Gentamicin Dosing

Background

1. Powerful antibiotic for gram negative and staph infections
2. Duration of treatment should be limited to minimise nephro and ototoxicity. Review prescription at 48 hours and continue treatment only on advice of microbiologist or ID consultant.
3. Gentamicin can usually be switched to an alternative antibiotic by 3 days in most cases.
4. There are very few indications for continuing gentamicin beyond 7 days.
5. Best to stop all other non-essential nephrotoxic drugs for duration of gentamicin therapy eg ACEIs, NSAIDs, frusemide

Daily or Alternate Daily Dosing

1. Recommended for most patients who require gentamicin by following steps below.
2. Not appropriate for patients with endocarditis, staphylococcal bone infection, pregnancy, major burns, ascites, age < 16 years, cystic fibrosis, patients on dialysis.
3. Most patients with oliguria, anuria or acute kidney injury can be given one or two doses of gentamicin if indicated without causing renal or ototoxicity, but thereafter it is always best to use a different antibiotic if possible – please seek renal advice.

Step 1: Calculate, Prescribe and Administer First Dose

1. The initial dose is based on actual body weight and estimated creatinine clearance (CrCl), summarised below. CrCl is easily estimated using an intranet creatinine clearance calculator. Creatinine Clearance Calculator
2. Note that estimated CrCl will overestimate renal function in patients with AKI in whom it is safest to assume CrCl <21ml/min
3. Lower doses sometimes used for synergy eg in treatment of endocarditis
   
4. Prescribe on gentamicin prescription sheet. Also write ‘Gentamicin – see prescriptions sheet’ on main drug kardex so that not overlooked on ward rounds. Dont circle a time on the kardex in case this confuses the nurses. Remember to write your phone number on the chart
5. Give the dose in 100ml saline 0.9% by infusion over 30min. Contact pharmacy for advice if different diluent required.

Step 2: Monitor Serum Creatinine and Gentamicin

If CrCl >21ml/min

1. Take a sample 6-14 hours after the start of the first infusion
2. Record exact time of all gentamicin samples on gentamicin prescription chart and on sample request form.
3. Record serum concentration on gentamicin prescribing chart and on the graph , reassessing dose/dose interval as indicated
4. This will indicate one of three options:
   1. Continue present dosage regimen.
   2. Adjust dosage interval.
   3. Withhold and resample after 24 hours.

If CrCl <21ml/min

1. Take a sample 24 hours after start of first gentamicin infusion
2. Record exact time of all gentamicin samples on gentamicin prescription chart and on sample request form.
3. If therapy to continue take additional samples at least every 24 hours and give further dose once serum level <1mg/l
   NB Note that it should rarely be necessary to take blood or give gentamicin between 2400 and 0800hrs

General Points

1. If blood sample is not taken, is lost or is taken at wrong time, or if there is any concern about the patient’s renal function, take a sample at 20-24 hours and wait for the result. Under these circumstances only give next dose if serum level <1mg/l.
2. If unsure how to interpret result, contact consultant microbiologist (33560) or pharmacist (33557). Both are available out of hours via switchboard
3. Patients usually need at least two doses to cover the first 48 hours, until antibiotic sensitivities are available.

If Serum Gentamicin Unexpectedly High or Low

1. Were dose and sample times recorded correctly?
2. Was correct dose given?
3. Was sample taken from line used to administer the drug?
4. Was sample taken during drug administration?
5. Has renal function declined or improved?
6. Does the patient have oedema or ascites?

If in doubt, take another sample before re-precribing and/or contact pharmacy for advice

Step 3: Assess Daily Need for Gentamicin and Signs of Toxicity

1. Take a further blood sample 6-14 hours after the dose, at least every 2 days.
2. If serum gentamicin unexpectedly high, or if renal function alters, daily sampling may be necessary.
3. To minimise risk of toxicity, duration of treatment should normally be limited to 72 hours.
4. Suitable alternative antibiotics can be determined by
   • checking microbiology results
   • referring to the IV to Oral Switch Policy
   • discussing the case with duty Microbiologist
5. All gentamicin prescriptions that continue beyond 3-4 days MUST be discussed with microbiology or ID consultant.

Nephrotoxicity

1. Extremely unlikely that patient will develop nephrotoxicity after a single dose, so safe to give one off gentamicin to patient with advanced renal failure after adjusting dose for CrCl – if in doubt in patient with AKI assume CrCl <21ml/min and give 2.5mg/kg to max of 180mg
2. Monitor creatinine daily and seek advice if renal function unstable eg change in serum creatinine >15-20%
3. Note that gentamicin toxicity may be non-oliguric
4. Predisposing factors include advanced age, preexisting renal insufficiency, dehydration, and concomitant use of other potentially nephrotoxic drugs.

Ototoxicity

1. Ototoxicity may be irreversible. Ask patient about new tinnitus, dizziness, poor balance, hearing loss, bobbing oscillopsia (vertical bouncing of surroundings).
2. Associated with prolonged gentamicin use, usually >10 days but may be >3 days, and is due to drug accumulation in inner ear
3. Patients continuing on gentamicin beyond 7 days should be referred to Audiology.

Links

• SAPG Guidance on Gentamicin
• NHSD&G Antimicrobial Handbook – Gentamicin and Vancomycin
• Scottish Antimicrobial Prescribing Group Gentamicin Calculator
• Helpful Hints on Gentamicin Prescribing