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Journals / Kocatepe Tıp Dergisi / 2026 / Cilt: 27 Sayı: 4

ANTIMICROBIAL SUSCEPTIBILITY TESTING WITH THE VITEK 2 AST-XN21CARD: COMPARISON WITH DISK DIFFUSION IN KLEBSIELLA PNEUMONIAE AND PSEUDOMONAS AERUGINOSA STRAINS AND THE EFFICACY OF NEW - β LACTAMS, INCLUDING CEFIDEROCOL

VITEK 2 AST-XN21 KARTI İLE ANTİMİKROBİYAL DUYARLILIK TESTİ: KLEBSIELLA PNEUMONIAE, PSEUDOMONAS AERUGINOSA SUŞLARINDA DİSK DİFÜZYON İLE KARŞILAŞTIRMASI VE SEFİDEROKOL DAHİL YENİ β-LAKTAMLARIN ETKİNLİĞİ

Selahattin Ünlü(AFYONKARAHISAR HEALTH SCIENCES UNIVERSITY, SCHOOL OF MEDICINE, DEPARTMENT OF BASIC MEDICAL SCIENCES, DEPARTMENT OF MICROBIOLOGY)0009-0002-9185-0109
Melahat Gürbüz(AFYONKARAHISAR HEALTH SCIENCES UNIVERSITY, SCHOOL OF MEDICINE, DEPARTMENT OF BASIC MEDICAL SCIENCES, DEPARTMENT OF MICROBIOLOGY)0000-0001-6290-1216
Pages
508–517
Publication Date
2026-10-12
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AI Summary & Key Takeaways

AI Insight
⚡ TL;DR(Too Long; Didn't Read)

Overall categorical agreement (CA) between XN21 and DD was 81.0% for CZA and 84.5% for C/T; sensitivity/specificity were 76.5%/87.5% for CZA and 56.3%/95.2% for C/T, respectively.

📌 3 Key Takeaways

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Objective & Hypothesis:OBJECTIVE: This study aims to evaluate the antimicrobial susceptibility results of novel β-lactams in Klebsiella pneumoniae (K. pneumoniae) and Pseudomonas aeruginosa (P. aeruginosa) strains using the VITEK 2 AST-XN21 panel; to compare the agreement between VITEK 2 and disk diffusion (DD) methods for ceftazidime/avibactam (CZA) and ceftolozane/tazobactam (C/T); and to investigate the in vitro activity of cefiderocol (FDC).
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Key Findings & Data:MATERIALS AND METHODS: In K. pneumoniae, susceptibility rates by disk diffusion (DD) were 51.2% for CZA, 4.9% for C/T, and 48.8% for FDC; using XN21, susceptibility was 43.9% for CZA and 2.4% for C/T.
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Conclusion & Impact:Overall categorical agreement (CA) between XN21 and DD was 81.0% for CZA and 84.5% for C/T; sensitivity/specificity were 76.5%/87.5% for CZA and 56.3%/95.2% for C/T, respectively.
🔬Design:Araştırma Makalesi
📍Scope:Comprehensive Sample
⚙️Variables:Klebsiella pneumoniae · Pseudomonas aeruginosa · VITEK 2 AST-XN21

Abstract

OBJECTIVE: This study aims to evaluate the antimicrobial susceptibility results of novel β-lactams in Klebsiella pneumoniae (K. pneumoniae) and Pseudomonas aeruginosa (P. aeruginosa) strains using the VITEK 2 AST-XN21 panel; to compare the agreement between VITEK 2 and disk diffusion (DD) methods for ceftazidime/avibactam (CZA) and ceftolozane/tazobactam (C/T); and to investigate the in vitro activity of cefiderocol (FDC). MATERIALS AND METHODS: In K. pneumoniae, susceptibility rates by disk diffusion (DD) were 51.2% for CZA, 4.9% for C/T, and 48.8% for FDC; using XN21, susceptibility was 43.9% for CZA and 2.4% for C/T. In addition, no susceptibility was detected to ticarcillin/clavulanate (T/K), whereas susceptibility to cefoperazone/sulbactam (CPS/SUL) was 2.4%. In P. aeruginosa, DD yielded susceptibility rates of 76.5% for CZA, 82.4% for C/T, and 88.2% for FDC, while XN21 showed 64.7% susceptibility to CZA and 58.8% to C/T. Overall categorical agreement (CA) between XN21 and DD was 81.0% for CZA and 84.5% for C/T; sensitivity/specificity were 76.5%/87.5% for CZA and 56.3%/95.2% for C/T, respectively. FDC was active against 83.3% of CZA+C/T–resistant P. aeruginosa strains and against 38.1% of K. pneumoniae strains resistant to all novel agents. RESULTS: The susceptibility rates for K. pneumoniae isolates determined by the disk diffusion (DD) method were 51.2% for CZA, 4.9% for C/T, and 48.8% for FDC. Using the XN21 method, the susceptibility rates were found to be 43.9% for CZA and 2.4% for C/T. Furthermore, while no susceptibility was detected for ticarcillin/clavulanate (T/K), the susceptibility rate for cefoperazone/sulbactam (CPS/SUL) was 2.4%. For P. aeruginosa isolates, the susceptibility rates obtained via the DD method were 76.5% for CZA, 82.4% for C/T, and 88.2% for FDC; meanwhile, the XN21 method yielded susceptibility rates of 64.7% for CZA and 58.8% for C/T. CONCLUSIONS: This study demonstrates that the VITEK 2 AST-XN21 panel shows an overall acceptable level of agreement for CZA and C/T; however, C/T results near the breakpoint in P. aeruginosa should be confirmed. In K. pneumoniae, the activity of ticarcillin/clavulanate (T/K) and cefoperazone/sulbactam (CPS/SUL) was low. Cefiderocol (FDC), evaluated by disk diffusion, was found to be effective against P. aeruginosa, and confirmatory testing using a reference method is recommended for borderline or discordant results.

Özet

AMAÇ: Bu çalışma, Klebsiella pneumoniae (K. pneumoniae) ve Pseudomonas aeruginosa (P. aeruginosa) izolatlarında yeni β-laktamların antimikrobiyal duyarlılık sonuçlarının VITEK 2 AST-XN21 paneli ile değerlendirilmesini; seftazidim/avibaktam (CZA) ve seftolozan/tazobaktam (C/T) için disk difüzyon (DD) yöntemiyle elde edilen sonuçlarla uyumun karşılaştırılmasını ayrıca sefiderokolün (FDC) in vitro etkinliğinin araştırılmasını amaçlamaktadır. GEREÇ VE YÖNTEM: 01.07.2024-30.09.2024 tarihleri arasında çeşitli klinik örneklerden izole edilen 58 tekrarsız suş (41 K. pneumoniae, 17 P. aeruginosa) dahil edildi. Tüm izolatlarda XN21 ile duyarlılık testi yapıldı. CZA ve C/T için ayrıca DD yöntemi uygulanarak kategorik uyum (CA), majör hata (ME) ve çok majör hata (VME) hesaplandı. FDC tüm suşlarda DD yöntemi ile değerlendirildi. BULGULAR: K. pneumoniae izolatlarında DD yöntemi uygulanarak saptanan duyarlılık oranları CZA için %51,2, C/T için %4,9 ve FDC için %48,8 iken; XN21 yöntemi ile bu oranlar CZA için %43,9 ve C/T için %2,4 olarak belirlenmiştir. Ayrıca, tikarsilin/klavulanat (T/K) için herhangi bir duyarlılık saptanmazken, sefoperazon/sulbaktam (CPS/SUL) duyarlılığı %2,4 idi. P. aeruginosa izolatlarında ise DD yöntemi ile duyarlılık oranları CZA, C/T ve FDC için sırasıyla %76,5, %82,4 ve %88,2 olarak; XN21 yöntemi ile CZA için %64,7 ve C/T için %58,8 olarak saptanmıştır. SONUÇ: Bu çalışma, VITEK 2 AST-XN21’in CZA ve C/T için genel olarak kabul edilebilir oranda uyum sağladığını, ancak P. aeruginosa’da sınır değerli C/T sonuçlarının doğrulanması gerektiğini göstermiştir. K. pneumoniae’da T/K ve CPS/SUL etkinliği düşüktür. DD ile değerlendirilen FDC P. aeruginosa’da etkili bulunmuş; sınır değerli/uyumsuz sonuçlarda referans yöntemle doğrulama testleri önerilmektedir.

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Cite This Article

Ünlü, S., Gürbüz, M. (2026). ANTIMICROBIAL SUSCEPTIBILITY TESTING WITH THE VITEK 2 AST-XN21CARD: COMPARISON WITH DISK DIFFUSION IN KLEBSIELLA PNEUMONIAE AND PSEUDOMONAS AERUGINOSA STRAINS AND THE EFFICACY OF NEW - β LACTAMS, INCLUDING CEFIDEROCOL. Kocatepe Tıp Dergisi, Cilt: 27 Sayı: 4, 508–517. https://doi.org/10.18229/kocatepetip.1884962

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