Mycoplasma pneumoniae is a common bacterium responsible for respiratory infections, frequently leading to “walking pneumonia.” This organism causes a milder, though sometimes prolonged, form of pneumonia. Medical professionals rely on antibody tests to determine if a person has been exposed to this pathogen. The presence of specific antibodies in the blood helps determine whether a current infection is present or if the person encountered the bacterium in the past.
Understanding Mycoplasma Antibody Testing
The immune system responds to infection by producing different types of antibodies. Serology testing for Mycoplasma pneumoniae primarily focuses on two classes: Immunoglobulin M (IgM) and Immunoglobulin G (IgG). These tests track the progression of the body’s defense mechanisms against the microbe.
IgM antibodies are the first line of defense, appearing early in the infection cycle. The presence of IgM usually indicates a recent or currently active infection, making them valuable markers for acute diagnosis. IgG antibodies develop later, forming a sustained and long-term immunological memory.
The timing of antibody appearance is predictable. IgM becomes detectable within about one to two weeks of symptom onset, and IgG follows within three to four weeks. While IgM levels rise quickly and then decline, IgG levels continue to build up over time. Measuring both IgM and IgG allows clinicians to distinguish between a brand-new infection and historical exposure.
The Duration of IgG Positivity
The central difference between the two antibody types is their persistence. IgM antibodies generally begin to wane after the infection peaks, typically becoming undetectable within three to six months post-infection. The IgG response is designed for long-term protection and memory.
Following a Mycoplasma pneumoniae infection, IgG antibodies persist for a significantly longer duration, often remaining detectable for years. Evidence suggests these antibodies may remain elevated for two to nine years. The exact duration of IgG positivity varies widely among individuals, influenced by the severity of the initial infection and the person’s overall immune competence.
A positive IgG result only confirms that the immune system has previously encountered Mycoplasma pneumoniae. The continued presence of these antibodies does not mean the patient has an active disease or requires treatment. This long-term persistence is a hallmark of adaptive immunity, providing a record of past exposure.
Interpreting Persistent Positive IgG Results
A positive IgG result without a positive IgM result is generally interpreted as evidence of past infection and likely immunity. This finding indicates a history of exposure to Mycoplasma pneumoniae but not an acute illness. This pattern is common, especially in adults who have a higher likelihood of previous exposure.
A more complex scenario arises when both IgG and IgM are positive, suggesting a recent or ongoing infection. To confirm an acute infection, doctors often look for a four-fold rise in the IgG antibody titer between two blood samples taken two to four weeks apart. This rising titer indicates an active immune response.
Clinical diagnosis requires correlation with a patient’s symptoms and other laboratory findings, not solely IgG presence. In older patients experiencing reinfection, the IgM response can be low or absent. Here, a positive IgG result combined with a positive molecular test may indicate the current illness. The persistent nature of IgG makes it a marker of immunological history rather than an indicator of active disease.