When Testing Really Changes Treatment
The main practical significance of this study is the choice of antibiotic. Mycoplasma does not have a cell wall, which means it is completely insensitive to penicillins, cephalosporins and other beta-lactams, which account for the bulk of prescriptions for pneumonia. Continuation of such therapy for mycoplasma infection has no effect and prolongs the disease.
A combination of signs allows one to suspect mycoplasma: young age, gradual onset, persistent dry cough, discrepancy between scanty physical data and noticeable changes on the radiograph, lack of effect from a standard antibiotic after 48–72 hours. In this situation, an IgM test provides an answer that justifies switching to a macrolide or doxycycline.
Limitation of the method in terms of time: IgM becomes detectable approximately a week from the onset of symptoms and reaches a peak in the second or third week. A negative result in the first days of illness does not exclude infection. In adults, especially with re-infection, IgM may not be produced at all - then the diagnosis is made by the increase in IgG or by PCR.
What do IgM and IgG mean?
The immune system produces different classes of antibodies at different times. Immunoglobulins M appear first, usually in the first or second week after infection, last for several weeks or months and then disappear. Their detection indicates an acute or recent process.
Immunoglobulins G appear later, after a few weeks, and persist for years, and often for life. They mean that a meeting with the pathogen took place in the past, and in many cases indicate formed immunity. Therefore, positive IgG in itself is not a reason for treatment.
Why dynamics are important, not just a number
A single result is often ambiguous. Antibody levels vary from person to person, with borderline levels falling into what is known as a gray area. A reliable sign of active infection is a fourfold increase in IgG titer in paired sera taken at intervals of 10–14 days.
This is why serology is rarely prescribed as a single test. The doctor either combines IgM and IgG, or repeats the test, or confirms the result with a direct method - PCR, which detects the DNA or RNA of the pathogen itself.
Limitations of the method
Antibodies do not appear immediately: in the first days of the disease, serology can be negative if the infection has already begun - this is the period of the serological window. In case of immunodeficiency and against the background of immunosuppressive therapy, antibodies may not be produced at all.
Cross-reactions with related pathogens are also possible, giving a false-positive result. Therefore, treatment is never prescribed “based on analysis” - only on the basis of a combination of clinical data and research results.