• If there are no contraindications to its use, once-daily dosing with gentamicin is safer, more convenient, and cheaper than multiple daily dose regimens

First dose

• Weigh patient and calculate ideal body weight (IBW). See Ideal body weight guideline. Choose which of these 2 weights to use according to instructions below
• If unfit to be weighed, estimate weight 

• Use lowest weight [actual (or estimated) or ideal] to select dose from Table 1
• Dilute gentamicin dose in 100 mL glucose 5% or sodium chloride 0.9% and administer by IV infusion over 1 hr
• Record time infusion started on drug chart 

Table 1: Dose banding for gentamicin 7 mg/kg (maximum dose 600 mg* daily) IV by infusion over 60 min

*In some situations the online calculator may recommend a maximum dose of 600 mg; this dose should not be exceeded

Monitoring of first dose

Measure concentration 6-14 hr after first infusion started

• Take blood samples for gentamicin (10 mL clotted blood) and creatinine 6-14 hr after start of infusion. Do not sample via cannula used for infusion 
• Request measurement of gentamicin concentration and document in patient record. It is imperative that time when infusion began and time when sample was taken are accurately documented on the microbiology request card; this will appear on the report
• Complete electronic microbiology request form on Medway as follows:
  
  • antimicrobial assay type - tick gentamicin box
  • dose frequency - tick once daily box
  • enter dose and date and time of last dose
  • sample(s) taken - tick random box for samples taken after 6-14 hr
  • enter date and time of random sample taken
  • enter date taken and time taken again at bottom of form

Interpretation and dose interval

• Check blood results for gentamicin level on iPortal
• Calculate time interval between start of gentamicin infusion and time level taken
• Plot time interval against gentamicin level to select dosing interval on Figure 1

Figure 1: Use values of plasma gentamicin concentration and time interval to find intercept

Example: a concentration of 6 mg/L after 10 hr yields a dose interval of 36 hr (i.e. give dose every 36 hr)

• For additional information on dose intervals and subsequent monitoring, see Gentamicin calculator or Table 2 if calculator not available
• Give next dose (7 mg/kg by infusion - see Table 1) at time after interval plotted in Figure 1

Table 2: Additional information on gentamicin dose intervals and subsequent monitoring

Further monitoring

• Check serum creatinine daily. Calculate CrCl from serum creatinine to check dose interval has not changed
• If dose interval has to be changed, check gentamicin concentration 6-14 hr after start of next infusion (note time of start of infusion and time of sampling) and use Figure 1 to verify correct dose interval

Gentamicin nomogram for multiple daily dose regimens

Nomogram for gentamicin dosage (devised by Prof. G. Mawer), which provides a loading dose (L), a maintenance dose (M), and a suitable interval between doses for an adult patient whose serum creatinine concentration (A), age (B) and body weight (D) are known

To use, join A to B with a line that crosses C; then join this intercept on C to D with a line that crosses M and L

Monitoring multiple daily dose regimens

• If gentamicin needs to be used in cystic fibrosis (CF) contact CF team for advice
• For patients with infective endocarditis, refer to Infective endocarditis guideline as target levels differ in this indication
• Measure serum gentamicin after 24 hr. Take a trough sample immediately before third dose, and a peak sample 1 hr after dose (doses are given by IV injection NOT infusion)
• Target peak concentration is 5-10 mg/L
• Trough concentration should be maintained <2 mg/L
• The relationship between maintenance dose and steady state concentration is linear. Doubling the dose will double peak and trough serum concentrations, assuming renal function stable

Measurement of trough and peak concentrations

• Take blood samples for gentamicin (10 mL clotted blood) just before IV injection for pre-dose trough concentration and 1 hr after IV injection for post-dose peak concentration. Do not sample via cannula used for IV injection 
• Request measurement of gentamicin concentration and document in patient record. It is imperative that time when IV injection was given and time when sample was taken are accurately documented on the microbiology request card; this will appear on the report
• Complete microbiology request form as follows:
  
  • antimicrobial assay type - tick gentamicin box
  • dose frequency - tick 8-hrly or 12-hrly box. If dosing interval longer than this (e.g. 24-hrly or 
    36-hrly), tick ….. hrly box and complete frequency
  • enter dose, date and time of the dose around which trough and peak are to be measured
  • sample(s) taken - tick pre-dose or post-dose box as applicable
  • enter date and time of sample taken
  • enter date taken and time taken again at bottom of form