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Antibiotics. Judith Coombes, University of Queensland. General Principles. Establish the need for antibiotic therapy When not to prescribe Viral or minor bacterial disease Viral diarrhoea Sore throat Sinusitis Common cold Are self limiting Common cold needs common sense campaign.

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Judith Coombes, University of Queensland

general principles
General Principles
  • Establish the need for antibiotic therapy
  • When not to prescribe
    • Viral or minor bacterial disease
    • Viral diarrhoea
    • Sore throat
    • Sinusitis
    • Common cold

Are self limiting

Common cold needs common sense campaign

antibiotic treatment can be
Antibiotic treatment can be
  • Prophylactic antibiotics prevent serious infection in specific situations (e.g. preventing the spread of meningococcal disease).
  • Presumptive antibiotic therapy is indicated in some circumstances (e.g. a human bite to the hand
  • Use empirical antibiotic therapy — which is aimed at the likely causative organism — to manage an infection until microbiological culture and susceptibility results are known
  • When the cause of an infection is confirmed, directed therapy is aimed at the specific pathogen.
mind me

M microbiology guides therapy wherever possible

I indications should be evidence-based

N narrowest spectrum required

D dosage appropriate to the site and type of infection

M minimise duration of therapy

E ensure monotherapy in most situations

5 antibiotics
5 Antibiotics
  • Benzylpenicillin or Amoxycillin
    • Streptococcis
  • Cloxacillin
    • Staphylococcis
  • Vancomycin
    • MRSA
  • Gentamicin
    • Pseudomonas
  • Metronidazole
    • anaerobes
sepsis definition
Sepsis- definition
  • In adults and older children, severe sepsis is the systemic response to an infection manifested by organ dysfunction, hypoperfusion or hypotension combined with 2 or more of the following:
    • fever,
    • tachycardia,
    • tachypnoea,
    • elevated white cell count
gentamicin revision
Gentamicin Revision
  • Gram negative bactericidal agent
  • Excellent anti-pseudomonal cover
  • Once daily dosing benefits vs tds or bd:
    • high peak level – excellent distribution
    • post antibiotic effect (>24-36 hours)
    • reduced monitoring and administration
    • reduced nephro and ototoxicity
    • easier monitoring (10-12 hours post dose)
severe risks of nephrotoxicity and ototoxicity
Severe risks of nephrotoxicity and ototoxicity

Mrs HR

78 years, wt 57kg

Admitted to outlying hospital acute exacerbation COPD

Baseline Cr 80mol/L, Urea 8.5

Charted gentamicin 160mg daily for 5/7 – no levels requested

Baseline CrCl = 45ml/min

patient progress
Patient progress

15/6 K+ = 6.7, Acidotic to ICU + haemodialysis

21/6 Gent level still 1.4!!!!

23/6 complaining of dizziness, unsteady feeling, vestibular symptoms, vomiting

1/7 Cr 0.21 recovering

14/11 Out patient – permanent Ototoxicity

local sri lankan hospital
Local Sri Lankan Hospital

75 yr old patient post amputation, septic

Gentamicin iv 80mg tds, Co Amoxyclav iv + metronidazole iv

Day 7 reviewed by anesthetists – pain control

Noted Urea increased from 7.1 – 15.2

Patient not septic - ? Need for antibiotic

Seen Day 12 post operative, Urea = 26 same dose gentamicin

aminoglycoside dosing and monitoring
Aminoglycoside dosing and Monitoring

Case continued…

Day 3:

  • Mr AD (67yrs) has now developed sever hospital acquired pneumonia
  • Ward round decisions
    • start gentamicin once a day
      • dose as per levels each night at 20:00
      • recall patient weighs 70 kg
      • creatinine has improved (now 140 micro mole/l)
    • start Co-Amoxiclav 1.2g IV q8h
creatinine clearance
Creatinine Clearance

Recall Cockcroft-Gault Formula:

CrCl (mL/min) = [140-age (years)] * ideal weight (kg)

[0.814 * serum creatinine (micromol/L)]

{♀ * 0.85 }

If patient 70 kg, 67 y.o. with serum creat~140micromol/L:

CrCl ≈ 42 mL/min (140-67 X 70) / (0.814 x 140)

gentamicin dose adjustment
Gentamicin Dose Adjustment

Day 4:

  • gentamicin level = 2.5

(taken 08:00, 12 hrs post dose)

  • The initial dose given in emergency department was 280mg (4mg/kg x approx 70kg)

Prescribe new gentamicin dose based on level

gentamicin dose adjustment1
Gentamicin Dose Adjustment
  • Level at 12 hours = 2.5 (ideal = < 2)
  • New Dose = Level Wanted (mg/L) x Dose Given (mg)

Level Achieved (mg/L)

In this case: (1.5/2.5) x 280 = 168 mg

  • round down to 160 (nearest multiple of 40 mg)
    • amps = 40 mg/mL
when not to take levels
When NOT to take levels

Do not take levels if:

Stat dose


Patient has Normal renal function and is only receiving 1 or 2 doses ie prophylactic

variable dose medication
Variable Dose Medication

Drug Level and Time Taken

Dose Time and Actual Time Given

use in moderate to severe renal function
Use in moderate to severe renal function

Where ever possible withhold other nephrotoxic drugs and ensure no other altenative

Ie unavoidable use of gentamicin:

Requires extended dose intervals 36, 48 or 96 hours

Effectively daily levels wait til < 1.0mmol/l then dose again