After the patient has been treated, how long will the Giardia antigen test remain positive?
It has been recommended that you test about 7 days after therapy, hopefully to avoid picking up residual antigen. However, if you wait too long (several weeks), you always run the risk of picking up antigen from a possible reinfection. Some also feel that low antigen levels can be found for up to about 2 weeks. A good time frame for retesting would probably be about 7-10 days after therapy. If the first specimen at 7 days is still positive, then you could retest at 10-12 days. Also, we know that the immunoassays (for diagnosis) may not pick up low antigen loads (organism shedding issues), thus the recommendation for diagnostic immunoassay testing for giardiasis has been changed and recommends performing immunoassay testing on one additional stool specimen (if the first one is negative). The testing on two different stool specimens should be performed within about 3-5 days.
