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Giardia tests: decoding
Last reviewed: 05.07.2025
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Suspicion of parasitic infection arises in the presence of persistent dyspeptic disorders. Lambliasis is a protozoan infection caused by the smallest intestinal parasites lamblia or giardia. The disease has been well studied, modern methods of its treatment give a 100% cure rate. The main thing is timely diagnosis of infection with these parasites, and the doctor can get the main information from the results of laboratory tests for the presence of lambliasis in the patient.
[ Preparation for Giardia Testing
A negative test result does not mean that there are no parasites. Tests for the presence of Giardia are usually taken repeatedly. Often, the patient himself is to blame for a dubious result. Following a few simple rules before taking the test will make the test results more accurate. Therefore, how to take a test for Giardia so as not to waste time?
If you intend to take tests, you should stop taking antihelminthic drugs at least seven days before the scheduled date, as well as drugs that can harm parasites: antibacterial (metronidazole, trichrpol), antacids (smecta). It is advisable, if possible, not to take any medications at all.
A blood test for antibodies to lamblia (enzyme immunoassay) is taken on an empty stomach in the morning from a vein. Before taking blood, you must stop eating and drinking anything except pure still water 10 hours before.
The stool for analysis is collected from the liquid fraction in six to seven places and sealed in a sterile container with a hermetic lid. It is advisable to collect it in the morning and deliver it to the laboratory as quickly as possible within two hours. If the stool is delivered within 20 minutes, then in this case the laboratory technician will be able to detect vegetative forms of parasites. Within 12 hours - cysts formed by them. A longer delivery time reduces the information content of the analysis. However, medical personnel often require that warm stool be delivered for analysis. This requirement is problematic to fulfill and it is illegal, no more than 12 hours should pass from the moment of defecation until the laboratory receives the analysis. You can ask the laboratory for a container with a preservative, which will extend the shelf life of the material.
The stool for antigen is collected in a sterile container, preferably delivered immediately to the laboratory. If this is not possible, the container can be stored at a temperature of 2-4°C (in the refrigerator) for one or two days. A single deep freeze (-20ºC) is also acceptable if longer storage is expected. The material for the study is collected within the first week from the onset of clinical symptoms.
The easiest way to answer the question "Where can I get tested for giardia?" is with a doctor who will write out a referral for the test. This test is done in all outpatient, hospital and commercial laboratories.
Another question patients often ask is: how long does it take to prepare a lamblia test? A stool test is prepared fairly quickly, as soon as the sample reaches the microscope. The result just needs to be processed, usually it is ready in the afternoon of the day the test is taken.
The blood test for Giardia antibodies will be ready in two days, the stool test for antigen - in a day. The PCR test result for Giardia is ready in 4-6 hours.
Comparative effectiveness of diagnostic methods
A blood test for antibodies to Giardia is an indirect diagnostic method, since they are formed in the blood as an immune response to parasitic invasion. They do not appear immediately, but within two or sometimes three weeks, so in the first days after infection the result will be false negative. The presence of immunoglobulins of class M indicates a recent infection, but they do not exist for long and are replaced by IgG, the presence of which confirms that the invasion took place. But they can persist for a long time after recovery and do not indicate the need for treatment, so they are not suitable for monitoring its effectiveness.
The indicator of the presence of antibodies depends on many factors, in particular, the state of the immune system and the intensity of infection. With low immunity or chronic recurrent giardiasis with a persistent course, antibodies may not be detected. False positive test results also occur in the case of invasion of other protozoa, for example, amebiasis, antibodies to which can be mistaken for antibodies to giardia.
A stool test for Giardia is more reliable. It involves examining a stool sample taken from several places under a microscope and visually searching for live specimens or their cysts. If the result is positive, then parasites are definitely present. A negative result does not mean that the patient is healthy. It is just that in the development of parasites there are so-called "blind" periods, their duration is 1-17 days, during which cysts are not excreted. If the analysis is collected and submitted during this period, the result will be negative even if parasites are present. Therefore, it is recommended to submit a stool test for Giardia cysts at least three times every three days. If the suspicion of the presence of parasites is serious, then the stool is monitored for a month or a month and a half, every week.
The two above blood and stool tests are the most commonly used, as most labs perform them. Both blood and stool tests are usually ordered. If one of the tests is positive, then an infestation can be concluded.
Giardia antigen analysis is more informative than stool microscopy, but its availability is limited, since it is performed only in large cities and not by all commercial laboratories. The study of the material is based on a one-stage immunochromatographic method, which allows detecting specific molecules (GSA-65 antigens) that are found exclusively in Giardia cells. Stool samples are mainly taken for the study, but sometimes a biopsy is used. The immunochromatographic method allows detecting Giardia even in the "blind" period. It is advisable to use it to monitor recovery, but after a two-week interval after discontinuing the medication, since the antigen can still be released during this time period.
The most informative method for detecting parasites at present is the PCR analysis for Giardia. Its main drawback is the lack of prevalence. It is not performed in almost any laboratory, even in large cities. This study allows, using the polymerase chain reaction, to determine fragments of Giardia deoxyribonuclease in feces even in the interval when cysts are not excreted. The accuracy of this analysis is the highest (up to 98%).
Deciphering the values of the analysis for giardia
The results obtained in determining serum antibodies to Giardia are compared with reference values. The lower limit of the norm for maintaining antibody activity is fixed at a ratio of 1:100. Immunoglobulin activity of less than 1:100 is assessed as a negative result. If the activity exceeds this ratio, a conclusion is made about the presence of giardiasis. An antibody titer of exactly 1:100 is interpreted as ambiguous. It is recommended to repeat the analysis, as well as the analysis of feces for giardia cysts.
The IgM positivity coefficient is greater than 1 and less than 2, indicating the level of antibody concentration in the blood serum, in combination with the absence of IgG, is diagnosed as the initial stage of giardiasis.
An immunoglobulin M positivity ratio of two in combination with the detection of cysts on stool microscopy is usually interpreted as acute giardiasis.
If cysts are detected in a stool analysis, there is no IgM, and the IgG concentration level is 1-2, chronic giardiasis is diagnosed.
A positive test for IgG in itself raises doubts about the unambiguous presence of Giardia in the body, since immunoglobulin class G is detected in the blood serum for another six months after recovery.
The norms for stool analysis for lamblia are the absence of any form of parasites. If there are living individuals or their cysts, the analysis is considered positive.
A negative result of the stool test for lamblia antigen should most likely indicate their absence. However, it can also be with a small number of cysts in the stool, and a repeated test can give a positive result. Therefore, if there are clinical manifestations, the test is repeated. A positive result indicates infection or carriage.
PCR analysis for Giardia can be positive when detecting the DNA of these parasites in biological material and negative when they are absent.
None of the above tests by themselves provide a 100% result, therefore, diagnostic interpretation is based on a comprehensive analysis of all available examination data.