In simple words: This test checks if a specific type of bacteria (mycobacteria) is resistant to a certain antibiotic. A lab technician grows the bacteria on a plate with and without the antibiotic. By comparing the growth, they can tell if the antibiotic is effective.

The lab analyst performs the technical steps to evaluate a mycobacteria culture isolate’s susceptibility, or sensitivity, to an antimicrobial agent (commonly called an antibiotic) using the proportion method. Following one or more separate procedures to culture and isolate a mycobacterium from the patient specimen, the lab analyst performs the technical steps of a proportion susceptibility study to determine if the mycobacterium is resistant to a specific antibiotic.The lab analyst may carry out steps such as preparing media (e.g., an agar plate containing the antimicrobial agent and an agar plate without the agent) and inoculating the plates with a specific dilution of the mycobacterium culture isolate. After incubating the growth media for a specific period, the lab analyst counts the colonies on the plates with and without the antimicrobial agent and calculates the ratio of resistant colonies on the antibiotic medium to colonies on the nutrient medium. The lab analyst compares the test ratio to an established ratio threshold for the specific antimicrobial agent (such as 1 percent for rifampicin) and reports the isolate as sensitive or resistant to the antimicrobial agent tested. Although not limited to use for a specific condition, clinicians may order this test to aid in the selection of an appropriate antibiotic to treat a mycobacterial infection such as tuberculosis (TB). Clinicians often continue to culture, isolate, and perform this test as long as the patient has an active TB infection because of the need to provide accurate TB drug resistance data for epidemiological purposes (the tracking of infectious diseases).

Example 1: A patient presents with suspected tuberculosis. A sputum sample is collected and sent to the lab for mycobacterial culture and sensitivity testing using the proportion method (87190) to guide antibiotic treatment., A patient with a known mycobacterial infection is not responding to the current antibiotic regimen. A culture is taken from the infected site, and 87190 is performed to identify potential antibiotic resistance and guide treatment adjustments., A patient with a history of multi-drug resistant tuberculosis is being monitored. Regular sputum samples are collected and tested using 87190 to track the development of any new antibiotic resistances.

Documentation should include the patient's medical history, the source of the specimen, the specific antimicrobial agent(s) tested, the results of the susceptibility study, and the interpretation of the results.