Introduction

Plazomicin is a new-generation aminoglycoside antibiotic, effective against multidrug-resistant Enterobacterales. It is recommended for complicated urinary tract infections (cUTIs) in adults.

Aim

The study evaluated VITEK 2 and VITEK 2 Compact systems for plazomicin susceptibility testing in Gram‑negative organisms and compared results with the Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) technique

Methods

• A total of 979 Enterobacterales isolates (869 clinical and 110 challenge isolates) were tested using the VITEK 2 AST‑GN plazomicin test and compared with the CLSI BMD reference method. 
• The study endpoint was to demonstrate essential agreement between VITEK 2 plazomicin MICs and CLSI broth microdilution per ISO criteria. 
• The other endpoints included FDA categorical agreement and error rates, species‑level performance, bias and trending analysis, reproducibility, quality control compliance, resistance detection, and time to result.

Results

ISO Performance  (overall clinical and challenged control)

• Overall essential agreement (EA) for combined clinical and challenge isolates tested in auto‑dilution mode was 97.8%, meeting ISO acceptance criteria. 
• Challenge isolates EA

• Auto‑dilution: 98.2%
• Manual dilution: 97.3%
• VITEK 2 Compact: 98.2%

Table 1 : Essential agreement for plazomicin susceptibility testing on VITEK 2 and VITEK 2 Compact systems according to ISO criteria

Type of Isolates	≤ −3	−2	−1	0	+1	+2	≥ +3	Essential Agreement
Overall: Clinical + Challenge (Auto‑dilution)	4	9	102	813	43	8	0	97.8%
Clinical (Auto‑dilution)	3	8	94	726	30	8	0	97.8%
Challenge (Auto‑dilution)	1	1	8	87	13	0	0	98.2%
Challenge (Manual dilution)	1	2	6	90	11	0	0	97.3%
Challenge (VITEK 2 Compact, manual)	1	1	9	88	11	0	0	98.2%

All EA values meet ISO acceptance criteria (EA ≥ 90%). 

• EA met acceptance requirements at both organism group and species level, except for Morganella morganii (EA 87.5%). 
• Despite this, the combined Proteus/Providencia/Morganella group EA was acceptable (90.5% with a 90% CI of 85.3–94.3). 

Table 2. Essential agreement by organism group for plazomicin testing on VITEK 2 under ISO criteria (auto‑dilution)

Organism Group	≤ −3	−2	−1	0	+1	+2	≥ +3	Essential Agreement
Enterobacter / Klebsiella grp	0	0	3	417	11	4	0	99.1%
Escherichia (E. coli)	0	3	62	259	4	0	0	99.1%
Other Enterobacterales	1	0	1	46	0	0	0	97.9%
Proteus / Providencia / Morganella	3	6	28	68	28	4	0	90.5%
Serratia marcescens	0	0	8	23	0	0	0	100%

 

• The ISO performance was determined as EA and Bias for the following organisms: C. freundii, C. koseri, E. cloacae, E. cloacae complex, E. coli, K. aerogenes, K. oxytoca, K. pneumoniae, P. vulgaris, M. morganii, P. mirabilis, P. stuartii, and S. marcescens.
• Overall bias was found to be: –35.9%, indicating a tendency for VITEK 2 MICs to be lower than BMD MICs ,primarily drivn by Escherichia coli
• 73.9% of evaluable E. coli isolates had MICs at least one doubling dilution lower than BMD

FDA Performance 

The overall combined clinical & challenge FDA performance was EA = 98.7% & CA = 99.4%, with one ME (0.1%) & no VME registered.

Clinical Isolates (n = 783)

• Essential Agreement: 98.6%
• Category Agreement (CA): 99.6%
• Errors observed
  • Very major errors (VME): 0
  • Major errors (ME): 1 
  • Minor errors (mE): 2 

EA and CA were ≥90% for every claimed species, meeting FDA acceptance criteria. 

Challenge Isolates (n = 75)

• Resistance‑enriched set: 74.7% resistant
• Auto‑dilution performance
  • EA: 100%
  • CA: 97.3%
  • No ME or VME; only minor errors observed
• Manual and Compact modes: Comparable performance, all with EA and CA ≥97%

These results confirm accurate resistance detection under controlled challenge conditions. 

Overall FDA Performance (Clinical + Challenge, n = 858)

• Essential Agreement: 98.7%
• Category Agreement: 99.4%
• Error rates: 
  • VME: 0%
  • ME: 0.1%
  • mE: 0.5%

Based on BMD

• 92.9% susceptible
• 0.5% intermediate
• 6.6% resistant

No FDA‑defined unacceptable error rates were observed. 

Trending analysis

Trending analysis identified three species requiring labeling notes due to systematic MIC shifts: 

• Klebsiella pneumoniae: MICs tended to be equal or one dilution higher than BMD
• Escherichia coli: MICs tended to be equal or one dilution lower
• Serratia marcescens: MICs tended to be equal or one dilution lower

Time of call

Reproducibility 

• Reproducibility ≥95% across all modes, exceeding FDA and ISO criteria and the new FDA requirement for worst-case ≥89% 
  1. Auto‑dilution: 97.0%

• Manual dilution: 95.6%
• VITEK 2 Compact: 97.4%

Quality Control (QC)

• QC strains (E. coli ATCC 25922, P. aeruginosa ATCC 27853) were within acceptable ranges ≥95% of the time
• Off‑scale low MICs for E. coli ATCC 25922 were anticipated and addressed through labeling notes
• All QC results met FDA and ISO expectations

Conclusion

• The study demonstrated a reliable correlation between the newly developed VITEK 2 AST-GN Plazomicin test for Enterobacterales and the CLSI BMD reference technique. 
• The results validate the use of the plazomicin AST test (cards) in conjunction with VITEK 2 and VITEK 2 Compact Systems using 9.04 software or higher as an automated in vitro diagnostic tool to determine MICs in Enterobacterales.

Reference 

J Clin Microbiol.2025 Sep 10;63(9):e0044925.