Journal Article

Disc diffusion susceptibility testing of <i>Haemophilus influenzae</i> by NCCLS methodology using low-strength ampicillin and co-amoxiclav discs

Pauliina Kärpänoja, Antti Nissinen, Pentti Huovinen and Hannu Sarkkinen

in Journal of Antimicrobial Chemotherapy

Published on behalf of British Society for Antimicrobial Chemotherapy

Volume 53, issue 4, pages 660-663
Published in print April 2004 | ISSN: 0305-7453
Published online April 2004 | e-ISSN: 1460-2091 | DOI:
Disc diffusion susceptibility testing of Haemophilus influenzae  by NCCLS methodology using low-strength ampicillin and  co-amoxiclav discs

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  • Medical Oncology
  • Critical Care


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Objectives: The objective of this multicentre study was to define the accuracy and reproducibility of the NCCLS disc diffusion method for Haemophilus influenzae against ampicillin and co-amoxiclav in Finnish clinical microbiology laboratories. Special attention was paid to the ability of the laboratories to detect β-lactamase-negative ampicillin-resistant (BLNAR) strains.

Methods: Three BLNAR and two β-lactamase-negative ampicillin-susceptible isolates (BLNAS)—originating from the American Type Culture Collection (ATCC) and UK National External Quality Assessment (UKNEQAS) schemes—were included in this study. Susceptibility tests for these isolates were performed in 26 clinical microbiology laboratories, in accordance with NCCLS guidelines. Additionally, low-strength discs for ampicillin (2 µg) and co-amoxiclav (3 µg) were tested.

Results: The low-strength discs for ampicillin and co-amoxiclav categorized more accurately BLNAR and BLNAS H. influenzae isolates than did the high-strength discs recommended by the NCCLS. In addition, the high-strength discs produced more major errors than the low-strength discs (22 versus six for ampicillin and 40 versus seven for co-amoxiclav). Great variation occurred in the method regardless of the antibiotic concentration of the discs.

Conclusions: The use of low-content ampicillin and co-amoxiclav discs is recommended for the susceptibility testing of H. influenzae. Interpretative criteria of S≥17 mm and R≤13 mm for both discs are suggested.

Keywords: Keywords: H. influenzae, BLNAR, susceptibility testing

Journal Article.  2204 words. 

Subjects: Medical Oncology ; Critical Care

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