Rapid Detection of Antibiotic Resistance

Vol. 15 •Issue 21 • Page 13
The Learning Scope:

Rapid Detection of Antibiotic Resistance

Editor’s note: This Learning Scope article offers a unique opportunity for clinical laboratory personnel to earn continuing education credit. It is offered to our subscribers six times a year. Simply read the peer-reviewed article and send your answers to the questions that follow to the American Society for Clinical Laboratory Science (ASCLS). As does this one, most Learning Scope articles provide 0.1 CEU or 1 contact hour if you score 70 or above. The credit is issued through ASCLS’ PACE program. The ASCLS is the oldest and largest non-registry, non-profit professional membership association representing non-physician clinical laboratory practitioners. You don’t need to be a member to earn the credit. Additional details concerning the CEU offer are listed to the left of the answer sheet on page 24. Tests may be submitted up to one year following publication in ADVANCE. The answer sheet may be copied for a fellow laboratorian to submit for credit.

Learning Objectives

At the conclusion of this article the participant will be able to:

• Define the target population, corresponding organisms and diseases most amenable and benefited by rapid detection methods for antibiotic resistance.

• List the current and emerging rapid detection methods for antibiotic resistance including antigen detection and real-time PCR.

• Define emerging algorithms for integrating clinical informatics and antibiotic resistance data to amplify more cost-effective empiric antibiotic selection.

• List methods of incorporating/implementing rapid antibiotic resistance for selected specimens via stratifying laboratory workflow.

• Describe the hospital beneficiaries from rapid resistance testing and expanded outcomes measurements.

Infections that are resistant to antibiotics not only can be deadly, they can have a financial impact. Resistant infections cost twice as much to treat as susceptible ones, amounting to an estimated $30 million-$100 million each year in the United States. As an example, treating resistant strains of Pseudomonas aeruginosa can cost $7,340 more than for a susceptible isolate.1

This can be directly attributable to the amount of time patients now spend in the ICU and the resistance trends within that unit, where antibiotics are obviously an important component of patient management. Recent data reveal that the incidence of antibiotic resistance increased approximately 23 percent in ICUs from 1989-1998, as opposed to 13 percent in non-ICUs. It is the norm in ICUs that, following 4 days within the unit, patients are on at least three antimicrobials and perhaps one antifungal, and will have as many as four lines for patient management. These include central venous lines, total parenteral nutrition, perhaps an endotrach for ventilation and several lines for peripheral access.

Device-related infections have become significant, particularly in the ICU. It is estimated by CDC that more than 80 percent of the infections in the ICU can be attributed to some form of lumenal abiotic surface to which organisms can be attached (e.g., sessile versus planktonic).

Table 1 lists some of the most recent data about the numbers of devices used and the potential infection rate. In the United States alone, 200 million catheters of all types are used annually. The estimated cost of treating one episode of a vascular catheter-related blood stream infection has been estimated at $28,000 for patients in the ICU and hospital stay prolonged by an average of 7 days.

Morbidity and mortality associated with antibiotic resistance is almost incomprehensible. Throughout the last 10-15 years, there has been a growing outcry from the public as well as specific professional groups (Reservoirs of Antibiotic Resistance Network and The Alliance for the Prudent Use of Antibiotics), based on the accumulation of hundreds of publications that antibiotic resistance is a global problem. The reasons are multi-factorial, including:

• sicker patients (case mix index [CMI]) or patient disease severity;

• increased length of stay/hospital versus ICU;

• non-prescription antimicrobials (e.g., soaps, bandages, mouth rinse); and

• use as growth promoters and veterinarian antimicrobial.

Although not often recognized, antimicrobial usage in non-prescription as growth promoters and veterinarian antimicrobials far outweighs antimicrobials used in management of patients. This has led to an international demand for the removal of antimicrobials as growth promoters, and several publications recently have addressed this issue.2

However, no one can dispute the use of antimicrobials in hospitals. Table 2 lists the recognized national trend in teaching institutions. It also lists key features defining this increased use.

WVUH: Antibiotic Resistant Trends

Given that a great deal of information has suggested that antibiotic resistance, although universal, is regional as well as different for each individual hospital, laboratory staff at West Virginia University Hospital (WVUH) tracked resistance in our institution via a number of different modalities. We are a member of The Surveillance Network (TSN) (Herndon, VA) and used an electronic surveillance that allowed us to tabulate a variety of “sort-parameters.” As a general signature or impression of resistance development, we measured eight antibiotics against 10 organisms over time (1995-2001). The organisms were retrieved from analysis of incident data for the entire institution, not segregated by any site or source. By looking at the organisms responsible and comparing them to the antibiotic usage, one can also ascribe this trend to a more selected population. On the other hand, there are selective bug/drug combinations that obviously presented the greatest potential cost benefit relative to patient management and rapid diagnostics.

Resistance analysis also includes percent resistance by year and the traditional antibiogram, but also highlights a more focused and appropriate method of selecting for the bug/drug combination that might be most beneficial if the results were available more rapidly. Table 3A highlights antimicrobial resistance (percent R) without reference to its relative weight given the frequency of appearance of that particular phenotype. Relative resistance, however, is based on the distribution of the resistance phenotypes as shown in the column next to it (Relative Resistance). This defines a phenotype prevalence data and antibiotic resistance relative to its appearance and frequency for which that resistance is recognized. This is very important. At WVUH, we not only establish the relative resistance for the entire institution (Table 3A), but subset it according to six patient locations: acute care, bone marrow transplant and cancer, ICU (Table 3B), pediatrics (Table 3C), outpatient and intensive care.

ART Instead of AST

The emergence and dissemination of resistance has made it imperative for laboratories to use in vitro tests that can detect resistance. The concept of antimicrobial resistance testing (ART) should replace antimicrobial susceptibility testing (AST). ART requires flexibility in testing systems and current AST methods are often handicapped. This is due to an over-emphasis on standardization, not accounting for the diversity and complexity of resistance mechanisms.

Limitations of AST Methods

AST methods were originally developed when antibiotic resistance was low. Techniques such as disc diffusion, microdilution and rapid automated systems are suited for testing rapidly growing bacteria. They were designed for convenience, rapid turnover of clinical samples and simplification of data handling.

Some of the underlying principles of these systems account for their limited ability to detect resistance. Due to the unstable antibiotic gradient in a disc test, zone sizes are directly influenced by inoculum and growth rates regardless of susceptibility. This makes the test only useful primarily for screening the susceptibility of rapidly growing aerobes.

Microdilution, restricted by volume and limiting nutritive capacity, tends to give false susceptible results. Rapid methods may not detect resistance because many mechanisms are not expressed within the 4-6 hour incubation period used. When using small inocula, a necessity in both manual and automated micromethods, the odds of capturing resistant subpopulation are minimal.

Preliminary Targeting

Prior to implementing any laboratory assay a variety of questions should be asked, not the least of which includes the following:

1. What is the target patient population and who would benefit most from the rapid result based on morbidity and mortality, length of stay and cost?

2. What is the hospital service area, specimen type and/or diseases most often associated with this target population?

3. What is the bug/drug combination (in the previous two instances) that would be most critical for antibiotic management, focused in a rapid laboratory protocol?

Analysis should address a cascade involvement.

Target patient population—The change in patient population is dramatically recognized by the CMI. Patients who are admitted to hospitals are sicker, have a need for immediate care and, because of the age of the population and the accumulated co-morbidities, need to be selected carefully. We use DRGs (Diagnostic Related Groups), which has a basis, national data for length of stay, costs and additional parameters.

Hospital service area—Patient location within the institution obviously defines the seriousness of the disease and the potential need for rapid diagnostics. Length of stay in the ICU has increased dramatically and an ever-increasing percent of WVUH patients now spend at least 1 day in the ICU. Antibiotic pressures have always been recognized as greatest in the ICU and the need for potential rapid intervention with appropriate antibiotics is higher than any other area of the institution. However, growing services such as bone marrow transplant/cancer and visits to the emergency department impact the decision on importance of location. In West Virginia, nearly 40 percent of the patients have no family physicians and thus regularly use the emergency department.

Target disease—What is the disease associated with a patient and particularly the patient location within the institution? This also directly impacts on the kind of specimen received in the laboratory. Selection of the target organism is critical in that it will describe the potential options available of a rapid diagnostic resistance measurement. It will also address potential measurement of a new workflow, given that specimens received in the laboratory may be selected for a particular type.

Target resistance profile—We attempted to unmask the potential resistance profiles by defining for each of six services the incidence of organisms within those services. We used three specimen sources: all, blood and urine. The incidence and antibiotic resistance for these organisms is posted on the Internet (www.wvuhlab.com). For each hospital service, we also establish relative resistance based on the total resistance for all markers.

Current and Emerging Rapid Detection Methods

Diagnostic procedures that could be classified as rapid and available to the diagnostic clinical laboratory are listed in Table 4. Although there is an extensive array of published literature describing rapid methods, these selected represent those that either are readily available or do not require extensive molecular methodology.3-12

The key, of course, is to amalgamate or link the test procedures to the most appropriate patient population and specimen type. Clearly, that will depend on the data discussed previously, including prevalence of the organisms, kind of patient population and the impact on the patient management. Do not forget, however, the impact of rapid reporting on infection control and pharmacy. These are often not in the initial decision tree and strategy, but should be.

Obviously, infection is concerned with patient location and protection for the institution as well as for the patient. Pharmacy’s formulary is often impacted by the ability to rapidly detect and utilize a particular antimicrobial. They clearly need to be part of the decision tree and any use of an antimicrobial assay at WVUH is always correlated with the opinions of the pharmacy department and clinical pharmacist.

Making the Case

We have chosen a case presentation to highlight the benefit of rapid reporting and highlight mechanisms of resistance. Three examples include:

1. Outpatient (community acquired pneumonia) — (Screen Positive for Penicillin Resistance)

2. Inpatient (Non-ICU) (Vancomycin-Resistant S. aureus [VRSA])

3. Inpatient (NICU) MDR (multi-drug resistant) Enterobacter cloacae (IBL – Inducible-b-Lactamase)

Case 1: Penicillin Screen-Resistant Pneumococcus

This is the case of a 34-year-old male who had leukemia, refracted to chemotherapy. He presented to the ED with fever and mental status changes. He was referred to the neurology service, which ordered a CAT scan that showed low-density lesions in the basal ganglia. There was some question whether this was metastasis or infectious etiology. Culture results are shown below.

• CSF: purulent

• CSF culture and gram stain: negative

• CSF cell count: 2,000 WBC/mL

• Four of four blood bottles positive for Gram positive coccus at 18 hours; identified as S. pneumoniae with the following traditional profile: oxacillin screen — 12 mm, interpretation – R, Subsequent results: EMIC — 0.03 mcg/mL — Interpretation; penicillin, S; Susceptible (Etest)

• Additional antibiotics tested by Etest included: erythromycin, clindamycin, cefepime, levofloxacin

The patient was treated with IV penicillin and gentamicin, blood cultures remained positive for 3 weeks and vancomycin was added. Patient expired on day 27.

There are several mechanisms of penicillin resistance. The most common is alteration of penicillin binding proteins (PBP-S). S. pneumoniae contains six penicillin-linking proteins: 1A, 1B, 2B, 2X and 3. If the alteration occurs in either 1A, 2B or 2X, it equals high resistance to penicillin. If alterations occur in 1A and 2X, there is a high resistance to the third generation cephalosporins, cefotaxime and ceftriaxone.

For mechanisms of macrolide resistance, there are basically two predominant mechanisms. The ERM AM is high-level resistance and deals with methalation of 23 S, RNA, and the mef E deals with mid-range resistance (1-32 mcg/mL) and is associated with an efflux pump.

There are several problems in the traditional method of performing resistance testing for S. pneumoniae. A preliminary screen for S. pneumoniae using an oxacillin disk has the potential to produce to false positive resistance due to a low prevalence resistance population. One-fifth of the isolates sent to reference laboratories are found not to be S. pneumoniae. Additionally, erythromycin (a macrolide) does not predict azithromycin (a supermacrolide) and the FDA and NCCLS have two distinct endpoints for resistance: 1.0 mcg/mL for the NCCLS, 2.0 ug/mL for FDA. Further, clinical reports suggest that 4 mcg would be the most reasonable breakpoint to establish efficacy. There are additional modifications with the breakpoints for penicillin-resistant pneumococcus: third-generation cephalosporins have two interpretations, a non-meningitis and a meningitis interpretation.

Hence, a rapid molecular method for addressing the mechanism resistance at the time of initial isolation would have been significant, in this case. It would have: 1) confirmed the mechanism of resistance and 2) confirmed the organism identification and treatment options earlier in the case.

A rapid antigen test could have been instrumental, also. A urine antigen test is also available and often part of a CAP algorithm. It is useful for both spinal fluid and urine.

Cost of the case to the institution was $53,000 and saved on a length of stay by 9 days, 4 in ICU. Both morbidity and mortality potentially could have been altered if the mechanism of resistance and early detection had been available.

Furthermore, antibiogram data for S. pneumoniae from the community (outpatient) based on relative resistance would have suggested an emperic therapy different from that chosen.

Case 2: Vancomycin-Resistant S. aureus

A 47-year-old diabetic with peripheral vascular disease and chronic renal failure was on dialysis 3 times a week. He had been previously treated for chronic foot ulcers and had a history of multiple antibiotic therapy, including vancomycin.

Due to a local infection, a toe amputation occurred while the patient was hospitalized; subsequently, methicillin-resistant S. aureus was isolated from the AB graft as well as simultaneously from four of four blood cultures. The patient was treated with vancomycin and rifampin, and duration of hospital stay was approximately 6 days. Two months later, the catheter exit site was cultured and VRSA was isolated with a vancomycin MIC ³128 mcg/mL. Although treatment was successful in eradicating the exit site, the foot ulcer continued to deteriorate. The foot ulcer was recultured a month later with the following results: vancomycin-resistant Enterococcus faecalis with a vancomycin MIC of 16 ug/mL, VRSA with a vancomycin MIC of 128 and, MDR Klebsiella oxytoca. Blood cultures drawn at time of the latest admission yielded four of four positive for VRSA.

The patient was treated with TMP/SMX and aggressive wound care.

There are three traditional methods available to identify methicillin-resistant/ vancomycin-resistant S. aureus, as a first step: 1) Kirby Bauer, disk diffusion, 2) automated MIC and 3) oxacillin screen agar. MRSA latex agglutination (LA for PBP2a) and PCR for mec-A are relatively new methods.

Each has very specific criteria and each has potential problems. The disk diffusion or Kirby Bauer with VISA (vancomycin intermediate S. aureus) is not recommended. An MIC of 8 mcg/mL is susceptible, S, by disk diffusion, even if held for 24 hours. The method of choice is the MIC for either VISA or VRSA, but it is important to use 24-hour detection and transmitted light to measure the zone. If a screening agar is used, such as BHI with 6 mcg/mL of vancomycin, both VISA and VRSA can be detected, both will grow on this medium if held for 24 hours and, as with all assays, it is extremely important to maintain quality assurance using S. aureus, ATCC 29213, E. faecalis ATCC 51299.

Staphylococcus interpretation for vancomycin is as follows: susceptible less than 4, intermediate 8-16 and resistant ³ 32 mcg/mL. A VISA has an MIC range of 4-16 mcg whereas VRSA is > 32 mcg/mL.

Interpretation for Staphylococcus to the b-lactams is very critical clinically. It is important to remember that oxacillin-resistant Staphylococcus is resistant to all b-lactams, and often an attachment to a culture result will include this information.

The two newer methods offer the laboratory the greatest opportunity. The LA test (Denka Suken, Mugata, Japan and Oxoid) is the most adaptable but, according to latest references, needs overnight induction. The mec A gene has several kit-forms, also now available, and may have greatest advocacy for coagulase-negative staphylococcus (CoNS). Some investigators are recommending that all staphylococcal isolates from blood or sterile sites be routinely tested for the presence of PBP2a.

The patient was treated for 14 days and subsequently released following aggressive wound management and the use of selected antimicrobials. The rapid antigen detection of the vancomycin resistance would have been very appropriate and had impact on not only the management, but the isolation policies and potential cross infection had the results been known at the time the blood culture was positive.

Case 3 – Inducible §-lactamases (IBL Enterobacter cloacae)

A 34-year-old male presented with a post-neurologic meningitis caused by E. cloacae. Initial susceptibility results are shown below as primary isolate and indicate a community-acquired, susceptible isolate. He was placed on cefotaxime and responded well. On day 7, however, the patient became febrile. Neurologic symptoms returned and additional blood cultures and CSF specimens were obtained. Secondary isolate results are shown below as well.

CSF cell count: 110 WBCs/mL, few red cells

Gram stain: Gram negative rods

Blood cultures: 4 of 4 bottles positive for gram-negative rod E. cloacae

Sensitivities were as follows:

Primary Isolate Secondary Isolate
Antibiotic Interpret Interpret
Cefotaxime S R
Ceftriaxone S R
Ceftazidime S R
Aztreonam S R
Ticar/Clav S R
Gentamicin R R
Tobramycin S R
Amikacin S S
Imipenem S S

This patient most likely had an inducible, chromosomal mediated b-lactamase or cephalosporinase. This chromosomal mediated response is often associated with the “SPICE” organisms: Serratia spp., Providentia spp., Inducible Proteus, Citrobacter freundii and Enterobacter spp. It also has been associated with Pseudomonas aeruginosa and Aeromonas spp. This is an IBL associated with the AmpC gene. AmpC gene detection is important because there are two interesting mechanisms for activation of this marker. It is recognizable inducible or produces a low-level resistance reversible upon removal of the stimulant. On the other hand, the low-level background may allow for selection of a specific mutant, which is irreversible and occurs in about 1 to every 106 organisms in the pool.

Clinical consequences are the multi-drug resistance that is associated with the cephalosporins (first-, second- and third-generation), cephamycins, monobactams and penicillins. A few characteristics are:

• b-lactamases Plasmid mediated AmpC: 1998-now

• Derived from IBLs

• Can be found in Klebsiella, E. coli, Salmonella, Proteus mirabilis

• Many types

• Unusual resistance patterns, including resistance to non-b-lactam drugs

• Prevalence increasing

Traditional laboratory tests to confirm the presence of these growing lists of chromosomal-mediated enzymes is technically difficult. The three-dimensional test is, at best, performed in reference laboratories under controlled conditions. The AmpC disk is available but, as part of the assay, must be determined under controlled conditions using stock cultures.

The alternative, of course, is to use a rapid detection method such as listed in Table 4.

Costs and length of stay associated with Amp C IBLs are considerably less than those associated with ESBLs (extended b-lactamases) because of the alternate therapy potential and the low likelihood of cross contamination given their chromosomal nature. Infection control needs to be informed, but the immediacy of cross-contamination is less likely, as is multi-drug resistance seen with ESBLs.

Medical Informatics to the Rescue

Clearly, there is significant impact of resistant isolates on morbidity and mortality and, just as clearly, laboratories have been rather reticent to change workflow to incorporate some of the newer diagnostic protocols that may impact on this cost. Part of that is associated with the tradition of microbiologists establishing workflows that have been held and created in a different economic and disease-oriented time. Patient type, location and specimen source should be used to define the patient most at risk to benefit from resistance trending:

• blood cultures from ICUs with a gram-positive coccus in two or more bottles, utilize a gene or the IPB antigen test performed; positive results called to the floor and to infection control

• wound specimens from ICU patients with a Gram stain suggesting infection; gram-positive colonies from a screen plate confirmed as MRSA or (possibly) MRSE, again infection control and physician notified

• Blood cultures/CSF with gram-negative rods and a changing susceptibility profile for the “SPICE” organisms, confirmation using Amp C (IBL).

Emerging Strategies Using Informatics

The field of computerized decision support for infection illnesses was initiated in the early 1970s with a publication describing the development of MYCIN, a rule-based computer program that guided therapy. In the 1980s and ’90s, investigators at Latter Day Saints Hospital in Salt Lake City developed an entire anti-infectives management program and reported quality of care improvements and cost of therapy reductions associated with its use. At WVUH, the bacterial susceptibility reports (Antibiograms) were published on the Internet in 1998. Others worldwide have taken note of the progress in this field, and computerized decision support systems have been developed for hospital-based infections in Israel and Germany and for urinary tract infections in Denmark. In addition, bioMŽrieux-Vitek (Durham, NC), the manufacturer of one of the most popular laboratory bacteriology and antimicrobial susceptibility detection systems, has developed an embedded EXPERT system that applies rules-based computer logic to microbiology results for the purpose of test QA and for the provision of simple therapy decisions. BioMŽrieux also cooperates with the TSN database of national bacterial culture results and antibiotic susceptibilities. This tool has been useful for microbiologists and infectious disease specialists to compare local bacterial resistance profiles versus national standard profiles, as described earlier but, again, does not project or predict clinical culture results to assist physicians choosing emperic antibiotic therapy.

Two groups have developed antibiotic therapy guidance programs for Palm personal digital assistants (Johns Hopkins University’s ABXGuide; Epocrates’ ID). These programs provide expert therapy opinions for infectious diseases, and are therefore similar to the numerous guidebooks available on the market. As these programs provide only generic advice and are not founded on local antibiotic susceptibility patterns, they are limited. Theradoc, a new computerized clinical decision support company based in Salt Lake City, markets a computerized “Antibiotic Assistant” that provides therapy recommendations based on guidelines; however, getting this guideline advice is quite expensive as the price for the Theradoc product is $1,000 per hospital bed (~ $359,000 for WVUH).

Antibiotic therapy is costly, and now comprises 25-40 percent of hospital pharmacy budgets. Expenses also have increased 40 percent since the advent of fixed reimbursement. Two studies mentioned above found that a computerized antibiotic decision support decreased costs of antibiotic therapy through selection of less expensive agents and use of shorter durations of therapy. The U.S. Congress Office of Technology Assessment published an estimate that hospitalization costs for infections caused by one resistant pathogen, methicillin-resistant S. aureus, amounted to at least $122 million in 1992. These data underscore the need to prevent the emergence of resistant bacteria by minimizing exposure of patients and bacteria to broad spectrum antibiotics. The gravity of this issue served as the foundation for the call for a national meeting on “Computer Decision Support for Antibiotic Prescribing” held last summer. The emergence of resistant organisms has contributed greater to escalations in the costs of antimicrobial therapy (Table 5).

Related Web Sites

West Virginia University Hospitals



CDC Division of Laboratory Systems

Multilevel antimicrobial susceptibility testing educational resource



The Surveillance Network (TSN)


Global Infectious Disease and Epidemiology Network (GIDEON)



1. Hoffken G, Niederman MS. Nosocomial pneumonia: The importance of a de-escalating strategy for antibiotic treatment of pneumonia in the ICU. Chest 2002;122(6):2183-2196.

2. Impacts of antimicrobial growth promoter termination in Denmark: The WHO international review panel’s evaluation of the termination of the use of antimicrobial growth promoters in Denmark. WHO. Foulum, Denmark, Nov. 2002.

3. Albert H, Heydenrych A, Mole R, et al. Evaluation of FASTPlaqueTB-RIF, a rapid, manual test for the determination of rifampicin resistance from Mycobacterium tuberculosis cultures. Int J Tuberc Lung Dis 2001;5(10):906-911.

4. Beck IA, Mahalanabis M, Pepper G, et al. Rapid and sensitive oligonucleotide ligation assay for detection of mutations in human immunodeficiency virus type 1 associated with high-level resistance to protease inhibitors. J Clin Microbiol 2002;40(4):1413-1419.

5.Garcia de Viedma D. Rapid detection of resistance in Mycobacterium tuberculosis: A review discussing molecular approaches. Clin Microbiol Infect 2003;9(5):349-359.

6. Garcia de Viedma D, del Sol Diaz Infantes M, Lasala F, et al. New real-time PCR able to detect in a single tube multiple rifampin resistance mutations and high-level isoniazid resistance mutations in Mycobacterium tuberculosis. J Clin Microbiol 2002;40(3):988-995.

7. Fluit AC, Visser MR, Schmitz FJ. Molecular detection of antimicrobial resistance. Clin Microbiol Rev 2001;14(4):836-871.

8. Francois P, Pittet D, Bento M, et al. Rapid detection of methicillin-resistant Staphylococcus aureus directly from sterile or nonsterile clinical samples by a new molecular assay. J Clin Microbiol 2003;41(1):254-260.

9. Hussain Z, Stoakes L, John MA, et al. Detection of methicillin resistance in primary blood culture isolates of coagulase-negative staphylococci by PCR, slide agglutination, disk diffusion and a commercial method. J Clin Microbiol 2002;40(6):2251-2253.

10. Moore JE, Millar BC, Yongmin X, et al. A rapid molecular assay for the detection of antibiotic resistance determinants in causal agents of infective endocarditis. J Appl Microbiol 2001;90(5):719-726.

11. Perez-Perez FJ, Hanson ND. Detection of plasmid-mediated AmpC beta-lactamase genes in clinical isolates by using multiplex PCR. J Clin Microbiol 2002;40(6):2153-2162.

12. Yang SJ, Park KY, Seo KS, et al. Multidrug-resistant Salmonella typhimurium and Salmonella enteritidis identified by multiplex PCR from animals. J Vet Sci 2001;2(3):181-188.

Dr. Thomas is professor and director, Microbiology-Virology, at West Virginia University Hospital. Dr. Bennett is a resident in the Department of Pathology, West Virginia University School of Medicine, Morgantown.

Learning Scope Questions

1. Continued increases in patient management have been associated with:

a) increased length of stay.

b) increased antibiotic resistance.

c) increased length of stay in the ICU.

d) all of the above.

2. The patient population dynamics have changed considerably over the past 15 years. This has resulted in:

a) an increased CMI.

b) an increased DRG.

c) a decrease in length of stay.

d) less comorbidity.

3. In addressing the potential benefit of rapid susceptibility testing or antigen detection, one should evaluate which of the following to focus the benefits?

a) ICU patient population and specimen source

b) size of institution

c) number of blood cultures

d) staffing for late shifts

4. Relative resistance vs. absolute resistance or susceptibility is advantageous in unmasking the significant bug/drug resistance profile because it:

a) integrates significance of incidence of the phenotype.

b) addresses the resistance population.

c) stratifies by percent within a resistance population.

d) all of the above.

5. A preliminary screen for Streptococcus pneumoniae using an oxacillin disk has the potential to produce to false positive resistance due to:

a) a low prevalence resistance population.

b) poor potency of oxacillin disk.

c) too heavy an inoculum.

d) use of CO2.

6. The AmpC gene associated with inducible b-lactamases has several key characteristics including:

a) it most commonly occurs on the plasmid.

b) it is involved in the two-step, the initial one reversible, the second irreversible.

c) increasing prevalence.

d) all of the above.

7. The most frequent clinical isolates generally recovered processing the AmpC include:

a) E. coli and Klebsiella.

b) “SPICE” organisms.

c) S. aureus.

d) S. penumoniae.

8. Laboratory cost analysis should emphasize patient outcomes including:

a) DRG and patient length of stay (total).

b) number of laboratory tests performed.

c) urban vs. rural setting.

d) age of patient.

9. The rapid identification of antibiotic resistance is particularly important in today’s health care climate because:

a) resistant infections cost much more to treat than susceptible ones.

b) of a decreasing length of stay in ICU.

c) of a decreasing usage of indwelling medical devices.

d) of a decreasing presence of ESBLs.

10. The principal reasons for the growing problem of antibiotic resistance include:

a) sicker patients by CMI.

b) a decreased length of stay in ICU.

c) a decreased usage of antimicrobials as growth promoters.

d) a decreased use of antibiotics in pediatric population.

11. What is the most common method of acquisition of penicillin resistance?

a) Alterations in membrane pores

b) Acquisition of plasmid-mediated b-lactamase production

c) Alterations in PBPs

d) None of the above

12. Which of these is not among the conventional methods available to identify MRSA?

a) Automated MIC

b) Kirby-Bauer disk diffusion

c) Oligonucleotide ligation assays

d) Oxacillin screen agar

13. Which of these is not among the problems inherent in the traditional method of resistance testing for S. pneumoniae?

a) Lack of concordance between NCCLS

and FDA endpoints for defining resistance

b) Lack of predictive value for shared resistance among different antibiotics in the macrolide class

c) Non-reproducibility of disk diffusion method among different observers

d) Relatively high percentage of isolates sent to reference laboratories not identified as S. pneumoniae

14. Factors impeding the process of implementing rapid detection methods of antibiotic resistance in clinical laboratories include:

a) tradition of microbiologists.

b) workflows that have been held and created in a different economic and disease-oriented time.

c) cost analysis based on lab not patient outcomes.

d) all of the above.

15. Which of the following bug/drug combinations has the highest relative resistance (%) in the ICU setting?

a) Methcillin-resistant coagulase negative Staph

b) Ciprofloxacin-resistant P. aeruginosa

c) Vancomycin-resistant E. faecium

d) Penicillin-resistant S. pneumonia

About The Author