Blood tests for antibody to M. pneumoniae
Two types of antibodies produced in response to an M. pneumoniae infection may be measured in the blood, IgM and IgG. IgM antibodies are the first to be produced by the body in response to infection. Levels of IgM rise for a short time period and then decline, often remaining detectable in the blood for several months. IgG antibody production follows IgM production, rising over time, and then stabilizing. Once a person has had a mycoplasma infection, they will typically have some measurable amount of mycoplasma IgG antibody in their blood for the rest of their life. In order to diagnose an active M. pneumoniae infection, a doctor may order both M. pneumoniae IgM and IgG antibody tests as acute samples and then collect another M. pneumoniae IgG test two to four weeks later as a convalescent sample. This combination of tests is ordered so that the change in the amount of IgG can be evaluated and because some people, especially infants and those with compromised immune systems, may not produce expected amounts of IgG or IgM.
M. pneumoniae detection involves finding the microorganism in the respiratory secretions, blood, fluid, or tissue sample. This can be done either by culturing the mycoplasma in a supportive environment or by detecting its genetic material (DNA).
A mycoplasma culture is the traditional method of detection, but it can be challenging and is not always successful. The test involves inoculating a nutrient media with the patient's sample and incubating the culture in a specialized growth media. There are specific nutritional needs that must be met to promote the growth of the microorganisms, and they can be slow to grow. For instance, a negative M. pneumoniae culture must be held for 3-4 weeks to confirm that mycoplasma is not present. Antibody testing, or sometimes DNA testing, is usually ordered in addition to, or instead of, a M. pneumoniae culture.
DNA testing is rapid and sensitive but is not widely used in the diagnosis of mycoplasma infections. This is due, in part, to the fact that it can be difficult to distinguish between a mycoplasma that is colonizing a person from one that is causing an infection and due to the fact that mycoplasma DNA may be detectable after the symptoms of infection have resolved and the organisms are no longer viable. M. pneumoniae DNA testing may sometimes be ordered, along with other tests, such as testing for Chlamydia pneumoniae, Bordetella pertussis, and Legionella species to help distinguish between these organisms as the cause of a respiratory infection.
Occasionally, testing may be used to determine if Mycoplasma hominis, Mycoplasma genitalium or Ureaplasma urealyticum is the cause of an infection of the genital or urinary tract. M. hominis and U. urealyticum genital samples are typically tested using a culture method that takes several days to recover the microorganisms, but M. genitalium, which can take 1-2 months to grow, may be more reliably detected with DNA testing.
The choice of tests and body samples collected depends on the age of the patient, their general health status and symptoms, and on the doctor's clinical findings and suspicions of organ involvement. A person with a suspected mycoplasma infection may be treated based upon clinical findings, and imaging studies with or without laboratory testing.