The long trace of the invasion
Class G antibodies appear later than others and remain in the blood for a long time - often from several months to a year or more after complete recovery. This determines both the possibilities and limitations of the analysis: it shows well that there was an encounter with Giardia, but poorly answers the question of whether there is a parasite in the intestines now.
An important practical conclusion follows from this: positive IgG in itself is not an indication for the prescription of antiparasitic drugs. The decision on treatment is made based on a combination of complaints and the results of direct methods - detection of cysts in stool or Giardia antigen using ICA.
For the same reason, it makes no sense to use IgG to monitor cure immediately after therapy: antibodies will still circulate, creating a false impression of treatment failure. Efficiency control is carried out by examining stool two to three weeks after the end of the course, and if necessary, repeat three times with an interval of several days, taking into account the intermittent release of cysts.
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.