Mycoplasma testing is primarily used to help determine if Mycoplasma pneumoniae is the cause of a respiratory tract infection. It may also be used to help diagnose a systemic infection that is thought to be due to mycoplasma.

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. Alternatively, in many laboratories the total antibodies produced may be measured together. 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 stabilising. 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 (initial) samples and then collect another M. pneumoniae IgG test two to four weeks later as a convalescent (subsequent) sample. This combination of tests is ordered so that the change in the amount of IgG antibody can be evaluated and because some people, especially infants and those with compromised immune systems, may not produce expected amounts of IgG or IgM.

Direct detection
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 for the laboratory and therefore is not often performed. 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 three to four weeks to confirm that mycoplasma is not present. Antibody testing or DNA testing, is usually ordered in addition to, or instead of, a M. pneumoniae culture.

DNA testing by polymerase chain reaction (PCR) is rapid and sensitive and is increasingly used in the diagnosis of mycoplasma infections. 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, DNA 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 one to two months to grow, is 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.

M. pneumoniae testing may be ordered when someone has respiratory symptoms that are not due to a typical bacterial infection, such as pneumococcal pneumonia. Some of these symptoms may include:

• nonproductive cough that may persist for several weeks
• fever
• sore throat
• headaches and muscle aches

Testing may also be done in order to find the cause of rash, arthritis, encephalitis, inflammation of the heart muscle or the lining that surrounds the heart or haemolytic anaemia.  It may also be ordered to help track and control the spread of M. pneumoniae infections during an outbreak.

In general, IgM and IgG testing is performed when a doctor suspects that someone has an active M. pneumoniae infection and another IgG test can be performed two to four weeks later to document a rise in antibody levels in response to an infection. A M. pneumoniae culture or a DNA test may also be ordered when an active infection is suspected.

Testing of genital samples is not often done because mycoplasmas are frequently part of the normal flora of the genital tract. However, a culture for M. hominis and U. urealyticum or a test for M. genetalium DNA is sometimes ordered when a sexually active man has inflammation of the urethra that is not due to gonorrhoea or chlamydia (non-gonococcal urethritis, NGU) or when a woman is suspected of having a genital or pelvic mycoplasma infection.

Mycoplasmas are frequent causes of infection of the aminotic fluid during pregnancy. This can lead to early labour, fever after birth and infections in the newborn. Testing for other species of mycoplasma may be performed, in addition to M. pneumoniae testing, when very young infants and those with compromised immune systems have lung and/or systemic infections or complications that could be due to a mycoplasma infection.

Antibody testing
Significant concentrations of M. pneumoniae IgM and/or a four-fold increase in IgG levels between the initial sample and the convalescent sample indicate an active or recent M. pneumoniae infection. Increases in IgG, without IgM, can also be seen with a re-infection.

If neither IgM nor IgG are present in detectable concentrations, it either means that a person does not have an active infection, has not had a mycoplasma infection (recent or in the past), or that the person's immune system has not produced antibodies in response to the microorganism.

Direct detection
The detection of one of the mycoplasmas or U. urealyticum in a cultured sample may indicate that someone has a mycoplasma infection, particularly if the sample is from a location in the body that is normally sterile, such as joint fluid or blood. However, if the sample is from the respiratory tract or the genital tract, a positive culture may also mean that the mycoplasma is present as part of the normal flora. For example, U. urealyticum is present in the genital tract of about 60 per cent of healthy women and M. hominis is present in about 20 per cent.

If mycoplasma is not detected in a culture, it may mean that the person concerned is not infected by that microorganism or that the organism was not present in sufficient quantity to be detected in the sample tested.

With DNA testing for M. pneumoniae, if the mycoplasma is present in the sample, the person may have M. pneumoniae or may be colonised by the organism. If it is not detected, they may not have a M. pneumoniae infection or else the microorganism was present in numbers too low to be detected.

Mycoplasma infections often cause symptoms that resemble viral infections, but they respond to antibiotic treatment.

Having a mycoplasma infection does not confer immunity. A person can become re-infected.

Mycoplasmas cannot be seen under the microscope on a Gram stain - a test that is often used to help identify bacteria.

An older test called cold agglutinins may sometimes be ordered to help detect a M. pneumoniae infection. It is based on the concept that during an active mycoplasma infection an antibody is produced in the blood that will cause red blood cells to clump together when cooled. This test is not specific for mycoplasma but more than half of those with a M. pneumoniae infection will have significant amounts of cold agglutinins.

They are a common but often unidentified cause of respiratory infections. Like the viruses that cause the common cold they tend to cause mild to moderate, nonspecific cold symptoms in most people, and in most cases they are self-limited, resolving without treatment or with prescribed antibiotics.

Mycoplasmas are very common in the environment, and it is not always possible to prevent infection. Those caused by outbreaks of Mycoplasma pneumoniae are transmitted through respiratory droplets and can be avoided through good hand washing, covering the nose and mouth when coughing or sneezing, and avoiding close contact with sick people. Mycoplasmas that are passed through sexual contact can be prevented in the same manner as other sexually transmitted diseases (STDs). Those passed from mother to baby are difficult to predict or prevent.