Here you will find answers to frequently asked questions about our products and concepts plus a list of useful links to other related/relevant sites.

Product FAQs:

Why have Rosco replaced the coloured Neo-Sensitabs by the unicoloured print-coded Neo-Sensitabs?
The print-coded Neo-Sensitabs still secure safe identification and the lettercode helps quick identification of the tablet. The size of the Neo-Sensitabs will make recognition of the letters both visually and optically easier than if the tablets were smaller. The lettercode has been made to achieve optimal recognition with automatic optic reading systems.

Why are most Neo-Sensitabs stable at room temperature when paper disks are not?
The Neo-Sensitabs are manufactured with the aid of microbially inert auxiliary substances by a dry process using crystalline antimicrobials. This procedure results in very uniform tablets, homogenous in their content of active ingredients, and with a long acting stability at room temperature for most of the Neo-Sensitabs.

Why is the diameter of Neo-Sensitabs 9 mm when paper disks are 6 mm and will it affect the zone size?
Through all the years Neo-Sensitabs have been marketed as 9 mm tablets. Now this permits identification codes with bigger letters and therefore easier identification than on 6 mm paper disks. The size of the inhibition zone depends on the potency of the tablet/disk more than on the size of the tablet/disk.

How do I find Quality Certificates for the individual LOT of Neo-Sensitabs?
All QC's are available on this website. You need to know the REF number and the LOT number of a specific product, and then simply write the numbers in the search box. For further instructions go to Search

Why use manual systems when automatic systems are easier and faster?

Automatic systems are often rapid, but do not forget to think yourself. Rapid is not always best, especially if the result is wrong or misleading. The lack of flexibility is one of the largest problems in the automatic systems. You cannot choose the antimicrobials that you wish to test. Using the agar disk (tablet) diffusion method with Neo-Sensitabs you have inherent flexibility of drug selection.

What is the CLSI (NCCLS) and are Neo-Sensitabs approved by CLSI?
The Clinical Laboratory Standards Institute (CLSI) (formerly NCCLS) is a private organization that convenes groups of experts from industry, academics, and government agencies on different topics and develops guidelines for clinical laboratory testing. The documents M2 and M7 are about how to perform antimicrobial susceptibility testing and they provide instructions for reference methods. CLSI's role is to develop generic reference-testing procedure, i.e. not to evaluate, endorse or recommend commercial antimicrobial susceptibility test systems.

Is there a European organization equivalent to CLSI (NCCLS)?
There is no European rules equivalent to the CLSI (NCCLS). The EUCAST is trying to reach a consensus on European MIC breakpoints, and recommendations for fluoroquinolones, aminoglycosides, glycopeptides and oxazolidinones are available (Dec. 2004). At least 7 or 8 systems are being used in Europe, based on different MIC breakpoints. In the User's Guide Neo-Sensitabs you will find MIC breakpoints in the following systems: CLSI (NCCLS), CRG (Holland), SRGA (Sweden), AFA (Norway), SFM (France), DIN (Germany), and BSAC (UK).

How can I screen Enterobacter spp. for ESBL (extended-spectrum beta-lactamase). Can I use the same procedure as for screening and confirmatory tests for ESBLs in E. coli and Klebsiella spp.?
CLSI (NCCLS) has not given recommendations yet for the detection of ESBL in Enterobacter spp. The same technique as for E. coli, and Klebsiella spp. cannot be used for detecting ESBL production in strains producing Amp C beta-lactamases.

Rosco recommends the use of cefepime and cefepime+clavulanate or the double disk (tablets) test with cefepime and amox+clavulanate (synergism). Cefepime is used because it is hardly affected by Amp C beta-lactamases from Enterobacter spp.

How can I screen E. coli and Klebsiella spp. for ESBL and is it in accordance with the recommendations from CLSI (NCCLS)?
One possibility of screening for ESBL is the use of lower MIC breakpoints and another possibility is the double disk (tablet) synergy test. More information on ESBL and further readings please see User's Guide.

Rosco recommends for phenotypic confirmatory testing Cefepime+Clavulanate and Ceftazidime+Clavulanate. An increase in zone diameter of > 5 mm for the combination with clavulanate compared to the Neo-Sensitabs without clavulanate is confirmatory of the presence of an ESBL. More information on ESBL and further readings please see User's Guide.

For screening and confirmatory tests for ESBL in E. coli and Klebsiella spp. CLSI (NCCLS) recommends the use of cefpodoxime, ceftazidime, aztreonam, cefotaxime og ceftriaxone with lowering of the inhibition zones because some of these ESBL strains will show zones below the normal susceptible population but above the standard breakpoints. The use of more than one antimicrobial agent improves the sensitivity of detection. CLSI (NCCLS) recommends ceftazidime in combination with ceftazidime-clavulanic acid and cefotaxime in combination with cefotaxime-clavulanic acid for confirmatory test.

Pseudomonas aeruginosa strains resistant to piperacillin but in-vitro susceptible to piperacillin/tazobactam, how should the report be?
Only the extended spectrum enzymes TEM-42, SHV-2a and PER-1 are inhibited by clavulanic acid and tazobactam and these enzymes are very rarely found in clinical specimens. Most other enzymes found clinically in Ps. aeruginosa, such as chromosomal cephalosporinases (Amp C), broad-spectrum penicillinases, oxacillinases and carbapenemases are not inhibited by tazobactam or clavulanic acid.

As a consequence, test a carbapenem and be careful in recommending piperacillin+tazobactam for a Ps. aeruginosa showing resistance to piperacillin, especially when treating serious infections.

How shall I test Stenotrophomonas maltophilia?
The CLSI (NCCLS) has established a subcommittee, which is trying to standardize susceptibility tests for S. maltophilia and Burkholderia cepacia. Incubation at 30 degrees seems to produce susceptibility results more in accordance with clinical effectiveness, while false susceptibility results are obtained with aminoglycosides and other drugs when incubated at 37 degrees. Only minocycline, levofloxacin and trimethroprim-sulphamethoxazole are recommended to be reported by CLSI (M100-S15, 2005). Trimethroprim + sulphamethoxazole is the drug of choice.

Are zones for polymyxin depending on the media?
Yes, colistin is a drug whose activity is influenced by the medium used for testing. MICs on agar media are always higher than in broth, because colistin binds to agar and consequently there is less free colistin available for acting against bacteria. Another factor is the divalent cation content of the media used. The higher the content of cations, the lower the activity of colistin.

Testing of anaerobes against amoxicillin or ampicillin?
Amoxicillin and ampicillin have the same antimicrobial spectrum of activity against anaerobes. There is cross-resistance and consequently the results obtained with one drug are valid for the other drug. The same MIC breakpoints are valid for both ampicillin and amoxicillin. If you use amoxicillin+clavulanate for anaerobes for practical reasons in any dilution method, it is convenient to use amoxicillin instead of introducing a new drug (ampicillin).

Hemolytic streptococci resistant to penicillin - is it possible?
Penicillin resistant haemolytic streptococci have not been isolated or described in any country.

Direct Beta-lactamase testing - will it detect all beta-lactamases?
Direct Beta-lactamase testing is useful in Enterococcus spp., Haemophilus influenzae, Moraxella catarrhalis, Neiseria gonorrhoeae, Staphylococcus spp. and some anaerobic bacteria. Different beta-lactamase detection tests are available and the one most used is chromogenic cephalosporins (e.g. nitrocefin). However, this test is not very sensitive to detect staphylococcal penicillinase and ROB-1, an uncommon plasmid mediated enzyme of haemophili.

None of the direct beta-lactamase test are helpful for determining beta-lactamase activity in isolates of other species and the beta-lactamase tests cannot be used to detect ESBLs, Amp C beta-lactamases or carbapenemases.

If the isolate is beta-lactamase positive, it should be reported resistant to amoxicillin, ampicillin, carbenicillin, penicillin, piperacillin and ticarcillin.

How can I test Haemophilus influenzae if I want to detect beta-lactamase negative ampicillin resistant strains (BLNAR)?
In order to detect BLNAR Rosco recommends using Ampicillin 2.5 mg Neo-Sensitabs. The low content tablet works better than the 10 mg disk or other Neo-Sensitabs (Ampicillin 33 mg or Amoxillin 30 mg) for testing susceptibility to ampicillin of Haemophilus spp. However, the most frequent resistance mechanism against ampicillin is beta-lactamase producing H. influenzae (plasmidic TEM or ROB-1), which can be detected by a beta-lactamase test e.g. Beta-Lactamase (acido) Diagnostic Tablets (Diatabs) or using Amoxycillin+Clavulanate Neo-Sensitabs compared to Amoxycillin alone. Synergism (larger zone with Amoxycillin+Clavulanate) will be seen in the presence of beta-lactamase.

Can I use the Beta-lactamase (acido) Diagnostic Tablets from Rosco to detect beta-lactamase producing strains?
The beta-lactamase test (acidometric) is suitable for detecting beta-lactamase from Haemophilus spp., Neisseria gonorrhoeae, and staphylococci. A positive test predicts resistance to penicillin, ampicillin and amoxicillin among Haemophilus spp. and Neisseria gonorrhoeae, and resistance to penicillin as well as amino-, carboxy-, and ureidipenicillins among staphylococci. A negative beta-lactamase test results does not rule out beta-lactamase resistance to other mechanisms and the method is not useful to test Moraxella catarrhalis and anaerobes.

An E. coli susceptible to gentamicin but resistant to tobramycin – are these results plausible?
Yes, this phenotype is quite possible. The enzyme AAC(6´)-I does not modify gentamicin but it modifies tobramycin, netilmicin and amikacin. Therefore the result susceptible to gentamicin should not be changed to resistant.

Why should gentamicin resistant staphylococci be reported as resistant to tobramycin, netilmicin, amikacin and isepamicin?
These strains possess the double enzyme AAC (6')-APH (2''), which confers resistance to most aminoglycoside antibiotics currently used in clinical practice. It is the same double enzyme that confers enterococci HLR (high level resistance).

Is E. faecium intrinsic resistant to synergy combinations beta-lactams/aminoglycoside?
E. faecium is known to harbour a chromosomally mediated enzyme AAC (6')-I and is consequently naturally resistant to all beta-lactam/aminoglycoside synergy combinations except gentamicin, isepamicin and streptomycin. Therefore Rosco recommend reporting HLR to all aminoglycosides except gentamicin, isepamicin and streptomycin without testing for E. faecium strains.