Gentamicin dosage and administration

# Dosage and Administration
Gentamicin injection may be given IM or IV.  The patients pretreatment body weight should be obtained for calculation of correct dosage.  The dosage of aminoglycosides in obese patients should be based on an estimate of the lean body mass.  It is desirable to limit the duration of treatment with aminoglycosides to short term.

### Patients With Normal Renal Function
The recommended dosage of gentamicin injection for patients with serious infections and normal renal function is 3 mg/kg/day, administered in three equal doses every eight hours (Table I).
For patients with life-threatening infections, dosages up to 5 mg/kg/day may be administered in three or four equal doses.  This dosage should be reduced to 3 mg/kg/day as soon as clinically indicated (Table I).
It is desirable to measure both peak and trough serum concentrations of gentamicin to determine the adequacy and safety of the dosage.  When such measurements are feasible, they should be carried out periodically during therapy to assure adequate but not excessive drug levels.  For example, the peak concentration (at 30 to 60 minutes after IM injection) is expected to be in the range of 4 to 6 mcg/mL.  When monitoring peak concentrations after IM or IV administration, dosage should be adjusted so that prolonged levels above 12 mcg/mL are avoided.  When monitoring trough concentrations (just prior to the next dose), dosage should be adjusted so that levels above 2 mcg/mL are avoided.  Determination of the adequacy of a serum level for a particular patient must take into consideration the susceptibility of the causative organism, the severity of the infection and the status of the patients host-defense mechanisms.
In patients with extensive burns, altered pharmacokinetics may result in reduced serum concentrations of aminoglycosides.  In such patients treated with gentamicin, measurement of serum concentrations is recommended as a basis for dosage adjustment.
6 to 7.5 mg/kg/day (2 to 2.5 mg/kg administered every eight hours).
7.5 mg/kg/day (2.5 mg/kg administered every eight hours).
5 mg/kg/day (2.5 mg/kg administered every 12 hours).
For further information concerning the use of gentamicin in infants and children, see gentamicin injection (pediatric) product information.
The usual duration of treatment for all patients is 7 to 10 days.  In difficult and complicated infections, a longer course of therapy may be necessary.  In such cases monitoring of renal, auditory and vestibular functions is recommended, since toxicity is more apt to occur with treatment extended for more than 10 days.  Dosage should be reduced if clinically indicated.

### For Intravenous Administration
The IV administration of gentamicin may be particularly useful for treating patients with bacterial septicemia or those in shock.  It may also be the preferred route of administration for some patients with congestive heart failure, hematologic disorders, severe burns or those with reduced muscle mass.  For intermittent IV administration in adults, a single dose of gentamicin injection may be diluted in 50 to 200 mL of sterile isotonic saline solution or in a sterile solution of dextrose 5% in water; in infants and children, the volume of diluent should be less.  The solution may be infused over a period of one-half to two hours.
The recommended dosage for IM and IV administration is identical.
Gentamicin injection should not be physically premixed with other drugs, but should be administered separately in accordance with the recommended route of administration and dosage schedule.

### Patients With Impaired Renal Function
Dosage must be adjusted in patients with impaired renal function to assure therapeutically adequate, but not excessive blood levels.  Whenever possible serum concentration of gentamicin should be monitored.  One method of dosage adjustment is to increase the interval between administration of the usual doses.  Since the serum creatinine concentration has a high correlation with the serum half-life of gentamicin, this laboratory test may provide guidance for adjustment of the interval between doses.  The interval between doses (in hours) may be approximated by multiplying the serum creatinine level (mg/100 mL) by 8.  For example, a patient weighing 60 kg with a serum creatinine level of 2 mg/100 mL could be given 60 mg (1 mg/kg) every 16 hours (2 x 8).
In patients with serious systemic infections and renal impairment, it may be desirable to administer the antibiotic more frequently but in reduced dosage.  In such patients, serum concentrations of gentamicin should be measured so that adequate but not excessive levels result.  A peak and trough concentration measured intermittently during therapy will provide optimal guidance for adjusting dosage.  After the usual initial dose, a rough guide for determining reduced dosage at eight-hour intervals is to divide the normally recommended dose by the serum creatinine level (Table II).  For example, after an initial dose of 60 mg (1 mg/kg), a patient weighing 60 kg with a serum creatinine level of 2 mg/100 mL could be given 30 mg every eight hours (60  2).  It should be noted that the status of renal function may be changing over the course of the infectious process.
It is important to recognize that deteriorating renal function may require a greater reduction in dosage than that specified in the above guidelines for patients with stable renal impairment.
In adults with renal failure undergoing hemodialysis, the amount of gentamicin removed from the blood may vary depending upon several factors including the dialysis method used.  An eight-hour hemodialysis may reduce serum concentrations of gentamicin by approximately 50%.  The recommended dosage at the end of each dialysis period is 1 to 1.7 mg/kg depending upon the severity of the infection.  In children, a dose of 2 mg/kg may be administered.
The above dosage schedules are not intended as rigid recommendations but are provided as guides to dosage when measurement of gentamicin serum level is not feasible.
A variety of methods are available to measure gentamicin concentrations in body fluids; these include microbiologic, enzymatic and radioimmunoassay techniques.
Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.