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Microbiologic Diagnosis of Diabetic Foot Infections

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Regarding the best microbiological techniques to monitor the microbiology of wounds. ASM Manual Clinical Microbiology states 'A swab is not the specimen of choice... – PowerPoint PPT presentation

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Title: Microbiologic Diagnosis of Diabetic Foot Infections


1
Microbiologic Diagnosis of Diabetic Foot
Infections
  • Albert T. Sheldon, Jr. Ph.D.
  • Microbiology Team Leader
  • Division of Anti-infective Drug Products

2
Guidance for Industry- Foot Infections in
Patients with Diabetic mellitus
  • Microbiological Considerations
  • All patients should have pre-therapy cultures.
  • Gram stain and cultures should be obtained from
    acceptable sources using acceptable methods.
  • We prefer cultures obtained by leading edge
    needle aspiration, soft tissue and joint
    aspiration, bone biopsy and/or surgical
    debridement.
  • Microorganisms isolated should be assessed as
    true pathogens, colonizers, or contaminants.
  • Only microorganisms designated as true pathogens
    should be considered in determining
    microbiological evaluability of enrolled subject.

3
Factors that Influence Infection Rates
  • Risk of Wound Infection varies according to the
    following equation
  • Dose of bacterial contamination x Virulence
  • Resistance of the Host
  • Altemeire, W.A., W.R. Culbertson (1965)
    Surgical Infection In Moyer, C. et .al. (eds.)
  • Surgery, Principles and Practices, 3rd ed.
    Philadelphia, Lippincott Co.

4
Host Factors that Influence Infection Rates
  • Diversity and abundance of microorganisms present
    in the wound are also influenced by host factors
  • Wound type, depth, location, and quality
  • Presence of nonviable exogenous contamination
  • Peripheral blood insufficiency
  • Immune competence of the host

5
Manual of Clinical Microbiology
  • The use of specimens for bacteriological
    analysis requires that specific clinical
    material be collected, stabilized, and
    transported according to exacting specifications
    to insure valid results.
  • Murray, P. R., E. J. Baron, J. H. Jorgensen, M.
    A. Pfaller, and R. H. Yolken. 2003. Manual of
    Clinical Microbiology, 8th Edition. ASM Press,
    Washington D.C.

6
Clinical Microbiology
  • Implicit in this definition are two issues that
    are of interest to the discussion of decubitus
    foot infections.
  • The methods used to collect the clinical sample
    and
  • the validity of the results to assess the
    involvement of an organism in the etiology of the
    disease.

7
1. Methods used in Collection of Microbiological
Wound Samples
  • Deep Tissue Techniques
  • Biopsy and/or surgically debrided tissue
  • Leading edge Needle aspirate
  • Joint fluid or synovium
  • Bone specimen
  • Blood
  • Surface Sampling Techniques
  • Swab
  • Curettage
  • Dermabrasion
  • Velvet pad surface imprint
  • Published methods used in decubitus ulcer
    sampling.
  • Method recommended in Agency guidance document
    are deep tissue techniques.

8
Comparison of Sampling MethodsSapico F.L., et.
Al. (1984) Rev Infect Dis. 6S171-S176
9
Comparison of Sampling MethodsSapico F.L., et.
Al. (1984) Rev Infect Dis. 6S171-S176
10
Relationship Between Biopsy Swab Burn Wounds
Thomson, P.D., (1994) Amer J Surgery 1677S-10S
11
Relationship Between Quantitative Culture Swab
Breidenbach W,C., S. Trager (1994) Plats.
Reconstr. Surg. 95860-865
  • Study purpose
  • To determine the relationship between the
    quantity of bacteria and infection in complex
    extremity wounds, and
  • To compare the predictive value for wound
    infection of quantitative cultures versus other
    factors considered to have predictive value for
    wound infection.
  • I will focus on the latter purpose of the study.

12
Relationship Between Quantitative Culture Swab
(cont) Breidenbach W,C., S. Trager (1994)
Plats. Reconstr. Surg. 95860-865
  • Evaluated 50 patients with complex wounds-defined
    as a soft-tissue defect that required flap for
    closure.
  • Quantitative cultures (biopsy) compared to
  • Clinical parameters (factors considered to have
    predictive value in wound infection wound
    position, mechanism of injury, fracture type)
  • Laboratory test (Swab culture)
  • 28 patients had quantitative cultures obtained
    after debridement and high pressure wash prior to
    flap closure. 16 patients had swab cultures. 2-5
    samples obtained per wound.

13
Criteria for Positive/Negative Test, Infection,
and PrevalenceBreidenbach W,C., S. Trager
(1994) Plats. Reconstr. Surg. 95860-865
Test Positive test criteria Infection Prevalence Negative test criteria Infection Prevalence
Quantitative gt104 cfu/g 8/9 89 lt104 cfu/g 1/19 5
Swab Positive 5/13 38 Negative 1/3 33
No culture --- --- --- --- 3/6 50
Wound position Lower extremity 7/13 54 Upper extremity 11/37 30
Mech. of injury Farm or lawn mower 4/11 36 Other than F or LM 14/39 36
Fracture type Type IIIB or IIIC fracture 10/28 36 Type I, II, IIIA, or none 8/22 36
14
Criteria for Positive/Negative Test, Infection,
and PrevalenceBreidenbach W,C., S. Trager
(1994) Plats. Reconstr. Surg. 95860-865
Test Positive test criteria Infection Prevalence Negative test criteria Infection Prevalence
Quantitative gt104 cfu/g 8/9 89 lt104 cfu/g 1/19 5
Swab Positive 5/13 38 Negative 1/3 33
No culture --- --- --- --- 3/6 50
Wound position Lower extremity 7/13 54 Upper extremity 11/37 30
Mech. of injury Farm or lawn mower 4/11 36 Other than F or LM 14/39 36
Fracture type Type IIIB or IIIC fracture 10/28 36 Type I, II, IIIA, or none 8/22 36
15
Predictive values, Sensitivity, Specificity of
Tests Studied
Test PPV NPV 100-NPV Sensitivity Specificity
Quantitative culture 89 (56-98) 95 (75-99) 5 89 95
Swab culture 38 (18-64) 67 (21-94) 36 83 20
Wound position 54 (29-77) 70 (54-83) 33 81 19
Mechanism of injury 36(15-65) 64 (48-77) 30 22 78
Grade III fracture 36 (21-54) 64 (43-80) 36 56 44
Note Values following each represent range of 95 confidence interval PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence interval PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence interval PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence interval PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence interval PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence interval PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection).
16
Predictive values, Sensitivity, Specificity of
Tests Studied
Test PPV NPV 100-NPV Sensitivity Specificity
Quantitative culture 89 (56-98) 95 (75-99) 5 89 95
Swab culture 38 (18-64) 67 (21-94) 36 83 20
Wound position 54 (29-77) 70 (54-83) 33 81 19
Mechanism of injury 36(15-65) 64 (48-77) 30 22 78
Grade III fracture 36 (21-54) 64 (43-80) 36 56 44
Note Values following each represent range of 95 confidence limits. PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence limits. PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence limits. PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence limits. PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence limits. PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection). Note Values following each represent range of 95 confidence limits. PPV positive predictive value posttest probability of a positive result (infection) NPVnegative predictive value 100-NPVposttest probability of a negative result (no infection).
17
2. Interpretation of Microbiologic Diabetic foot
infection Samples (or Qualitative Microbiology)
  • Diabetic foot ulcers are polymicrobic.
  • S. aureus is the predominant aerobic species
    followed by S. epidermidis, Streptococcus spp.,
    P. aeruginosa, Enterococcus spp., and coliform
    bacteria.
  • The predominant anaerobic species are
    Peptostreptococcus spp., Bacteroides spp., and
    Prevotella spp.
  • Application of good microbiological techniques
    will allow isolation of anaerobes from up to 95
    of decubitus ulcers. However, such studies are
    not usually performed due to the labor intensive
    nature of anaerobic microbiological studies.
  • Bowler, P.G., et. al. (2001) Clin Microbiol Rev
    14244-269.

18
Schools of Thought
  • Although microorganisms are responsible for wound
    infections, there is controversy regarding their
    role. Published literature is inconclusive!
  • The density of microorganisms is the critical
    factor in determining whether a wound is likely
    to heal.
  • The presence of specific pathogens is of primary
    importance in delayed healing.
  • Microorganisms are of minimal importance in
    delayed healing.
  • Debate whether a wound should be sampled, the
    value of the results and the method that should
    be used.

19
Conclusions
  • Widespread controversy still exists
  • Regarding the exact mechanisms by which
    microorganisms cause wound infection,
  • Regarding the significance of microorganisms in
    nonhealing wounds that do not exhibit signs of
    clinical infection, and
  • Regarding the best microbiological techniques to
    monitor the microbiology of wounds.
  • ASM Manual Clinical Microbiology states A swab
    is not the specimen of choiceSince a swab
    specimen of a decubitus ulcer provides no
    clinical information.

20
Conclusions
  • A regulatory agency must require microbiological
    methods that provide us with confidence and data
    necessary to assess the response of
    antimicrobials for their intended uses.
  • We describe, in our guidance document, what we
    consider to be relevant methods. These are the
    deep tissue techniques discussed in a previous
    slide.

21
Louis Pasteur
  • The germ is nothing. It is the terrain in
    which it is found that is everything.
  • Pasteur, L. (1880) De lattenuation virus du
    cholera des poules. CR Acad. Sci. 91 673-680.

22
Obtaining a Swab Culture
  • Before taking the specimen
  • Cleanse the wound with water or saline
  • Remove excess necrotic debris
  • Compress edges to elicit new drainage
  • Use swab tip to swab the healthiest tissue
  • Do not swab exudate, pus, eschar, or heavily
    fibrous tissue.
  • Gram stain helps define quality of specimen as
    determined by polymorphonuclear cells, squamous
    epithelial cells and bacteria on gram stain.
  • ASM Manual Clinical Microbiology states A swab
    is not the specimen of choiceSince a swab
    specimen of a decubitus ulcer provides no
    clinical information.

23
Microbiology of noninfected wounds
24
Microbiology of Infected Wounds
25
Useful Wound Definitions
  • Contamination-Presence of bacteria on the wound
    surface.
  • Colonization-Presence of and multiplication of
    surface microbes/bacteria contaminants without
    infection.
  • Infection-Invasion and multiplication of
    microorganisms in body tissues resulting in local
    cellular injury.
  • Distinction between definitions not so clear in
    the clinical setting.
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