What Causes a False-Positive Chagas Disease Blood Test?

It is possible for a blood test checking for chronic Chagas disease to be falsely positive. A false positive means the test indicates you have the infection when you actually do not. The most common cause for a false-positive Chagas test is a phenomenon called “cross-reactivity.” This happens if you have been exposed to other, similar parasitic infections—most notably Leishmaniasis ^[1][2]. Other harmless parasites, unrelated infections (like malaria or syphilis), or even certain autoimmune conditions can also sometimes trigger false positives ^[1][3]. Because of this risk, standard medical guidelines require at least two different types of blood tests to confirm a chronic Chagas diagnosis, and sometimes a specialized third test if the first two disagree ^[4][5].

(Note: This guide specifically covers testing during the chronic phase of Chagas disease. During the acute phase right after infection, doctors typically use different methods, like looking for the parasite directly under a microscope.)

Why Do False Positives Happen?

During the chronic phase of Chagas disease, the parasite burden in the blood is very low ^[6]. Therefore, doctors diagnose the infection by looking for antibodies—proteins your immune system makes to fight off the Trypanosoma cruzi parasite ^[7]. Serological blood tests are designed to detect these specific antibodies ^[4].

However, Trypanosoma cruzi belongs to a family of parasites that share very similar physical features and proteins ^[1]. If you have been exposed to a related parasite, such as the Leishmania species (which causes Leishmaniasis), your immune system creates antibodies for that specific infection. Because the proteins in Leishmania and Trypanosoma cruzi look so much alike, a Chagas test might accidentally mistake the Leishmania antibodies for Chagas antibodies, triggering a positive result ^[1][3].

The likelihood of a false positive can also depend on the exact type of test used:

The “Two-Test” Rule for Diagnosis

Because no single blood test is perfectly accurate in every situation, the World Health Organization (WHO) gold standard for diagnosing chronic Chagas disease requires two different serological tests ^[4][5].

Relying on just one test can lead to misdiagnosis due to the limitations in how sensitive or specific a single test might be ^[4]. By using two different testing methods (for example, one whole-cell test and one recombinant test), doctors can cross-check the results. If both tests come back positive, the diagnosis of chronic Chagas disease is confirmed.

The “Tie-Breaker” Test

Sometimes, the results of the first two tests are discordant, meaning one test comes back positive and the other comes back negative ^[5]. When this happens, a third, entirely different “tie-breaker” test is required to resolve the uncertainty ^[5][4].

This third test is often a highly specialized assay that must be processed by a reference laboratory, such as the Centers for Disease Control and Prevention (CDC). Waiting for a third test result is completely normal and does not represent a dangerous delay in your care. The chronic phase of Chagas disease progresses very slowly over decades, so taking the time to get an accurate diagnosis is the safest approach. This careful, multi-step protocol ensures that patients are not incorrectly diagnosed with—or treated for—Chagas disease due to a simple false-positive result.