MICRO: Antimicrobials 2 Flashcards
What are the main types of misuse of antibiotics?
- No infection present
- Selection of incorrect drug
- Inadequate or excessive dose
- Inappropriate duration of therapy
- Expensive agent used when cheaper is available
Which patients most benefit from antibiotics? What about bacteraemia?
- About 50% of people with bacteraemia would get better by themselves
- Patients who are hypotensive tend to do quite badly and mortality is higher- these patients need antibiotics quickly
What is the % of patients who get adverse events with antibiotics?
- About 5% of hospitalised people who are given an antimicrobial will experience an adverse event
What is the most common SE of antimicrobials?
GI upset/diarrhoea
NB: about 60% of diarrhoea post-antibiotics is due to SE and not C. diff.
What are the common adverse events with antibiotics?
- GI upset (i.e. diarrhoea)- MOST COMMON
- Fever and rash
- Renal dysfunction
- Acute anaphylaxis
- Liver dysfunction (abnormal LFTs seen), Hepatitis
Benefits should outweigh the risks
Where has antibiotic prescribing been most reduced in healthcare?
GP and dentists
What 4 factors should be considered when prescribing antibiotics?
-
Choice of the correct antimicrobial depends on the CHAOS:
- Host characteristics (e.g. age, pregnancy, renal/ liver failure, other medications, tetracyclines deposit in bone so cannot be used in children
- Antimicrobial susceptibilities (local policies)
- Organism itself
- Site of infection (e.g. bone, CSF, urine)
Why can giving narrow spectrum antibiotics be challenging? Why is it important?
Easier to give narrow spectrum if you have sensitivities/culture but this is not commonly done
Use narrow sprectrum + bactericidal if possible e.g. penicillin for tonsillitis
What 3 pharmacological factors should be considered when prescribing antibiotics?
- Pharmacokinetics (absorption, distribution, elimination)
- Route of administration - IV for serious infection or if the patient is not absorbing PO
- Dosage (age, renal/ hepatic function, drug monitoring)
When should you consider IV over PO antibiotics?
- Sepsis - BP is low, hence perfusion is low and subsequent drug absorption PO may be compromised
- If accessing a deep site (e.g. endocarditis, osteomyelitis) or CNS (only a small proportion get into the CNS effectively)
- If antimicrobial is not absorbed well orally e.g. aminoglycosides
Name 2 methods for susceptibility testing.
- Gradient MIC method
- Agar disc diffusion method
Define MIC and break point.
MIC (minimum inhibitory concentration) = this is the least amount of drug required to inhibit the growth of the organism in a culture
Break point = the point above the MIC that determines whether the organism is sensitive or resistant to the antimicrobial
Describe the agar disc diffusion method for measuring MIC.
- The disc is impregnated with antibiotic (which diffuses out from the disc)
- Distance from the disc ↑ = conc of antibiotic ↓logarithmically
- The border of the clear zone is the MIC
- The zone of inhibited antibiotic is measured and compared to guidance (as below) and reported as sensitive or resistant.
What is the difference between empirical and definitive treatments?
EMPIRICAL THERAPY: If it is necessary to treat the patient on an empirical basis, a broad-spectrum agent that is likely to ‘cover’ the most likely organism is used. Collect specimen before starting antibiotic
DEFINITIVE THERAPY: Empirical cover can be then changed based on culture results
How are break points determined?
Susceptibility is set by standard bodies e.g. European Committee on Antimicrobial Susceptibility Testing
reported as a table which gives breakpoints i.e. if MIC is greater than the breakpoint then it will be resistant. If the MIC is 8 and the breakpoint is less than or = to 8 then it will still be intermediate or sensitive.
What are the advantages of empirical therapy in nosocomial infections?
- higher survival rates
- shorter hospital stays
- lower healthcare costs
(nosocomial = hospital)
What are the only clnical situations where empirical therapy has been shown to improve survival?
Septic shock (e.g. hypotension) is a particularly important indication for broad-spectrum antibiotics
Patient age, bacteraemia, neutropenia, causative organism and source of infection are not associated with better outcome with empirical treatment at infection onset in this study.
When can gram staining(3) and rapid antigen detection (2) be used in the identification of infecting organisms?
-
Gram-staining
- CSF
- Joint aspirate
- Pus
-
Rapid Antigen Detection
- Immunofluorescence
- PCR
What 3 factors can affect the concentration of the antibiotic locally?
- pH of site
- lipid solubility of drug
- ability to penetrate BBB e.g. in CSF infections
What evidence can point towards whether a patient needs antimicrobials?
- Evidence of a systemic response, e.g.
- Fever
- Raised CRP
- High WCC (mainly neutrophils)
- NOTE: in SEVERE infections, you can get a low WCC
- Also:
- Duration of symptoms
- Underlying risk factors
- Likely source of infection
- Exclude other pro-inflammatory disease
When should IM antimicrobial administration not be used?
- Irritant antimicrobials
- If long term use required
When should you switch to PO antimicrobials? When is this not appropriate?
- IV to PO switch is recommended in hospital for most infections if the patient has stabilised after 48 hours of IV treatment
In CNS infections and severe infections such as osteomyelitis and endocarditis, you may NOT switch to PO
Describe the main factors to consider in terms of pharmacokinetics/pharmacodynamics of antimicrobials graphically.
- Peak concentration above MIC
- Time above the MIC
What are the three patterns of activity of antimicrobials? Give an example of each.
Type I - concentration dependent killing and prolonged persistent effects
Type II - time-dependent killing and minimal persistent effects
Type III - time-dependent killing and moderate-prolonged persistent effects
- Aminoglycosides, fluoroquinolones
- Carbapenes, cephalosporins, erythromycin
- Azithromycin, tetracyclines
