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Reading MICs and breakpoints

16 min

Skills you'll build4 skills
  • Read the minimum inhibitory concentration from a valid dilution series
  • Assign a susceptibility category to an MIC with the organism-drug breakpoint
  • Distinguish growth inhibition from the bactericidal endpoint
  • Flag a susceptible result for a drug the species is intrinsically resistant to

Try first

Try first

A broth microdilution series runs 0.5, 1, 2, 4, 8, 16 and 32 µg/mL. The growth control well is turbid. Visible growth appears through 4 µg/mL, and the wells from 8 µg/mL up are clear. What is the MIC?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Reading the MIC

MIC = the lowest tested antimicrobial concentration that prevents visible growth under the specified test conditions, usually reported in µg/mL. A susceptibility category comes only from the matching organism-drug-method breakpoint, with its exposure or site qualifications. The series is read only when the drug-free growth control shows growth.1,2 A gradient strip reading between two doubling dilutions is rounded up to the next dilution before it is compared.2

Breakpoints and categories

An MIC is interpreted only by comparing it with a breakpoint. Each breakpoint applies to one organism group, one drug and one method, and some also to one infection site. Pneumococci, for example, have separate meningitis and nonmeningitis breakpoints.2

CategoryMeaning
Susceptible (S)Standard dosing is likely to work
Susceptible-dose dependent (SDD)Success needs a higher dosing regimen, which the breakpoint names
Intermediate (I)A buffer around the breakpoint, or a drug that concentrates at the site
Resistant (R)Standard dosing is likely to fail
Nonsusceptible (NS)Above the only breakpoint published, for drugs with no resistant category yet

The laboratory records which breakpoint version it uses and reviews it for currency, because the Clinical and Laboratory Standards Institute (CLSI) revises the tables each year.3

Inhibition and killing

A clear well shows that growth was inhibited. It can still contain many viable organisms. The minimum bactericidal concentration (MBC) comes from subculturing the clear wells to drug-free agar. It is the lowest concentration that kills at least 99.9% of the starting inoculum, so up to 0.1% may survive.4

Intrinsic resistance

Intrinsic resistance is present in every isolate of a species. Klebsiella is resistant to ampicillin. Proteus, Providencia and Morganella are resistant to nitrofurantoin, tetracyclines and colistin, and Serratia to colistin.2 A susceptible reading for one of these suggests a wrong identification, a mixed culture or a testing error, and it is never released as susceptible. Some drugs also appear active in vitro and are not effective clinically. For enterococci, cephalosporins, clindamycin and trimethoprim-sulfamethoxazole are never reported susceptible.2

Follow the bench demonstration

Sources for this step
References
  1. Clinical and Laboratory Standards Institute. M02, Disk Diffusion 14th ed. and M07, Dilution 12th ed. CLSI; 2024.
  2. Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. 36th ed. CLSI supplement M100. CLSI; 2026. Accessed September 27, 2026. https://clsi.org/shop/standards/m100/
  3. Clinical and Laboratory Standards Institute. Breakpoint implementation toolkit. Accessed September 27, 2026. https://clsi.org/resources/breakpoint-implementation-toolkit/
  4. Deventer AT, Stevens CE, Stewart A, Hobbs JK. Antibiotic tolerance among clinical isolates: mechanisms, detection, prevalence, and significance. Clin Microbiol Rev. 2024;37(4):e00106-24.

Watch one

Read each decision and its explanation.

An Escherichia coli from a blood culture is tested against cefepime by broth microdilution. The growth control is turbid and the sterility control is clear. Growth appears through 2 µg/mL, and the wells from 4 µg/mL up are clear. The laboratory's current cefepime breakpoints for Enterobacterales are susceptible 2 µg/mL or less, susceptible-dose dependent 4 to 8 µg/mL, and resistant 16 µg/mL or more.

What category is reported?

  1. Check the controls first. The growth control grew and the sterility control stayed clear, so the series can be read.

    A series without growth in its control well cannot show inhibition.

  2. Read the first clear well. The MIC is 4 µg/mL.

    The MIC is the lowest concentration without visible growth.

  3. Match the table. E. coli belongs to the Enterobacterales, the drug is cefepime, and the method is broth microdilution, so this row applies.

    A breakpoint applies only to its own organism group, drug and method.

  4. Compare. An MIC of 4 µg/mL falls within the 4-to-8-µg/mL band.

    The category comes from where the MIC falls in the stated ranges.

  5. Report it as susceptible-dose dependent. Calling it susceptible hides the dosing requirement, and calling it intermediate or resistant misstates the breakpoint.

    SDD indicates that success depends on a higher-dose regimen.

MIC 4 µg/mL, reported as cefepime susceptible-dose dependent (SDD).

Practice this skill

Your turn

Problem 1 of 3 · Hints available

In a valid broth dilution series, growth occurs at 1 and 2 µg/mL. Wells at 4, 8, and 16 µg/mL are clear. What is the MIC?

The 2-µg/mL well still contains visible growth. It is below the inhibitory endpoint.

The first concentration without visible growth is 4 µg/mL. A clinical category requires the applicable breakpoint.

The MIC is the lowest concentration that inhibits growth, so the clear 4-µg/mL well is the endpoint.

Hint
  1. Find the wells that still show growth.
  2. The endpoint is the lowest concentration where growth stops.

Review Dilution and gradient-diffusion methods

Problem 2 of 3 · Hints available

A Klebsiella pneumoniae from a urine culture reads ampicillin MIC 4 µg/mL on the automated panel. The laboratory's ampicillin breakpoint for Enterobacterales is susceptible 8 µg/mL or less. What do you do?

Klebsiella is intrinsically resistant to ampicillin. A susceptible report would claim activity the species does not have.

Released a susceptible result the species cannot have

Every isolate of a species shares its intrinsic resistance, so a susceptible reading for ampicillin in Klebsiella or colistin in Serratia points to a wrong identification, a mixed culture, or a testing error. Releasing it reports a drug as active when it will fail.

Resistant is the correct category for Klebsiella. The impossible reading also suggests a wrong identification or a mixed culture, which makes every other result on the panel questionable.

An impossible susceptible result suggests a wrong identification, a mixed culture or a testing error. Each is checked before any result is released.

Hint
  1. Ask what every isolate of this species does with ampicillin.
  2. If the reading cannot be true for K. pneumoniae, consider what else could be in the well.

Review Testing selection, intrinsic resistance, and reporting policy

Problem 3 of 3 · On your own

An isolate has an MIC of 2 µg/mL. The starting inoculum was 5 × 105 CFU/mL. The clear wells are subcultured to drug-free agar with these counts:

  • 2 µg/mL: too numerous to count
  • 4 µg/mL: 2,000 CFU/mL
  • 8 µg/mL: 300 CFU/mL
  • 16 µg/mL: no growth

What is the MBC?

Show the answer

8 µg/mL

Killing 99.9% of 5 × 105 CFU/mL leaves no more than 0.1%, which is 500 CFU/mL. The 4-µg/mL subculture grew 2,000 CFU/mL, above that limit. The 8-µg/mL subculture grew 300 CFU/mL, within it. The MBC is 8 µg/mL. The 16-µg/mL well also passes, and the MBC is the lowest concentration that does.

Review Bactericidal testing

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • A Proteus mirabilis grows at more than 100,000 CFU/mL from a clean-catch urine.
  • The automated panel prints nitrofurantoin MIC 32 µg/mL, susceptible.
  • It also prints ciprofloxacin MIC 0.5 µg/mL, susceptible.
  • The panel software still uses an outdated ciprofloxacin breakpoint, susceptible 1 µg/mL or less.
  • The laboratory's current, verified breakpoints for Enterobacterales ciprofloxacin are susceptible 0.25 µg/mL or less, intermediate 0.5 µg/mL, and resistant 1 µg/mL or more.
Decision 1 of 2

What happens to the nitrofurantoin result?

Proteus is intrinsically resistant to nitrofurantoin. A susceptible report would claim activity this species does not have.

Released a susceptible result the species cannot have

Every isolate of a species shares its intrinsic resistance, so a susceptible reading for ampicillin in Klebsiella or colistin in Serratia points to a wrong identification, a mixed culture, or a testing error. Releasing it reports a drug as active when it will fail.

Every Proteus isolate is resistant to nitrofurantoin, regardless of the panel result. The laboratory reports it resistant or suppresses it by procedure.

A repeat cannot make the species susceptible. Proteus is intrinsically resistant, so the result is reported resistant.

Review Testing selection, intrinsic resistance, and reporting policy

Decision 2 of 2

How is ciprofloxacin reported?

An MIC of 0.5 µg/mL is intermediate under the laboratory's current table. The software's old table is updated through the laboratory's verification process.

The panel applied an outdated breakpoint. The measured MIC is correct, and the old table assigns the wrong category.

Applied the wrong breakpoint to the measured MIC

Each breakpoint applies to one organism, drug, method, and sometimes infection site, as with the separate meningitis and nonmeningitis pneumococcal breakpoints. Applying the wrong table to a correct MIC gives the wrong category, and a report released that way needs a corrected report.

Rounding a category toward resistance misstates the result. The MIC falls in the intermediate band.

The report needs a category, and a current breakpoint exists for this organism and drug.

Review Interpretation: MIC, breakpoints, and categories

The problem is in the categories the software printed beside the MICs. Both MICs were measured correctly. The nitrofurantoin category ignored the species' intrinsic resistance, and the ciprofloxacin category came from an outdated breakpoint table.

For reference

Sources checked