Antimicrobial susceptibility testing

Microbiology (B.Sc) · Medical microbiology · free study notes for exams, placements and interviews

In short

Antimicrobial susceptibility testing (AST) measures whether a bacterial or fungal isolate from a patient is inhibited by a given antimicrobial at concentrations achievable in the body, and reports it as susceptible (S), intermediate or susceptible-dose-dependent (I/SDD), or resistant (R). It converts a culture report into treatment advice, drives hospital antibiotic policy and feeds national resistance surveillance. In India, clinical AST follows CLSI M100 breakpoints (updated annually; some labs use EUCAST), NABL ISO 15189 quality requirements, and the ICMR AMR Surveillance Network protocols; antifungal AST follows CLSI M27/M38. It is a core unit of B.Sc Microbiology medical microbiology papers, and the single most asked practical topic in interviews for clinical labs (Dr Lal PathLabs, SRL, Metropolis, Apollo, AIIMS/PGI labs), pharma microbiology QC and CRO/diagnostics R&D roles.

Explained

Why test, and what the report means

Purpose: guide definitive therapy, de-escalate empirical broad-spectrum drugs, detect emerging resistance, and generate a cumulative antibiogram for empirical policy.
Breakpoint: the MIC (or zone diameter) value that separates categories. It is set from MIC distributions (wild-type vs non-wild-type), PK/PD targets (e.g. %T>MIC for beta-lactams, AUC/MIC for fluoroquinolones and vancomycin) and clinical outcome data — not from the zone size alone.
Categories (CLSI M100 current usage): S - likely to respond at the usual dose; I - buffer zone, response likely only at sites where drug concentrates (e.g. urine) or at higher dose; SDD - susceptible only at a specified higher dosing regimen (used for cefepime and piperacillin-tazobactam with Enterobacterales, ceftaroline with *S. aureus* and daptomycin with *E. faecium* in recent editions); R - unlikely to respond; NS (nonsusceptible) where only a susceptible breakpoint exists.
Intrinsic (natural) resistance must be known and reported as R regardless of the result - e.g. *Klebsiella* vs ampicillin, *Proteus/Providencia* vs nitrofurantoin and colistin, *Serratia* vs colistin, anaerobes vs aminoglycosides, *Enterococcus* vs cephalosporins and clindamycin, *Stenotrophomonas* vs carbapenems.
Selective reporting / cascade reporting: first-line drugs reported routinely; reserve drugs (carbapenems, colistin, linezolid) released only when needed - part of Antimicrobial Stewardship Programmes recommended by ICMR and NCDC's National Action Plan on AMR (NAP-AMR; second phase from 2025).

Disc diffusion (Kirby–Bauer) - step by step

Pure culture only. Pick 3–5 well-isolated colonies of the same morphology from an 18–24 h plate.
Inoculum: suspend in sterile saline/broth to 0.5 McFarland = approx 1–2 × 10⁸ CFU/mL for bacteria; verify by spectrophotometer (OD₆₂₅ 0.08–0.13) or against a Wickerham card. Use within 15 minutes.
Medium: Mueller–Hinton agar (MHA), depth 4 mm (approx 25–30 mL in a 90 mm plate), pH 7.2–7.4, cation-adjusted for broth (CAMHB: Ca²⁺ 20–25 mg/L, Mg²⁺ 10–12.5 mg/L). MHA + 5% defibrinated sheep blood for streptococci; Haemophilus Test Medium (HTM) for *Haemophilus*; GC agar + 1% defined growth supplement for *N. gonorrhoeae*.
Lawn culture: dip swab, press out excess, streak the entire surface in three directions rotating the plate 60°, rim last. Dry 3–5 min, not more than 15.
Discs: place within 15 min of inoculation using a dispenser or sterile forceps, press down; maximum 12 discs on a 150 mm plate, 5 on a 90 mm plate, centres ≥24 mm apart, no disc within 15 mm of the rim. Discs are single-use, stored at 2–8 °C (or ≤ −14 °C for beta-lactams), brought to room temperature before opening.
Incubation: inverted, 35 ± 2 °C in ambient air for 16–18 h (Enterobacterales, staphylococci, pseudomonads, enterococci). Exceptions: 24 h for oxacillin/cefoxitin-staphylococci and vancomycin-enterococci (to catch slow-growing resistance); 5% CO₂ for streptococci and *N. gonorrhoeae* (20–24 h) and *Haemophilus* on HTM (16–18 h).
Reading: measure the diameter of complete inhibition in mm with calipers or a ruler on the back of the plate against a dark background with reflected light; blood agar plates are read from the front with the lid off. Ignore faint growth/swarming; for sulfonamides and trimethoprim ignore slight growth (≤20%) and read the obvious margin. Colonies within the zone = mixed culture or resistant mutants - repeat.
Interpret against the current CLSI M100 table for that organism group; never carry over last year's chart.

Dilution methods, MIC and MBC

Broth microdilution (BMD) - reference method. Doubling dilutions of drug in CAMHB in a 96-well plate, final inoculum 5 × 10⁵ CFU/mL, 35 °C, 16–20 h. MIC = lowest concentration with no visible growth. Mandatory (and the only CLSI-accepted method) for colistin and polymyxin B - discs and gradient strips are invalid because polymyxins diffuse poorly in agar.
Agar dilution - drug incorporated into molten agar at each concentration, spot 10⁴ CFU per spot; reference method for *N. gonorrhoeae* and anaerobes; allows many isolates per plate.
MBC (minimum bactericidal concentration): subculture clear wells onto drug-free agar; MBC = lowest concentration giving ≥99.9% kill (≥3 log₁₀ reduction) of the starting inoculum. MBC/MIC ≤4 = bactericidal; >4 (usually ≥32) = tolerance.
Gradient diffusion (E-test / MIC strip): plastic strip with an exponential antibiotic gradient; MIC read where the ellipse of growth intersects the strip. Convenient for fastidious organisms, vancomycin MIC on MRSA, and for drugs with no disc. Read-up to the next doubling dilution when comparing with BMD.
Automated systems: VITEK 2, BD Phoenix, MicroScan - miniaturised broth dilution with turbidimetric/fluorescence growth detection and expert-rule software; results in 6–18 h. Must still be verified against reference methods for new resistance mechanisms.
Rapid and molecular approaches: MALDI-TOF for rapid ID (not susceptibility, except specific assays), rapid AST from positive blood culture bottles (direct BMD, EUCAST RAST disc diffusion read at 4/6/8 h), PCR/cartridge assays for *mecA*, *vanA/B*, *bla*<sub>KPC/NDM/OXA-48</sub>, and *rpoB*; Xpert MTB/RIF Ultra and line probe assays are used nationally under NTEP for TB drug resistance, alongside liquid culture (MGIT 960) phenotypic DST at the critical concentration.
Other quantitative tools: MPC (mutant prevention concentration), time-kill curves, post-antibiotic effect, synergy testing by checkerboard (FIC index) or time-kill.

Resistance mechanisms and the phenotypic tests that detect them

Beta-lactamases: penicillinase (staphylococci - detected by nitrocefin or the penicillin disc zone-edge test), ESBL (*bla*<sub>CTX-M/SHV/TEM</sub>) - combined disc test: ceftazidime and cefotaxime ± clavulanate, increase in zone ≥5 mm with clavulanate = ESBL positive; or the double-disc synergy test (keyhole effect). AmpC (inducible/derepressed in *Enterobacter, Serratia, Citrobacter freundii, Morganella, Providencia, P. aeruginosa*) - cefoxitin resistance with no clavulanate effect; boronic acid or cloxacillin-based tests.
Carbapenemases: screen with meropenem/ertapenem; confirm with mCIM (modified carbapenem inactivation method - meropenem disc incubated with the isolate, then tested on an *E. coli* ATCC 25922 lawn; zone ≤15 mm or pin-point colonies = positive) with eCIM (EDTA) to distinguish metallo-beta-lactamase (NDM, VIM, IMP) from serine (KPC, OXA-48). NDM-1 is the dominant carbapenemase in India, so ceftazidime-avibactam often fails while aztreonam-avibactam or ceftazidime-avibactam + aztreonam is used.
MRSA: *mecA*/*mecC*-mediated PBP2a. Screen with the cefoxitin 30 µg disc (surrogate, better inducer than oxacillin): *S. aureus* zone ≤21 mm = MRSA (≥22 mm susceptible); confirm by *mecA* PCR or PBP2a latex if needed. Oxacillin MIC ≥4 µg/mL = resistant for *S. aureus*.
Inducible clindamycin resistance (MLSb): D-test - erythromycin and clindamycin discs 15–26 mm apart; flattening of the clindamycin zone (D shape) = inducible resistance; report clindamycin as resistant.
Vancomycin resistance: *vanA/vanB* in enterococci; disc diffusion is unreliable for VISA/VRSA - use MIC method and the BHI-vancomycin 6 µg/mL screen agar.
High-level aminoglycoside resistance (HLAR) in enterococci: gentamicin 120 µg and streptomycin 300 µg discs - loss of synergy with cell-wall agents, so combination therapy will fail.
Others: efflux pumps (tetracyclines, fluoroquinolones), porin loss (OmpK35/36), target mutation (*gyrA*, *rpoB*, 23S rRNA for linezolid), *mcr*-mediated plasmid colistin resistance, methylases (16S rRNA) for aminoglycosides.

Quality control, standards and Indian practice

QC strains (reference, traceable): *E. coli* ATCC 25922, *E. coli* ATCC 35218 (beta-lactamase producer, for inhibitor combinations), *S. aureus* ATCC 25923 (disc diffusion) and ATCC 29213 (dilution), *P. aeruginosa* ATCC 27853, *E. faecalis* ATCC 29212, *K. pneumoniae* ATCC 700603 (ESBL), *K. pneumoniae* ATCC BAA-1705/1706 (KPC positive/negative), *H. influenzae* ATCC 49247, *S. pneumoniae* ATCC 49619.
QC frequency: daily until 20 or 30 consecutive days are in range, then weekly (with corrective action and reversion to daily on failure). Record each new medium lot, disc lot, and after instrument service.
Documents: CLSI M02 (disc diffusion), M07 (dilution), M100 (tables, annual), M45 (infrequent/fastidious), M27/M38/M60/M61 (antifungal); EUCAST breakpoint tables and RAST. In India, ICMR's AMR Surveillance and Research Network (AMRSN) publishes annual data and standard operating procedures; NCDC runs NARS-Net; NABL accreditation to ISO 15189 governs competence, EQAS participation and traceability. Schedule H/H1 of the Drugs and Cosmetics Rules, 1945 restricts OTC sale of listed antibiotics - H1 requires a prescription register.
Biosafety: AST on routine isolates at BSL-2 with a biosafety cabinet for aerosol-generating steps; *M. tuberculosis* DST requires BSL-3; discard as biomedical waste under the Biomedical Waste Management Rules, 2016 (as amended) - yellow category after autoclaving/incineration.
Antifungal AST: CLSI M27 (yeasts) / M38 (moulds) broth microdilution in RPMI-1640 with 0.2% glucose, MOPS buffered (2% glucose is used only for the agar/gradient-strip modifications), read at 24 h (*Candida*) or 48 h; endpoints are 50% inhibition for azoles/echinocandins and 100% for amphotericin B; *Candida auris* (widely reported from Indian ICUs) needs tentative CDC breakpoints as CLSI has none for most drugs.

Comparing the methods - strengths and limits

Disc diffusion: cheap (approx ₹8–20 per disc), flexible, no instrument; but qualitative, not valid for colistin/polymyxins or for slow-growers, and sensitive to inoculum, agar depth and timing.
MIC methods: quantitative, needed for endocarditis, meningitis, osteomyelitis, bacteraemia, dose escalation and for drugs with no reliable disc; costlier and more laborious; ±1 doubling dilution is acceptable inter-method variation.
Automated: fast, standardised, interfaced with LIS; high capital and per-card cost, fixed drug panels, and can miss low-level or inducible resistance.
Molecular: fast and detects the gene, but a gene present is not always expressed, and absence of a known gene does not prove susceptibility.
Error grading in method comparison: very major error = reported S but truly R (most dangerous); major error = reported R but truly S; minor error = involves the I/SDD category. Essential agreement = MIC within ±1 dilution; categorical agreement = same S/I/R call.

Key points

AST - in-vitro test of inhibition of an isolate by antimicrobials, reported S/I/SDD/R against current breakpoints.
0.5 McFarland - standard inoculum, approx 1–2 × 10⁸ CFU/mL, used within 15 minutes.
Mueller–Hinton agar - the reference medium: low thymidine/thymine, low cations, 4 mm deep, pH 7.2–7.4.
Zone of inhibition - diameter of complete growth inhibition in mm; inversely related to MIC, not a measure of drug potency across different drugs.
MIC - lowest concentration preventing visible growth; MBC - lowest giving ≥99.9% kill; MBC/MIC >4 indicates tolerance.
Broth microdilution - reference method and the only valid method for colistin and polymyxin B.
Cefoxitin disc - surrogate for *mecA*-mediated MRSA; *S. aureus* zone ≤21 mm = MRSA.
D-test - detects inducible clindamycin (MLSb) resistance; a D-shaped clindamycin zone means report clindamycin R.
ESBL combined disc test - ≥5 mm zone increase with clavulanate confirms ESBL.
mCIM/eCIM - phenotypic confirmation of carbapenemase and separation of metallo- from serine-type; NDM predominates in India.
HLAR screen - gentamicin 120 µg and streptomycin 300 µg discs in enterococci; predicts failure of synergy therapy.
Intrinsic resistance - must be reported R irrespective of the zone (e.g. *Klebsiella*–ampicillin, *Proteus*–nitrofurantoin/colistin).
Breakpoints - from MIC distributions + PK/PD + clinical outcome; CLSI M100 revises them annually, so charts must be the current edition.
QC strains - ATCC reference strains run daily, then weekly after 20–30 in-control days; failures trigger corrective action.
Cumulative antibiogram - annual %S by organism and drug, used for empirical policy under antimicrobial stewardship.
Error grading - very major (S reported, truly R) is the most serious; essential agreement is ±1 doubling dilution.
Indian framework - ICMR AMRSN and NCDC NARS-Net surveillance, NAP-AMR, NABL/ISO 15189 accreditation, Schedule H1 prescription control.

Formulas and examples

MIC = lowest antimicrobial concentration (µg/mL or mg/L) showing no visible growth after standard incubation
MBC = lowest concentration giving ≥99.9% (≥3 log₁₀) reduction in viable count vs the starting inoculum (µg/mL)
Doubling dilution series: Cₙ = C₀ / 2ⁿ, where C₀ = highest concentration, n = number of two-fold steps
C₁V₁ = C₂V₂ (dilution of stock; C in µg/mL, V in mL)
Stock potency: mass to weigh (mg) = (Volume in mL × Concentration in µg/mL) / Potency in µg/mg
Log₁₀ reduction = log₁₀(CFU/mL initial) − log₁₀(CFU/mL final)
FIC index = (MIC of A in combination / MIC of A alone) + (MIC of B in combination / MIC of B alone); ≤0.5 synergy, >0.5–4 indifference, >4 antagonism
Zone diameter and MIC are inversely related; a regression of log₂MIC against zone diameter defines the breakpoint correlates
Percentage susceptible in an antibiogram = (number of S isolates / total isolates tested) × 100 (first isolate per patient per year)

Example 1 - weighing an antibiotic stock from a potency-labelled powder. Need 10 mL of a 1,280 µg/mL gentamicin stock; the vial is labelled potency 620 µg/mg.

Mass of active drug needed = 10 mL × 1,280 µg/mL = 12,800 µg = 12.8 mg.
Powder to weigh = 12,800 µg ÷ 620 µg/mg = 20.65 mg, dissolved in 10 mL sterile water.
Check: 20.65 mg × 620 µg/mg = 12,803 µg in 10 mL = 1,280 µg/mL. Correct.

Example 2 - setting up a broth microdilution range. Highest well 128 µg/mL, doubling dilutions down a 10-well row.

Series: 128, 64, 32, 16, 8, 4, 2, 1, 0.5, 0.25 µg/mL (Cₙ = 128/2ⁿ, n = 0…9); well 11 = growth control (no drug), well 12 = sterility control.
Growth always appears at the low-concentration end. Suppose growth in wells 6–10 (4 µg/mL and below) and no growth in wells 1–5 (8 µg/mL and above): MIC = 8 µg/mL.
Interpretation for *E. coli* with gentamicin: CLSI lowered the Enterobacterales aminoglycoside breakpoints in M100 (33rd edition, 2023) to S ≤2, I 4, R ≥8 µg/mL, so 8 µg/mL = Resistant (under the older S ≤4, I 8, R ≥16 it would have been Intermediate — the reason an old chart must never be used).

Example 3 - MBC and the bactericidal ratio. Starting inoculum 5 × 10⁵ CFU/mL. Subculture of the clear wells gives: 16 µg/mL → 2,000 CFU/mL; 32 µg/mL → 400 CFU/mL; 64 µg/mL → 20 CFU/mL.

99.9% kill threshold = 5 × 10⁵ × 0.001 = 500 CFU/mL.
16 µg/mL (2,000) fails; 32 µg/mL (400) is below 500 → MBC = 32 µg/mL.
Log₁₀ reduction at 32 µg/mL = log₁₀(5 × 10⁵) − log₁₀(400) = 5.699 − 2.602 = 3.10 logs.
If MIC = 8 µg/mL, MBC/MIC = 32/8 = 4 → bactericidal (ratio >4 would indicate tolerance).

Example 4 - ESBL combined disc test. Zones for a *K. pneumoniae* from urine: ceftazidime 30 µg = 14 mm; ceftazidime-clavulanate 30/10 µg = 21 mm; cefotaxime 30 µg = 17 mm; cefotaxime-clavulanate 30/10 µg = 20 mm.

Ceftazidime increase = 21 − 14 = 7 mm (≥5 mm → positive).
Cefotaxime increase = 20 − 17 = 3 mm (<5 mm → not confirmatory on its own).
Either drug showing ≥5 mm is sufficient → ESBL confirmed. Report all penicillins, cephalosporins and aztreonam as resistant irrespective of measured zones (as per the confirmatory-test reporting convention); carbapenems or beta-lactam/inhibitor combinations as tested.

Example 5 - synergy by checkerboard (FIC index). MIC alone: meropenem 32 µg/mL, colistin 2 µg/mL. In combination the lowest inhibitory pair is meropenem 4 µg/mL + colistin 0.5 µg/mL.

FIC(meropenem) = 4/32 = 0.125; FIC(colistin) = 0.5/2 = 0.25.
FIC index = 0.125 + 0.25 = 0.375 → synergy (≤0.5), which is why carbapenem–colistin combinations are studied for carbapenem-resistant *Acinetobacter*.

Method	Output	Cost/throughput	Key limitation
Disc diffusion (Kirby–Bauer)	S/I/R	Very low cost, high throughput	No MIC; invalid for colistin; sensitive to technique
Broth microdilution	MIC	Moderate, labour-intensive	Manual plate preparation; needs CAMHB QC
Agar dilution	MIC	Good for many isolates, one drug	Laborious; drug-in-agar stability
Gradient strip (E-test)	MIC	High per-test cost	Reading errors; not for polymyxins
Automated (VITEK/Phoenix)	MIC + expert rules	High capital, fast	Fixed panels; may miss inducible/low-level resistance
Molecular (PCR/cartridge)	Gene detected	Fast, high cost	Gene ≠ expression; unknown mechanisms missed

Test	Target resistance	Positive criterion
Cefoxitin 30 µg disc	MRSA (mecA)	S. aureus zone ≤21 mm
D-test	Inducible MLSb	D-shaped clindamycin zone
Combined disc (± clavulanate)	ESBL	Zone increase ≥5 mm
mCIM / eCIM	Carbapenemase / MBL	E. coli lawn zone ≤15 mm; eCIM ≥5 mm increase = MBL
BHI-vancomycin 6 µg/mL	VRE / VISA	Any growth after 24 h
Gentamicin 120 µg disc	HLAR in enterococci	Zone ≤6 mm resistant (7–9 mm inconclusive → MIC)

Where it is used

Clinical diagnostic laboratories: hospital microbiology (AIIMS, PGIMER, CMC Vellore, Tata Memorial) and chains (Dr Lal PathLabs, SRL, Metropolis, Thyrocare) - roles: microbiology technologist, AST bench in-charge, NABL quality officer.
Infection control and stewardship teams: generating cumulative antibiograms, ICU-specific resistance reports, restricted-antibiotic approvals, outbreak investigation of MRSA/CRE/*C. auris*.
Public health surveillance: ICMR AMR Surveillance Network nodal centres, NCDC NARS-Net, NTEP culture and DST laboratories (MGIT and LPA for rifampicin/isoniazid and second-line drugs).
Pharmaceutical and biotech R&D: MIC screening of new molecules, natural-product and phytochemical extracts, formulation claim support at companies such as Wockhardt, Cipla, Bugworks and Orchid; CRO services.
Pharma QC and preservative efficacy: antibiotic potency assay by cylinder-plate/turbidimetric method as per the Indian Pharmacopoeia, disinfectant and antiseptic efficacy, growth-promotion and media QC.
Veterinary, food and agriculture: AST of poultry and dairy isolates, antibiotic residue and resistance monitoring under FSSAI limits and ICAR-led AMR work in the food chain.

Interview questions

Q: Why is Mueller–Hinton agar the medium of choice for disc diffusion? A: It is a well-standardised, low-thymidine/thymine medium (so sulfonamides and trimethoprim are not antagonised), has reproducible lot-to-lot performance, supports most non-fastidious pathogens, and its cation content is controlled so aminoglycoside and tetracycline results are valid.
Q: What happens if the agar is poured too thick or too thin? A: Thicker than 4 mm gives falsely small zones (apparent resistance) and thinner gives falsely large zones (apparent susceptibility), because the drug diffuses in three dimensions.
Q: Why must the inoculum be standardised to 0.5 McFarland and used within 15 minutes? A: A heavy inoculum shrinks zones and raises the apparent MIC (inoculum effect, marked for beta-lactam/inhibitor combinations); a light one enlarges zones; after 15 minutes the suspension density changes by growth or settling.
Q: Why is cefoxitin used instead of oxacillin to detect MRSA? A: Cefoxitin is a better inducer of *mecA* expression and gives clearer, more reproducible zones, so it is the recommended surrogate; confirmation is by *mecA*/PBP2a detection.
Q: A *S. aureus* is erythromycin-resistant but clindamycin-susceptible. What next? A: Perform the D-test; a D-shaped clindamycin zone indicates inducible MLSb resistance and clindamycin must be reported resistant, otherwise therapy may fail by selecting constitutive mutants.
Q: Why can colistin not be tested by disc diffusion or E-test? A: Polymyxins are large, highly cationic molecules that diffuse poorly through agar, so zones do not correlate with MIC; CLSI and EUCAST accept only broth microdilution (in plain, surfactant-free polystyrene).
Q: Differentiate MIC and MBC, and define tolerance. A: MIC is the lowest concentration that prevents visible growth; MBC is the lowest that kills ≥99.9% of the inoculum; an MBC/MIC ratio greater than about 4 indicates tolerance, where the drug inhibits but does not kill.
Q: How do you distinguish an ESBL from an AmpC producer? A: ESBL activity is inhibited by clavulanate (≥5 mm zone increase in the combined disc test) and cefoxitin is usually susceptible; AmpC is cefoxitin-resistant, not inhibited by clavulanate, and is inhibited by boronic acid or cloxacillin.
Q: What is mCIM and what does adding EDTA (eCIM) tell you? A: mCIM detects carbapenemase by the isolate's ability to hydrolyse meropenem in a disc, which then fails to inhibit a susceptible *E. coli* lawn; EDTA chelates zinc, so restoration of the zone identifies a metallo-beta-lactamase such as NDM or VIM.
Q: Which error in a method comparison is the most serious and why? A: A very major error - calling an organism susceptible when it is resistant - because the patient receives an ineffective drug.
Q: How is a breakpoint decided? A: From the wild-type MIC distribution and epidemiological cut-off, PK/PD target attainment at the approved dose, and clinical outcome data; it is drug-, organism-, dose- and site-specific, and is revised periodically.
Q: Why should a laboratory not use last year's interpretive chart? A: CLSI M100 is revised annually - breakpoints have been lowered for carbapenems, fluoroquinolones, cephalosporins and others - so an outdated chart can report resistant isolates as susceptible.

Common mistakes

Using a mixed or non-pure culture, or picking colonies from a selective/differential medium that distorts growth, instead of a fresh pure subculture.
Eyeballing turbidity instead of standardising to 0.5 McFarland, or letting the suspension stand well beyond 15 minutes before use.
Measuring the zone to the outer haze or including the disc incorrectly - the reading is the diameter of complete inhibition, including the 6 mm disc.
Crowding too many discs on a plate so zones overlap and cannot be measured, or reading overlapping zones by guesswork.
Reporting disc results for colistin, or reporting an intrinsically resistant combination as susceptible (*Klebsiella*–ampicillin, *Proteus*–nitrofurantoin).
Reading oxacillin/cefoxitin and vancomycin plates at 18 h instead of a full 24 h, thereby missing heteroresistant MRSA and VRE.
Treating a larger zone for one drug as proof it is the "stronger" antibiotic - zone sizes are only comparable within the same drug's own breakpoints, since disc contents and diffusion rates differ.
Omitting ATCC QC strains or continuing to report after a QC failure without corrective action - a direct NABL non-conformity.