A blood test, combined with a standard questionnaire and an artificial intelligence tool, could help change the way leprosy is diagnosed in Brazil.
The strategy was tested by researchers at the University of São Paulo using blood samples collected during a Covid-19 population survey and showed potential for identifying the disease earlier, in the initial stages, when symptoms are still subtle and traditional laboratory tests tend to fail.
The new diagnostic method was evaluated in a study conducted by researchers from the Department of Clinical Medicine, Biochemistry, Immunology and Social Medicine at FMRP-USP (Faculty of Medicine of Ribeirão Preto, University of São Paulo), with FAPESP. Coordinated by the researcher, the work was in the magazine BMC Infectious Diseases.
“Leprosy is an ancient disease, but it still faces challenges typical of underprioritized health problems. There is still a lack of sensitive laboratory technologies for early diagnosis and many health professionals are not properly prepared to recognize the initial forms of the disease”says the biomedical scientist, one of the authors of the study.
Furthermore, the standard treatment currently used is basically the same for more than 4 decades, which contributes to cases of therapeutic failure and bacterial resistance.
It was precisely to address this bottleneck that researchers sought to identify new biomarkers and tests for early diagnosis. To do this, they used blood samples collected during a serological survey carried out during the covid-19 pandemic in Ribeirão Preto. The idea was to use this existing material to identify possible people exposed to the leprosy bacillus and, in this way, detect new cases earlier.
SCREENING
The study combined two screening tools. The 1st was the application of a clinical questionnaire on suspicion of leprosy, called “QSH”, which consists of 14 questions focused mainly on neurological signs and symptoms. This questionnaire was enhanced with an artificial intelligence system called “MaLeSQs”.
The second tool used by the researchers was to perform a blood test that detects the presence of antibodies against the Mce1A antigen, a key protein in the Mycobacterium lepraewhich facilitates the invasion and survival of bacteria in human cells. Currently, the antigen used in blood tests is PGL-I, a molecule that also facilitates the entry of bacteria into the nerve. The conventional method is less technically sensitive.
“Unlike the traditional test [anti-PGL-I]which assesses the presence of only one type of antibody, the new test [anti-Mce1A] analyzes 3 different classes of antibodies [IgA, IgM e IgG]which increases sensitivity and helps differentiate exposure to the bacillus, active infection and previous contact”says Lima.
According to the researcher, the traditional test only tends to be positive in the most serious forms of the disease, when the bacillus has already proliferated and the lesions already exist. “Mce1A makes it possible to identify contact with the bacillus and the active disease much earlier”it says.
INVITATION, QUESTIONNAIRE AND EXAMINATION
To reach the results, the researchers invited the approximately 700 people included in the population survey on Covid-19 to join the study on leprosy. In total, 224 agreed to participate and responded to the digital questionnaire and 195 had blood samples analyzed. All of them were invited to undergo a face-to-face clinical evaluation with specialist doctors, a fundamental step for diagnostic confirmation.
Of these, 37 attended the in-person consultation. When crossing data from the questionnaire, examination and clinical evaluation, the result caught our attention: 12 new cases of leprosy were diagnosed, equivalent to around a third of the individuals evaluated. “These are people who had no obvious symptoms, did not suspect they were sick and were diagnosed thanks to the project”says Lima.
According to the researcher, among the laboratory tests, the IgM antibody against the Mce1A antigen showed the best performance, identifying 2/3 of the new confirmed cases. When the researchers combined laboratory analysis with the artificial intelligence tool, the method reached 100% sensitivity, that is, it was able to flag all suspected cases of leprosy, confirmed in the face-to-face consultation.
“The blood test alone does not confirm the diagnosis of leprosy, but it is an important tool to indicate who really needs to be evaluated by a specialist”says the researcher. According to Lima, the test can strengthen diagnostic screening in the public health network and, in terms of cost, the difference in relation to tests already used is minimal.
“From a laboratory point of view, they are very similar techniques, low cost and easy to perform. Any clinical analysis laboratory has the technical capacity to carry them out. In practice, what changes is only the molecule analyzed”.
In addition to the early diagnosis of leprosy, the study also used a georeferencing map and analyzed the spatial distribution of identified cases. Mapping revealed a diffuse pattern of exposure to the bacillus.
“This can be justified because we were unable to clinically evaluate all participants. But our results show that leprosy is randomly distributed in the city, there is no specific region with the highest concentration. Today we see the disease diagnosed in patients of different socioeconomic profiles”says Lima.
PUBLIC HEALTH PROBLEM
Leprosy is an infectious disease that mainly affects the skin and peripheral nerves, which can cause light or reddish spots, loss of sensitivity and muscle weakness. According to the World Health Organization, more than 200,000 new cases are registered annually, especially in low- and middle-income countries.
Brazil ranks 2nd globally in number of cases, behind only India, and accounts for around 90% of notifications from the Americas. Among the most common symptoms are tingling, cramps, numbness and areas of the skin with reduced sensitivity.
At this initial stage, the most commonly used laboratory tests, such as sputum smear microscopy – which attempts to directly identify the bacteria on the skin – tend to be negative because the bacterial load is still very low. “More than 60% of our patients can have negative tests, even when they are sick”says Lima.
Treatment involves the use of antibiotics for periods of 6 months to a year, depending on the stage of the disease, which is currently classified as a DSD (socially determined disease), a term that has been replacing the expression “neglected disease” not Brazil.
The next step is to advance the validation of these tools for large-scale use, with the aim of incorporating them into the SUS and primary care. At the same time, the researcher is working on a new stage of the project aimed at increasing the specificity of the Mce1A marker. “Today, the test uses the entire protein of the bacteria. Now we are studying small parts of this same protein to evaluate whether it is possible to develop an even more sensitive and more accurate test”he states.
This text was originally published by on April 8, 2026 and adapted for publication by Poder360.
