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

• Published in: Journal of antimicrobial chemotherapy 2004-04, Vol.53 (4), p.660-663
• Main Authors: Kärpänoja, Pauliina, Nissinen, Antti, Huovinen, Pentti, Sarkkinen, Hannu
• Format: Article
• Language: English
• Subjects: Amoxicillin-Potassium Clavulanate Combination - pharmacology; Ampicillin - pharmacology; Antibiotics. Antiinfectious agents. Antiparasitic agents; Biological and medical sciences; BLNAR; Dose-Response Relationship, Drug; Finland; H. influenzae; Haemophilus influenzae; Haemophilus influenzae - drug effects; Haemophilus influenzae - isolation & purification; Humans; Medical sciences; Microbial Sensitivity Tests - methods; Microbial Sensitivity Tests - standards; Penicillin Resistance - drug effects; Penicillin Resistance - physiology; Pharmacology. Drug treatments; Sensitivity and Specificity; susceptibility testing
• ISSN: 0305-7453; 1460-2091; 1460-2091
• DOI: 10.1093/jac/dkh134

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.