Lowest concentration of an anti-TB medicines in vitro that will inhibit the growth of 99% of phenotypically wild type strains of M. tuberculosis complex.

Lowest concentration of a drug that inhibits growth of 95% of M. tuberculosis strains isolated from patients who have never been treated with/exposed to that drug (i.e. presumably susceptible isolates), while at the same time not inhibiting growth of strains isolated from patients non-responsive to therapy with that drug (i.e., presumably resistant to that drug)

Lowest concentration of drug that will inhibit 95% (90% for pyrazinamide) of wild strains of M. tuberculosis that have never been exposed to drugs, while at the same time not inhibiting clinical strains of M. tuberculosis that are considered to be resistant (e.g. from patients who are not responding to therapy)

The critical concentration is defined as the lowest concentration of an anti-TB agent in vitro that will inhibit the growth of 99% of phenotypically wild type isolates of M. tuberculosis complex.