Interpreting Minimum Inhibitory Concentration (MIC)

Minimum Inhibitory Concentration (MIC)

The lowest concentration (in μg/mL) of an antibiotic that inhibits the growth of a given strain of bacteria.

Kirby-Bauer method

A disk diffusion method used to determine the susceptibility of bacteria to antibiotics based on the size of the growth-free zone.

VITEK® 2

An automated platform used in microbiology laboratories for rapid and accurate quantitative antibiotic susceptibility test (AST) reporting.

Antibiogram

A report that includes the identity of the organism and the appropriate antibiotic sensitivity pattern against each organism, often including MICs.

Interpretation of MIC

The susceptibility interpretation next to each antibiotic: S (sensitive), I (intermediate), or R (resistant), followed by the MIC in μg/mL.

Sensitive

Implies that the organism is inhibited by the serum concentration of the drug that is achieved using the usual dosage.

Intermediate

Implies that the organisms are inhibited only by the maximum recommended dosage.

Resistant

Implies that the organisms are resistant to the usually achievable serum drug levels.

Clinical and Laboratory Standards Institute (CLSI)

The organization that establishes interpretive standards for antibiotic susceptibility testing.

Amoxicillin

An antibiotic with an MIC of ≤2, interpreted as sensitive.

Amoxicillin/clavulanic acid

An antibiotic with an MIC of ≤2, interpreted as sensitive.

Cephalexin

An antibiotic with an MIC of 8, interpreted as sensitive.

Cefpodoxime

An antibiotic with an MIC of ≤0.25, interpreted as sensitive.

Ceftiofur

An antibiotic with an MIC of ≤1, interpreted as sensitive.

Cefovecin

An antibiotic with an MIC of ≤2, interpreted as sensitive.

Ceftazidime

An antibiotic with an MIC of ≤4, interpreted as sensitive.

Imipenem

An antibiotic with an MIC of 2, interpreted as sensitive.

Amikacin

An antibiotic with an MIC of ≤2, interpreted as sensitive.

Gentamicin

An antibiotic with an MIC of ≤0.5, interpreted as sensitive.

Tobramycin

An antibiotic with an MIC of ≤1, interpreted as sensitive.

Enrofloxacin

An antibiotic with an MIC of 0.5, interpreted as sensitive.

Marbofloxacin

An antibiotic with an MIC of 0.5, interpreted as sensitive.

Ciprofloxacin

An antibiotic with an MIC of ≤1, interpreted as sensitive.

Doxycycline

An antibiotic with an MIC of ≥16, interpreted as resistant.

Nitrofurantoin

An antibiotic with an MIC of 32, interpreted as sensitive.

Chloramphenicol

An antibiotic with an MIC of 8, interpreted as sensitive.

Trimethoprim/sulfa

An antibiotic with an MIC of ≤10, interpreted as sensitive.

Class-reference antibiotics

Antibiotics used to determine the susceptibility of other antibiotics in the same class.

Further application of Amoxicillin

Predicts susceptibility of ampicillin.

Further application of Amoxicillin/clavulanic acid

Predicts susceptibility of Clavamox®.

Further application of Cephalexin

Predicts susceptibility of all first-generation cephalosporins, except cefazolin.

Further application of Clindamycin

Predicts susceptibility of lincomycin; should not be used in horses, rabbits, and other herbivores.

Further application of Erythromycin

Predicts susceptibility of azithromycin and clarithromycin.

Erythromycin

Predicts susceptibility to azithromycin and clarithromycin.

Oxacillin

Predicts susceptibility to methicillin.

MIC

Minimum inhibitory concentration; indicates antibiotic effectiveness.

Breakpoint

Dilution where bacteria show resistance.

In vitro efficacy

Laboratory measurement of antibiotic effectiveness.

Gram-negative bacteria

Bacteria with thin cell walls, often resistant.

Gram-positive bacteria

Bacteria with thick cell walls, generally susceptible.

Species considerations

Certain antibiotics can be toxic to specific species.

Lipid-soluble drugs

Reach higher tissue levels than serum levels.

Kidney excretion

Drugs reach higher bladder levels than serum.

Antibiotic cost

Financial consideration when selecting treatment.

Consultant support

Available for interpreting antibiotic test results.

Florfenicol

Antibiotic effective at lower concentrations topically.

Polymyxin B

Used for gram-negative infections, not urine cultures.

Customer support services

Assistance for practices regarding diagnostics and treatments.

Clinical discretion

Use of judgment based on patient evaluation.