Interpreting Microbiology reports for better Clinical Decisions Interpreting Antibiogrammes Prof C. Wattal Hon. Sr. Consultant & Chairman Dept. of Clinical Microbiology Sir Ganga Ram Hospital New Delhi
AST to change outcomes Strike early and strike hard Golden first two hours of sepsis control
Essentials of a meaningful AST Speed, Reliability, Reproducibility & Follow up of AST in break through infections
Interpretation of susceptibility test results CLSI: publishes interpretive criteria for MICs of all relevant antibiotics for most bacterial genera (reviewed and updated every year) susceptible result: Patient s organism should respond to therapy with that antibiotic using the dosage recommended normally for that type of infection and species resistant result: Not inhibited by the concentrations of the antibiotic achieved with the dosages normally used with that drug intermediate result: microorganism falls into a range of susceptibility in which the MIC approaches or exceeds the level of antibiotic that can ordinarily be achieved and for which clinical response is likely to be less than with a susceptible strain CLSI M100-S24: Jan 2015; CID 2009;49:1749-55
Interpretive Criteria
% Resistance of Tigecycline - Blood Isolates - Ward & ICU 100 90 80 70 60 50 40 30 20 10 0 2008 2009 2010 E.coli Acinetobacter spp. Klebsiella spp. Shift from Disk Diffusion to MIC from 2010 onwards
Correlation between in vitro and in vivo situations: The Inoculum effect Refers to the decreasing efficacy of an antibiotic with increasing bacterial density Unique strategy of antibiotic tolerance Can complicate design of effective antibiotic treatment of bacterial infections Antibiotics that elicit the inoculum effect can lead to poor response to antibiotic treatment: The treatment efficacy drastically diminishes at ribosomal level of treatment Can cause overestimation of in vitro findings Increases generation of resistant pathogens
Case 1 A 40 year diabetic male developed fever & pain in right thigh since 1 week. On examination patient had fluctuant swelling over right thigh. Further excision drainage of the abscess was done and sent for microbiological examination. On Grams. Staining abundant gram positive cocci in pairs and bunches with many PMN cells were seen. On culture pure and heavy growth of Staph aureus was obtained which was subjected to AST. Blood culture also grew similar organism.
Antibiogram of Staphylococcus aureus Antimicrobial MIC Interpretation Remarks β lactamase NEG - Benzyl Penicillin < 0.12 S Oxacillin 0.5 S Erythromycin 0.5 S Clindamycin 0.25 S Trimethoprim sulfmethoxazole <2 S Ciprofloxacin <1 S Implications and Treatment options : Drug of choice : IV Penicillin/ Cloxacillin, Patients allergic to penicillin: Erythomycin or Clindamycin Erythromycin resistant and clindamycin sensitive- Ask for D-test All reported Vancomycin β lactamases negative methicillin 2 sensitive S S. aureus Report (MSSA) to be suppressed Vancomycin & Teicoplanin susceptibility report to be suppressed unless indicated MIC interpretation guideline : CLSI M100-S25 (2015)
Staphylococcus aureus clindamycin erythromycin oxacillin penicillin vancomycin S R R R S If clindamycin-s and erythromycin-r, do not report clindamycin-s without performance of D Test
D Test positive reaction Inducible clindamycin resistance (erm-mediated) 15-26 mm another example Photos courtesy of J. Jorgensen and K. Fiebelko
Inducible Clindamycin Resistance - Incidence Varies considerably geographically Community-associated MRSA Frequently erythromycin-r clindamycin-s Often msra-mediated mechanism (NOT inducible) USA report 2002 61.7% S. aureus erythromycin-r clindamycin-s 50% NOT inducible resistance Fiebelkorn et al. 2003. JCM. 41:4740.
Antibiogram of Staphylococcus aureus Antimicrobial MIC Interpretation Antimicrobial Β lactamase POS + Cefoxitin screen POS + Benzyl Penicillin >= 0.5 R Oxacillin >=4 R Erythromycin >=8 R Clindamycin >4 R Ciprofloxacin >4 R Vancomycin 1 S MRSA resistant to all betalactam group of antibiotics including quinolones, aminoglycosides and macrolides irrespective of its in-vitro results. All reported Implications and treatment options Look for Vancomycin/ Teicoplanin susceptibility and manage Alternative drugs for management : Linezolid and Daptomycin MIC interpretation guideline : CLSI M100-S25 (2015)
What does CLSI Guidelines Say
THE MUTANT SELECTION WINDOW: Resistant Mutants are selected within a concentration Range (Mutant Prevention Concentration) The concentration range (MSW) extends from where the Inhibition begins approximated by the MIC up to the MPC Can be reduced : (Minimum Inhibitory Concentration) 1. Change in Antibiotic structure 2. Dosing Regimens 3. Combination Therapy Xilin Zhao et al. Restricting the Selection of Antibiotic-Resistan Mutants: A General Strategy Derived from Fluoroquinolone Studies Clinical Infectious Diseases 2001; 33(Suppl 3):S147 56
Vancomycin intermediate and resistant S. aureus When to consider h VISA? MRSA with vancomycin treatment failure Disc diffusion methods not reliable MIC testing to be done Rising Vancomycin MIC > 2 ug/ml while on treatment indicative h VISA
The Vanco Creep Sourced from. Gould IM, 2008, International Journal of antimicrobial Agents.
Case 2 A 15 year old adolescent was admitted with high grade continuous fever since five days. He also has cough with expectoration. Sputum and blood cultures were sent and both grew Streptococcus pneumoniae. The same was subjected to AST.
Antibiogram of Streptococcus pneumoniae Non meningitis isolates Antimicrobial Zone diameters (mm) MIC (ug/ml) Interpretation Penicillin x MIC test <2 S Cefotaxime recommended if <0.5 S Oxacillin zone <19mm Ceftriaxone <0.5 S Erythromycin 21 <0.25 S Clindamycin 19 0.12 S Levofloxacin 17 <2 S Meropenem x <0.25 S Vancomycin and Linezolid 17 <1 S To be reported routinely for isolates other than from CSF Report to be suppressed routinely for penicillin susceptible isolates Reliable disk diffusion tests do not exist For nonmeningitis isolates Penicillin MIC of 0.06 ug/ml can predict susceptibility to (oral and parenteral) ampicillin, ampicillin-sulbactam, amoxicillin, amoxicillin-clavulanate, cephalosporins, and carbapenems In vitro susceptibility test to be performed using MIC method for penicillins, cephalosporins and carbapenems For penicillin resistant isolates Cefotaxime or ceftriaxone for penicillin non allergic Reserve drugs : Linezolid (so not reported routinely) Clinical Infectious Diseases 2001; 33(Suppl 3):S147 56
Antibiogram of Streptococcus pneumoniae For CSF isolates only Antimicrobial MIC Interpretation Penicillin >0.12 R Cefotaxime <0.5 S Ceftriaxone <0.5 S To be reported routinely for CSF isolates Meropenem <0.25 S Vancomycin No disc diffusion criteria for penicillin, cephalosporins and carbapenems MIC testing to be performed and only meningitis interpretations to be reported Susceptible ranges different from non meningitis isolates No FDA approved indication for use of cefepime for meningitis Drug choice for penicillin resistant isolates Ceftriaxone or cefotaxime Carbapenem Vancomycin for β lactam allergic <1 S Disk diffusion can be valid >17mm
Penicillin R in and around India 70 60 57.9 60.8 50 40 30 20 10 0 India (< 1% R) 3.8 0 Philipn. Thiland Malaysia 23.1 9 Singapore Vietnam Srilanka 41.2 Pak 9 Indonesia 21 US 33 CID 2001;32(15 May):1463-1469; Tropical Med Int. Hlth.2005;10(3):234-239
Case 3 A 26 year old male was admitted with continuous fever since three weeks. On examination patient was febrile, anemic had tachycardia and a left systolic murmur. On echocardiography mitral valve prolapse with large vegetation was seen. Patient was diagnosed as having bacterial endocarditis. Six blood cultures were drawn which grew Enterococcus faecium. AST for the same was done using Vitek system.
Antibiogram of Enterococcus faecium Antimicrobial Zone diameter (mm) Ampicillin <16 >=16 R Gentamicin High level (HLAR) 6 Growth present R Vancomycin <14 1 S Synergistic activity between ampicillin/ penicillin and an aminoglycoside can be predicted using high level aminoglycoside screening test Recommended routinely for bacterial endocarditis cases Not recommended on urinary isolates HLAR Resistant: not synergistic with cell wall active agent HLAR Susceptible: synergistic with cell wall active agent MIC (ug/ml) Interpretation Remarks For synergistic activity Linezolid <20 2 S Reserve drugs not Daptomycin Not reliable <4 S reported routinely Ampicillin resistant Vancomycin susceptible Enterococci MIC interpretation guideline : CLSI M100-S25 (2015)
Antibiogram of Enterococcus faecium Antimicrobial Zone diameter (mm) Ampicillin <16 >=16 R Gentamicin High level (HLAR) 6 Growth present R Vancomycin >17 >32 R E.gallinarum & E.casseliflavus : Intrinsic low level resistance to vancomycin Should not be treated with vancomycin Therapeutic options Linezolid Quinopristin-dalfopristin (E.faecium only) Others Daptomycin, fosfomycin, tigecycline MIC (ug/ml) Interpretation Remarks For synergistic activity Linezolid <20 2 S To be reported as Daptomycin Not reliable <4 S isolate is VRE Vancomycin resistant Enterococci MIC interpretation guideline : CLSI M100-S25 (2015) hvre: Organisms with intermediate zones be tested by MIC method
Enterococcus faecalis Ampicillin susceptibility can be used to predict imipenem susceptibility provided the species is confirmed to be E. faecalis. M100-S14 (M2, M7); Table 2D
Case 4 A 56 year old male after two weeks of having undergone liver transplant was admitted to the ICU with difficulty in breathing and fever. After admission patient s condition deteriorated and had to be intubated. After one week of intubation he developed VAP. His endotracheal secretions were sent for culture which grew Klebsiella pneumoniae. The same was subjected to AST.
Antibiogram of K.pneumoniae β lactamases Antimicrobial MIC Interpretation Antimicrobial MIC Interpretation Ampicillin >= 32 R Imipenem <= 0.25 S Amoxicillin Clavulanate <=2 S Amikacin <= 2 S Cefuroxime 2 S Gentamicin < = 1 S Ceftriaxone <= 1 S Ciprofloxacin < = 0.25 S Cefepime < = 1 S Preferable to avoid 3 rd and 4 th generation cephalosporins & carbapenems Report can be suppressed Trimethoprim/Sul famethoxazole Aztreonam <= 4 S Tigecycline Ertapenem <= 0.5 S Colistin < = 20 S Criteria not provided by CLSI EUCAST criteria followed CLSI criteria for Acinetobacter adopted β- lactamase postive: can be treated with B lactam- b lactam inhibitor combinations MIC interpretation guideline : CLSI M100-S25 (2015)
Antibiogram of K.pneumoniae ESBL Antimicrobial MIC Interpretation Antimicrobial MIC Interpretation Remarks Ampicillin >= 32 R Imipenem <= 0.25 S Amoxicillin Clavulanate <=2 S Amikacin <= 2 S Cefuroxime >= 32 R Gentamicin < = 1 S Ceftriaxone >=4 R Ciprofloxacin >1 R Cefepime <2 R Trimethoprim/Sulf amethoxazole Aztreonam >=16 R Tigecycline Ertapenem <= 0.5 S Colistin < = 20 S Criteria not provided by CLSI EUCAST criteria followed CLSI criteria for Acinetobacter adopted Report to be suppressed MIC interpretation guideline : CLSI M100-S25 (2015)
Extended spectrum B lactamases Treatment options Implications of ESBL production Antibiotic choice for infections with such organisms is reduced Plasmids bearing the genes encoding ESBLs frequently also carry genes encoding resistance to aminoglycosides, quinolones and trimethioprim/sulfamethoxazole Treatment options βlactam/β lactamase inhibitor combinations Carbapenems (including Imipenem, Meropenem, and Ertapenem) Cephamycins ESBL-producing organisms may lose outer membrane proteins leading to resistance to the cephamycins not related to β lactamase production
MIC interpretation guideline : CLSI M100-S25 (2015) Antibiogram of Klebsiella pneumoniae AmpC Antimicrobial MIC Interpretation Antimicrobial MIC Interpretation Ampicillin >= 32 R Amikacin <= 2 S Amoxicillin Clavulanate >= 32 R Gentamicin < = 1 S Cefuroxime >= 64 R Ciprofloxacin >1 S Ceftriaxone >= 64 R Tigecycline* <= 0.5 S Cefepime 2 S Colistin** < = 0.5 S Aztreonam 4 S Ertapenem <= 0.5 S Imipenem <= 0.25 S Resistant to β- lactam and β- lactam- inhibitor combinations, cephalosporins and sometimes carbapenems but spares cefepime Treatment options: Carbapenems Fluoroquinolone Tigecycline Trimethoprim/S ulfamethoxazole < = 20 S Piperacillin tazobactam >= 128 R * EUCAST criteria for tigecycline followed **Colistin breakpoints in CLSI for Acinetobacter adopted
Antibiogram of Klebsiella pneumoniae carbapenemases Antimicrobial MIC Interpretation Antimicrobial MIC Interpretation Ampicillin >= 32 R Amikacin <= 2 S Amoxicillin Clavulanate >= 32 R Gentamicin 8 I Cefuroxime >= 64 R Ciprofloxacin >1 R Ceftriaxone >= 64 R Tigecycline* <= 0.5 S Cefepime >=16 R Colistin** < = 0.5 S Aztreonam >= 16 R Trimethoprim/Sul famethoxazole >=4 R Ertapenem >=2 R Piperacillin tazobactam >= 128 R Imipenem >=4 R Organism resistant to therapy with carbepenems, regardless of in vitro status Treatment options Colistin Tigecyline Fosfomycin (Only for Urinary isolates) * EUCAST criteria for tigecycline followed **Colistin breakpoints in CLSI for Acinetobacter adopted MIC interpretation guideline : CLSI M100-S25 (2015)
H. influenzae Only results of testing with Ampicillin, 3 rd - generation cephalosporins and meropenem are appropriate to report routinely in CSF Amoxicillin-clavulanate, azithromycin, cefaclor, cefdinir, cefixime, cefpodoxime, cefuroxime, clarithromycin, are oral agents that may be used as empiric therapy for respiratory tract infections. The results of susceptibility tests with these antimicrobial agents are often not useful for management of individual patients.
A good predictor of antimicrobial efficacy Area under curve (AUC)/MIC ratio Concentration dependent & independent antibiotic concept Recognise the resistance potential particular antibiotic class Selective use of antibiotics Infectious Diseases Clinics of North America.:Aug 2003
Internationally Accepted guidelines to interpret AST for better outcomes
CLSI: Enterobacteriaceae Zones MM MIC For fosfomycin MIC determination, the medium must be supplemented with glucose-6-phosphate to a final concentration of 25 mg/l. Fosfomycin guidelines only for E.coli in urinary isolates
CLSI: Fosfomycin intrinsically resistant to A. baumannii and P. aeruginosa
CLSI: Enrerococci Zones MM MIC Guidelines available only for E. fecalis urinary isolates
EUCAST: Enterobacteriaceae MIC µgm Zones MM No zone sizes Fosfomycin guidelines not available for enteroocci and non-fermenters in EUCAST
Incompletely understood PK/PD. Tigecycline : blood isolates Daptomycin/ Linezolid: Urinary VRE Antibiotic not recommended for AST e.g Tigecycline : Urinary AND Pseudomonas
Understanding of Resistance can help interpret antibiogrammes
Case 5 A 60 year old female on 10 th day of her post renal transplant day developed low grade fever. She was was investigated and only blood culture was found to be positive for Candida non-albicans. The same was subjected to further identification and sensitivity.
Antibiogram of Candida non albicans C krusei Drug MIC Interpretation Remarks Fluconazole 32 R Intrinsically resistant Voriconazole <=0.12 S Amphotericin B* 0.5 S Caspofungin <=0.25 S *The CLSI breakpoints for Amphotericin B have not been defined In vitro studies in animal models have shown that if an amphotericin B MIC of >1ug/ml is obtained for a candida species isolate, then the isolate is likely to be resistant to amphotericin B CLSI document M27-S4 :2012
Antibiogram of Candida non albicans C galbrata Drug MIC Interpretation Remarks Fluconazole 2 S Voriconazole No guideline available Amphotericin B* 0.5 S Caspofungin <=0.25 S *The CLSI breakpoints for Amphotericin B have not been defined In vitro studies in animal models have shown that if an amphotericin B MIC of >1ug/ml is obtained for a candida species isolate, then the isolate is likely to be resistant to amphotericin B C. glabrata: Current data are insufficient to demonstrate a correlation between in vitro susceptibility testing and clinical outcome using voriconazole Susceptible criteria not provided by CLSI CLSI document M27-S4 :2012
Antifungal Sensitivity interpretation guidelines Difficult organisms C.haemulonii and C.auris No guidelines by CLSI Studies extrapolate interpretative criteria of C.albicans for these isolates Drug MIC Interpretation Remarks Fluconazole 32 I Shows variable susceptibility pattern Voriconazole <=0.12 S Amphotericin B 8 R MIC usually raised Caspofungin 0.5 S
Thanks