الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
A vital role of a microbiology laboratory in patient care is to aid physician to identify the etiologic agents of several infectious diseases and help them to determine the antibiotic of choice for treatment of such infections by means of AST.
Disc diffusion method is the most practical susceptibility testing. The accuracy of results of disc diffusion test can be affected by multiple factors, which include the media, antimicrobial discs, inoculum size, plate reading, incubation condition and the competence of the medical laboratory personnel. Therefore, it is crucial that each variable in the procedure should be standardized and carefully controlled. One of the most critical components of the disc diffusion test is the medium used. The most famous and commonly followed AST standardizing organization CLSI and EUCAST both recommend the use of MH agar as a susceptibility test medium.
A pilot study was carried out covering 30 different laboratories to evaluate the adherence to the CLSI guidelines during AST specially the use of MH agar. Agar diffusion tests was not conducted correctly in many laboratories and instead of MH agar, NA was used to perform AST.
Therefore, the present study was carried out to evaluate nutrient agar and MH agar for antibiotic for their validity in antibiotic susceptibility test.
Each of the three reference strains; S. aureus ATCC® 25923, E.coli ATCC®25922 and P. aeruginosa ATCC®27853 and 149 clinical bacterial isolates (49 P. aeruginosa, 50 S. aureus and 50 Enterobacteriaceae) were tested for their antibiotic sensitivity by the Kirby-Bauer technique using the two media following CLSI guidelines. Inhibition zones obtained from testing the three reference strains as a quality control measure over 30 consecutive days were compared to standard inhibition zones in CLSI zone size interpretative tables together with inhibition zones obtained from testing the clinical isolates.
A pilot study was carried out to evaluate AST compliance of 30 randomly selected microbiology laboratories revealed that:
1-Gram stain technique for identification of the causative pathogen isolated from clinical samples was carried out in all laboratories, followed by a battery of biochemical tests used according to standard scheme for identification of the isolate; performed in the majority of the laboratories. In addition, 14(46.70%) laboratories used API system and 3(10%) laboratories used Vitek system to verify isolates.
2-Muller Hinton agar was used only by 13(43.3%) laboratories while NA and Tryptic Soya agar were used by 11(36.70%) and 6(20.00%) laboratories respectively.
3-Inoculum preparation by direct colony suspension was observed in 14(46.7%) laboratories where only 12(40%) used McFarland turbidity standard.
4-Overcrowding of antibiotic discs (> 5 discs/100 mm plate) was observed in 21 (70%) laboratories.
5-CLSI zone size interpretative tables for AST results interpretation were used by 14 (46.70%) laboratories.
6-Quality control strains were used by 7 (23.30%) laboratories only where 2 (28.60%) laboratories used them properly while the other laboratories used them in the wrong manner.
Using NA instead of MH agar for AST revealed the following:
7- All antibiotic organism (reference strains) challenges (450) on NA compared to the gold standard MH agar were unacceptable (>3 out of limit zones in 30 consecutive days).
8- Frequency of very major errors (false sensitive), major errors (false resistant) and minor errors of AST for clinical isolates on NA compared to MH was highest in case of P. aeruginosa (8.98%, 4.08% and 14.7% respectively) followed by S. aureus (7.6%, 6% and 8.8% respectively). On the other hand, the least frequency of errors were in case of Enterobacteriaceae with 0% very major errors, 0.4% major errors and 3.2% minor errors.
Conclusions
1.Lack of compliance with CLSI guidelines for AST in some microbiology laboratories.
2.All AST results of antibiotic/reference bacterial strains combination were unacceptable with the use of NA.
3.Use of NA for AST of clinical isolates was associated with remarkable very major, major and minor errors.
4.Quality control of AST is essential for validation of AST results