How to interpret a positive IgG
Respiratory chlamydia is very widespread: by adulthood, IgG antibodies are found in a significant proportion of people, most of whom had a mild infection and did not even notice it. Therefore, an isolated positive IgG in itself is not a reason to prescribe antibiotics.
The level and its change over time have diagnostic significance. A fourfold increase in titer in paired sera taken at intervals of two to four weeks indicates an active process. A persistently high titer in combination with a prolonged cough, recurrent bronchitis, or poorly controlled asthma may indicate a persistent infection.
The connection of Chlamydia pneumoniae with chronic diseases - bronchial asthma, chronic obstructive pulmonary disease, atherosclerosis - has been discussed for a long time, but there is no clear evidence of the benefit of treatment in asymptomatic carriers. The decision on therapy is made by the doctor based on the clinical picture, and not on the number on the form.
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.