Antimicrobial MIC Calculator
Calculate minimum inhibitory concentration (MIC), MBC/MIC ratio, and susceptibility interpretation. Compare MIC to clinical breakpoints for antibiotic susceptibility testing (AST) and resistance determination.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Antimicrobial MIC Calculator | — | MBC/MIC ratio >= 4 indicates bactericidal | ug/mL |
Step-by-Step Examples
MIC=0.06 ug/mL, MBC=0.12 ug/mL. Clinical breakpoint S<=0.12 ug/mL.
- MBC/MIC = 0.12/0.06 = 2.0 (<4 = bactericidal)
- MIC 0.06 <= breakpoint 0.12: SUSCEPTIBLE
- Penicillin is bactericidal for this strain
Oxacillin MIC=4 ug/mL. S. aureus breakpoint S<=2, R>=4.
- MIC 4 >= R breakpoint: RESISTANT
- MIC at or above resistance threshold = MRSA
- Requires vancomycin or alternative therapy
Tetracycline MIC=2 ug/mL, MBC=32 ug/mL.
- MBC/MIC = 32/2 = 16 (>= 4 = bacteriostatic)
- Bacteriostatic: inhibits growth but does not kill
- Higher MBC/MIC ratio typical of protein synthesis inhibitors
Real-World Applications
Common Mistakes to Avoid
Clinical breakpoints incorporate pharmacokinetics (drug concentrations at site of infection), not just MIC. A low MIC means nothing if drug can't reach the infection.
MIC depends on inoculum size, growth conditions, and medium. Twofold serial dilutions mean true MIC is between two dilutions.
MIC = minimum concentration that inhibits visible growth. MBC = minimum concentration that kills 99.9% of bacteria. For critical infections (endocarditis), bactericidal drugs (low MBC/MIC) are preferred.