptomatic but has low or undetectable levels of IgG and/or IgM, it may mean that they either have a condition other than parvovirus B19 or that their immune system is not responding normally – not producing an adequate amount of antibody even if parvovirus is present. This may be true for immunocompromised . people, and DNA testing may need to be done.

Viral detection
If a parvovirus B19 DNA test is positive, then the person is currently infected with parvovirus B19. A negative result does not rule out the infection. The virus may not be present in sufficient amount in the sample to be detected.

Sometimes a reticulocyte test may be performed along with parvovirus B19 testing to evaluate red blood cell (RBC) production. This test measures new immature RBCs in the blood that still contain genetic material. Since parvovirus B19 disrupts RBC production, the number of reticulocytes will decrease during an active infection.

Parvovirus B19 infections are usually self-limiting in otherwise healthy people. Treatment may be given to relieve symptoms and, when necessary, to address anaemia.

Pregnant women who pass parvovirus B19 infection to their foetus will be closely monitored for several weeks with periodic ultrasounds. In most cases, the fetus will be healthy. When hydrops fetalis or severe anaemia develops, it can sometimes be addressed by giving the foetus a blood transfusion. The parvovirus B19 infection is not known to cause birth defects.