Mycoplasma tests

No special preparation is needed for most mycoplasma tests. The sample collected depends on the type of infection being investigated. 

For a respiratory infection, the healthcare professional will take one of the following: 

• a blood sample from a vein in your arm (for antibody testing) 
• a throat swab 
• a sputum (phlegm) sample — you will be asked to cough up a sample into a pot 

For a genital infection, the sample may be: 

• a urine sample — you will be asked to provide a sample in a pot 
• a swab of the cervix or urethra (the tube that carries urine out of the body) 

Tell your doctor about any antibiotics you have recently taken, as these can affect test results. 

Mycoplasma bacteria are unusual. They are the smallest free-living bacteria known. Unlike most bacteria, they have no cell wall. This means they cannot be seen with a standard Gram stain test (a common method for identifying bacteria). It also means they do not respond to some antibiotics, such as penicillin. When M. pneumoniae infects you, it can cause symptoms that feel more like a viral illness than a typical bacterial chest infection. 

There are three ways laboratories can detect mycoplasma bacteria, and different tests may be used depending on your symptoms, the type of infection, and your local laboratory:

When you are infected with M. pneumoniae, your immune system produces proteins called antibodies. Two types are measured: 

• IgM antibodies — these appear first, usually within one to two weeks of infection. They rise quickly and then fall but can remain detectable for several months. 
• IgG antibodies — these appear later and tend to stay in the blood for life. A large rise in IgG between two samples taken two to four weeks apart can confirm an active or recent infection. 

PCR (polymerase chain reaction) tests detect the genetic material (DNA) of the bacteria directly in your sample. This is a faster and more sensitive method than culture. PCR is now the most commonly used test for M. genitalium in UK sexual health clinics and is increasingly used for M. pneumoniae respiratory infections.

Some laboratories still use culture — placing the sample in a nutrient-rich environment to see if bacteria grow. This method is reliable but slow, taking several days to weeks. Culture remains useful for some species, particularly those where PCR testing is not routinely available. 

Your results will be interpreted alongside your symptoms and clinical history. A positive result alone does not always confirm the cause of your symptoms — your doctor will consider everything together. 

A positive IgM result, or a significant rise in IgG between two samples taken two to four weeks apart, suggests an active or recent infection with M. pneumoniae. 

IgG antibodies without IgM usually indicate previous exposure. However, in some people with reinfection, IgM may be absent and diagnosis relies on PCR or demonstrating a significant rise in IgG between paired samples. If neither IgM nor IgG is detected, this suggests either no current infection, no previous exposure, or — in some people with weakened immune systems or very young infants — a reduced ability to produce antibodies even when infected.

A positive PCR result means DNA from M. pneumoniae or M. genitalium was found in your sample.. This strongly supports an active infection, especially if you have matching symptoms, however, clinical correlation remains important. Occasionally, low-level DNA can be detected without causing active disease, so your doctor will always consider your results in context. 

A negative PCR result means bacteria were not found at a detectable level. This usually means you do not have a mycoplasma infection, though a very early infection may occasionally produce a negative result. 

A positive culture means living mycoplasma bacteria were grown from your sample. This is a reliable finding when the sample comes from a normally sterile body site such as blood or joint fluid. For samples from the respiratory or genital tract, the result is interpreted alongside your symptoms, as these bacteria can sometimes be found in people without active infection. 

A negative culture does not always rule out infection — bacteria may not have been present in sufficient numbers in the sample tested, or may have been affected by antibiotic treatment taken before the sample was collected.

If your test is positive, your doctor will prescribe an antibiotic. Mycoplasma bacteria do not respond to penicillin-based antibiotics. Commonly used antibiotics include doxycycline, azithromycin or a fluoroquinolone, depending on the type of infection and local guidance. 

For M. genitalium genital infections, antibiotic resistance is an increasing concern in the UK. Your doctor may carry out a resistance test (using your PCR sample) before prescribing or if treatment does not work. This helps ensure you receive an antibiotic that will be effective. 

If your test is negative but your doctor still suspects mycoplasma, treatment based on clinical findings may still be recommended. A follow-up blood sample for antibody testing may be requested two to four weeks later to look for a rise in IgG antibody levels. 

Mycoplasma infections usually improve with the correct antibiotic. Symptoms may take time to fully resolve even after treatment begins. If symptoms do not improve, tell your doctor so that alternative causes or antibiotic resistance can be investigated. 

• Antibiotic treatment before testing — if you have taken antibiotics before your sample was collected, this can reduce the number of bacteria in the sample and make culture or PCR less likely to detect an infection. 
• Early infection — antibodies may not be detectable in the first few days of illness. IgM antibodies usually appear within one to two weeks of infection. If the first antibody test is negative but symptoms persist, a second test is recommended. 
• Weakened immune system — people with a weakened immune system, including very young infants, may not produce detectable antibody levels even during an active infection. PCR or culture may be more reliable in this group. 
• Previous infection — IgG antibodies can remain in the blood for life. A positive IgG result alone does not confirm a current infection. 
• Laboratory method — different laboratories use different methods. Results should ideally be compared within the same laboratory. 
• Sample quality and timing — the sample type, how it was collected, and how quickly it reached the laboratory can all affect whether bacteria are detectable. 

• Chlamydia test — often requested alongside mycoplasma testing for genital infections 
• Gonorrhoea test — to rule out another common cause of genital infection 
• Influenza tests — to distinguish flu from other respiratory infections 
• RSV (respiratory syncytial virus) — another cause of respiratory illness, particularly in young children 
• Sputum culture — to look for other bacterial causes of a chest infection 

Mycoplasma pneumoniae causes respiratory infections ranging from a mild cold-like illness to walking pneumonia. Mycoplasma genitalium and related bacteria cause genital tract infections in adults. Most infections respond well to appropriate antibiotic treatment. For more information, visit UKHSA: Mycoplasma pneumoniae or speak to your GP or sexual health clinic.