INTERPRETATION OF MICROBIOLOGY REPORTS

INTERPRETATION OF MICROBIOLOGY REPORTS

INTERPRETATION OF MICROBIOLOGY REPORTS

When a microorganism is isolated from a patient, the microbiology lab will often perform susceptibility testing. There is often confusion about what these results mean and how it can be used by the clinician to guide the treatment of the patient.

Antimicrobial susceptibility reports provide the primary basis for determining the incidence of antimicrobial resistant infections on which longitudinal assessments of the problems severity depend and for determining the prevalence of antimicrobial resistance among clinical isolates of common bacterial species that crucially informs empirical antimicrobial therapy strategies.

The microbiology results will show the identity of the organism and the appropriate antibiotic sensitivity pattern against each organism. Historically, Microbiology antibiotic susceptibility testing was routinely performed by Kirby bauer disk diffusion method. The size of the clear zone determined whether the bacterium was susceptible, resistant, or intermediate to a particular antimicrobial. Although a useful guide for selecting an effective antibiotic, Kirby-Bauer testing could not tell the clinician the exact concentration of antibiotic needed to achieve a therapeutic result.

In Microbiology susceptibility report, Next to each antibiotic is the susceptibility interpretation: S (sensitive), I (intermediate), or R (resistant), followed by the MIC in μg/mL. Sensitive implies that the organism is inhibited by the serum concentration of the drug that is achieved using the usual dosage; intermediate implies that the organisms are inhibited only by the maximum recommended dosage; and resistant implies that the organisms are resistant to the usually achievable serum drug levels. These interpretive standards have been established by the Clinical and Laboratory Standards Institute (CLSI).

Most Antimicrobial susceptibility reports will include MICs to determine the most effective antibiotic that will result in effective treatment.?The Minimum inhibitory concentration (MIC) provides the ability to precisely determine the concentration of antibiotic required to inhibit growth of a pathogen. The MIC number is the lowest concentration (in μg/mL) of an antibiotic that inhibits the growth of a given strain of bacteria. ??An MIC number for one antibiotic cannot be compared to the MIC number for another antibiotic. The choice of antibiotic should be based on the MIC value, the site of infection, and an antibiotic’s breakpoint. Consider safety, ease of use, and cost when determining the optimum antimicrobial.

The CLSI/EUCAST tables will aid in MIC interpretation and antimicrobial selection. However, Breakpoints are not available for some antimicrobials. In such cases, Epidemiological Cut off Value is used for interpretation of susceptibility reports. The Epidemiological Cut off Value [ECV (CLSI) or ECOFF (EUCAST)], are measures of a drug MIC distribution that separate bacterial populations into those representatives of a wild type population, and those with acquired or mutational resistance to the drug. Epidemiological Cut off Value should be used as an indicator of possible resistance mechanisms that could affect response to therapy in absence of breakpoints rather than predictor of response to therapy.

The concentration range tested for a drug and the interpretative criteria for various categories are based on extensive studies that correlate with Breakpoints. An antibiotic breakpoint is the dilution where bacteria begin to show resistance. The breakpoint and range of dilutions differ by drug and bacterial species. Therefore, comparing MICs of different antibiotics is not based solely on the numerical value but on how far the MIC is from the breakpoint, the site of the infection, and other considerations, such as the age, species, and health of the animal. Possible side effects of the drug, price, frequency, and route of administration are also important factors.

For example: A strain of Escherichia coli has an?MIC of 2 μg/mL for amoxicillin and for cefovecin. Looking at the dilutions for amoxicillin, at 2 μg/mL, this strain of E. coli is four dilutions away from the breakpoint. For cefovecin, the same strain of E. coli at an MIC of 2 μg/mL is two dilutions away from the breakpoint. So, based on MICs, this strain of E. coli is more susceptible to amoxicillin than cefovecin. Other factors to take into consideration are the site of the infection, the patients clinical conditions, frequency and route of administration, and cost of the antibiotic.

Some antibiotics are used to determine the susceptibility of other antibiotics in the same class. For example, the presence of methicillin-resistant staphylococci (MRS) is tested in the laboratory with oxacillin and not methicillin. Also, antibiotics susceptibility results can be interpolated to susceptibility of other classes of antibiotics. For e.g. Methicillin resistance can be inferred as resistant to other beta lactam group of antibiotics like Penicillins, cephalosporins, carbapenems and monobactams. Always take in account intrinsic resistances and possible resistances which are not always expressed in vitro testing.

Below Rules could help the clinicians to interpret the microbiology culture and susceptibility reports:

Rule 1:

??Always start with a beta-lactam if possible, especially in severe infections

??They have the best data supporting their use and are in general excellent drugs; exception: atypical infections

Rule 2:

??Do not compare MICs between drugs

??Each antibiotic has different pharmacodynamic?parameters

Time dependent antibiotics vs Concentration dependent antibiotics vs AUC/MIC dependent antibiotics

Rule 3:

??If MIC “≤” Breakpoint; you can use the drug

o??(Note exceptions below)

??Exceptions

???????Drug doesn’t get to the site of action

???????Drug doesn’t achieve its goal pharmacodynamics parameters

???????Drug doesn’t have inducible resistance

???????Patient-specific factors

???????Drug cost

Rule 4:

??Microbiology always has more information than what is reported

???????They may have results before they are reported in the computer

???????Antibiotics may be suppressed?

???????They can perform additional testing


Molecular AST are effective direct methods that eliminate tedious bacterial cultures, long incubation, chances of contamination, and the spreading of deadly infections. PCR, DNA microarray and DNA chips, and loop-mediated isothermal amplification (LAMP) are some of the genotypic techniques for the detection of antibiotic resistance. Rapid detection of resistance gene within few hours?helps in making decision to use more specific antimicrobial agent rather than broad spectrum antimicrobial agent.

An important limitation to molecular testing is that resistance detection is not the same as susceptibility testing, i.e., a negative result does not necessarily imply susceptibility. A number of alternative resistance mechanisms can still cause microbial resistance and, hence, treatment failure. For instance, despite lack of carbapenem’s detection Gram-negative bacteria may be resistant due to reduced permeability or increased efflux. In contrast, phenotypic susceptibility testing such as MIC broth dilution or Kirby bauer disc diffusion is universal, mechanism-independent and allows exact phenotypic categorization with direct therapeutic relevance.

Susceptibility testing is an in vitro phenomenon and does not necessarily reflect or predict in vivo efficacy. Susceptibility testing is subject to great variability depending on pathogen tested, media used, conditions of incubation, and method of accessing bacterial growth.

Antibiotic Susceptibility testing (AST) :

??Does not identify infection vs colonization vs contamination

??Don’t treat colonization or contamination

??Does not tell you which antibiotic to use

In any case, results of AST need to be interpreted with some background knowledge on the biology of the microorganism and patient history before treatment is prescribed. The patient history is important; underlying disease and previous treatments attempted are of outmost importance. Drug choice needs to take in account the drug properties and distribution for predicting efficacy at target organs.


Take home message:

??Choosing the correct test, correct interpretation, rapid communication, and appropriate adjustment of antimicrobial regimens based on diagnostic test results is key to optimise antimicrobial prescribing.

??POCT/rapid assay enable early identification and initiation of antibiotic therapy BUT a negative POCT/Rapid assay does not mean absence of disease and a positive test may be false positive (requires careful clinical judgement)

??Biomarkers are good as rule out test and have a higher negative predictive value BUT cannot guide initiation of antibiotics

?

#MICROBIOLOGY LAB #CLINICAL MICROBIOLOGISTS #ANTIMICROBIAL STEWARDSHIP ?#ANTIMICROBIAL SUSCEPTIBILITY TESTINGS #ANTIMICROBIAL RESISTANCE


Dr Rahul S Kamble

MBBS, MD Microbiology

Diploma Infectious Diseases (UNSW, Australia)

Infection Control course (Harvard Medical School, USA)

International Clinical Tropical Medicine course

(CMC Vellore|Haukeland university|McGill university)

International Vaccinology course (CMC Vellore)

Six Sigma Black Belt (Govt of India certified)

Auditor: JCI|NABH|NABL|CSSD|RBNQA|Texila university

PGDBA|PGDHM|PGDCR|PGDMR|PGDOM|

PGDMLS|PGDIM|PGDHI|PGDBI|PGDHA|CCDHHO

Consultant Clinical Microbiologist & Infectious Diseases

Project Lead - Antimicrobial Stewardship

VIJAY JUVEKAR

Retired at not working

1 年

Good. Very nice.

回复
Minh DK Nguyen

Assessor Pathology & CPR/AED First Aid Multidisciplinary Pathology - Data Manager Clinical Trials Research (Medical Oncology-Sir Charles Gairdner Hospital)

1 年

Thks Dr Kamble, the article is excellent info to learn. Most of your articles posted are excellent and invaluable knowledges of microbiology. Very interested reading

VIJAY JUVEKAR

Retired at not working

1 年

Great

Dr Sumit Chavan

MBBS, MD Microbiology Consultant Clinical Microbiology and Infectious Disease; Hospital Infection Control and Prevention

1 年

Excellent session and many thanks for sharing the PPT along with notes

Kavitha Jinjil

Anaesthesiologist | Entrepreneur | Leader | Veteran | Student

1 年

Excellent information and explained very lucidly Dr Rahul

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