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BackgroundThe MODS is an important assay for early diagnosis of tuberculosis and drug susceptibility. MODS is based in the microscopic observation, underneath, of the characteristic cords of Mycobacterium tuberculosis colonies grown in liquid media. An inverted optical microscope is required to observe and interpret MODS cultures. Unfortunately, the cost of commercial inverted microscopes is not affordable in low resource settings in developing countries.MethodologyTo perform a diagnosis of tuberculosis using the MODS assay, images with modest quality are enough for proper interpretation. Therefore, the use of a high cost commercial inverted optical microscope is not indispensable. In this study, we designed a prototype of an optical inverted microscope created with a 3D printer and based on a smartphone. The system was evaluated by comparison of manual interpretations of 226 TB positive MODS culture images and 207 negative MODS culture images.SignificanceThe prototype resulted in a low-cost inverted optical microscope, with simple functioning, and whose parts have been manufactured using 3D printing techniques. The quality of the images was good enough and achieved a 100% concordance between the manual inspection with the developed microscope, and the standard diagnostics of MODS.