Introduction to Clinical Infectious Disease Practice

1
Introduction to Clinical Infectious Disease
Practice

• Applying Microbiology to the Practice of Medicine

2
Key Aspects of Clinical ID

• Determine if the patient infected, and with what
• Determine risk factors or exposures
• Assess if the patient is contagious/address
  public/preventive health issues
• Advise for/against invasive, dangerous, or
  expensive procedure(s)
• Guide empiric and definitive Rx, if any is
  needed, discontinue/avoid uneccesary
  antimicrobials
• Interpret micro lab reports contaminant vs.
  normal flora vs. pathogen
• Antimicrobial stewardship
• Infection control

3
Is the Patient Infected

• No symptom or sign is 100 specific for infection
• Easier to know what the patient has if you know
  what he/she doesnt have
• Common mimics of infection autoimmune diseases,
  hematologic malignancy, drug reactions

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• Why this particular organism e.g. look for
  occult colon cancer

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Infection Looking Beyond the Diagnosis

• Clinical scenario

• Additional consideration

• Occult colon cancer
• Complement deficiency
• Common variable immune deficiency
• Occult myeloma
• Contiguous osteomyelitis or prosthetic device
  infection

• Streptococcus gallolyticus bacteremia
• Neisseria meningitis
• Recurrent giardiasis
• Pneumococcemia without pneumonia
• Non-healing wound

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Risk Factor Assessment in ID

• Host immune status
• Exposures travel, animal, close contacts,
  occupation

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Risk Assessment in ID

• Endogenous immune deficiency

• Exogenous Immune deficiency

• Extremes of age
• Cellular immune defect(HIV, alcoholism,
  liver/renal disease)
• Humoral immune defect(HIV, myeloma, CVID)
• Anatomic defect(Atopic dermatitis,
  COPD/bronchiectasis, neutropenia, burns)
• Other immune defects (granulocyte defect or
  deficiency, complement deficiency, Fe overload,
  hyperglycemia/acidosis, pregnancy)
• Vaccine status

• Corticosteroids
• TNF antagonists
• Organ rejection inhibitors
• Cancer chemotherapeutics
• Infectious agents (cytomegalovirus, HTLV1,
  influenza)

common variable immunodeficiency
tumor necrosis factor
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Exposures for Clinical ID

• Indoor - hot tub, recruits/dormitory, GI virus,
  healthcare
• Outdoor woods, zoo, local vs. distant travel
• Animals obvious (pets, work) vs. less obvious
  (ticks, mosquitoes, rodents)
• Geography endemic mycoses, parasites
• Prior Rx antimicrobials (false negative
  cultures, allergic reaction, C. difficile), or
  mask key signs or symptoms (analgesics,
  antipyretics)

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Public Health and ID

• Immediate contagion risk TB, zoster, anthrax,
  plague, scabies
• Potential or future exposure influenza, group A
  strep, RMSF, MRSA, C. difficile, multi-drug
  resistant organisms
• Laboratory personnel coccidioides, tularemia,
  hemorrhagic fever viruses
• Tracking of exposed (PPD, STD partners)

Rocky mountain spotted fever Methicillin
resistant s. aureus
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Guiding Patient Evaluation Dollars and Danger,
Tissue is the Issue

• Decision to do a test or procedure can be
  influenced by financial gain, workload,
  incomplete understanding of diagnostic rationale
  or risk/benefit ratio
• Since ID docs dont perform the procedures and
  dont read the scans, minimal bias can exist
• Lack of specific diagnosis must be weighed
  against risk of test/procedure
• Specific diagnosis, removal or drainage of
  infected sites may improve healing but not be
  justified

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Micro Lab Results for the Clinician

• Many docs have unrealistic expectations or
  limited understanding of clinical labs
• Micro in particular so, because of added
  parameters of contaminants, normal flora,
  fastidious growth, effects of antimicrobials
• Serologic, histological and nuclear (PCR) testing
  can have varying sensitivity and specificity for
  different organisms and from different labs
• Susceptibility results require clinical
  correlation
• Human tendency to rely on technology to make the
  diagnosis e.g. nuclear scans, urinalysis

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Pitfalls in Micro Lab Culture Results

• Do not assume nurses or physicians know optimal
  specimen type, collection technique, or what
  organisms to pursue
• Tissue, fluid always preferred over swab
• No technique is beyond contamination, and few
  organisms are always clinically relevant by there
  mere presence
• Antibiotics can affect results even if given
  within weeks

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The Gram Stain
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Gram Stains of Common Bacterial Pathogens
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Gram Stains (Cont.)
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Acid Fast (Kinyoun) Stain
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Mixed Morphology on Culture Plate
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Mycology Lab Basics How Fungi Differ from Common
Bacteria

• Fungi generally grow slower
• Environmental contaminants common
• Candida spp. often expected flora
• Susceptibility testing far less reproducible or
  clinically correlated
• Many pathogenic molds rarely isolated from blood

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Virology Lab Basics

• Too inefficient to routinely culture many
  relevant viruses
• Viruses causing chronic infection (e.g. herpes
  CMV) can be detected by serology, histology or
  culture yet still not be clinically relevant
• PCR assays expanding beyond clinical correlative
  data
• Very little susceptibility testing outside of HIV

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Basic Serologic Responses
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Complement Fixation Serology
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Infection Control

• Goes along with antibiotic stewardship
• Recognize trends in drug-resistant isolates,
  nosocomial infections
• React to contagious pathogens, outbreaks (e.g.
  swine flu)

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General Rules for Antimicrobial Use

• Cheapest, narrowest spectrum agent with the most
  clinical data preferred
• Such a selection optimized by knowing exact Dx,
  and/or likely organisms involved
• No one or even 2 drugs can cover every
  possibility
• All antibiotic usage, no matter how efficient,
  will result in some degree of serious morbidity
  and promote drug-resistant isolates
• Empiric Rx. Must weigh risk of inappropriate or
  unnecessary use and above consequences

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Antimicrobial Stewardship

• Tempting, but impractical for ID docs to
  completely control use of certain drugs in hopes
  of minimizing inappropriate use
• Colleagues will learn from our approach-
  especially with new drugs communicate our
  rationale for Rx choices

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Consequences in Antimicrobial Rx

• Liberal or broad spectrum use

• Conservative or narrow spectrum use

• Less chance of not covering the etiologic
  agent(s)
• Improves outcomes in situations where any delay
  in starting Rx. increases mortality
• Greater costs, more side effects, drug resistance
• If patient improves on multiple IV drugs, but no
  definite diagnosis made many err on liberal side
  for continuing or completing

• May miss window for initiating Rx
• May be inadequate to cover resistant or
  unexpected etiology
• Minimizes cost and adverse drug reactions,
  resistance
• Allows non infectious, or self-limited
  infectious etiologies to be easily recognized

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Disk Diffusion MIC
-Inexpensive, -Cant be automated, -Good
clinical correlation except for
fastidious organisms
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E-Test for MICs
-Antibiotic gradient -Expensive -Best for 1 or 2
drug testing or fastidious organisms