Aminoglycosides may be administered once a day or even every other day using an extended interval dosing scheme. This method has proven to be as effective as more frequent dosing with no increase in toxicity, while being more convenient and less expensive to administer. This section handles administration of a higher dose of an aminoglycoside (7 mg/kg gentamicin or tobramycin), while the previous section handles the lower dose (5 mg/kg).


Step 1: Make sure that the patient is not excluded for extended interval dosing (see above)


Step 2: Calculate the patient's weight to use for dosing.


If the patient's body weight <= 1.2 * (ideal body weight in kilograms), then use the actual body weight.


If the patient's body weight is > 1.2 * (ideal body weight), then use the adjusted body weight.


adjusted body weight in kilograms =

= (ideal body weight in kg) + (0.4 * ((actual body weight) – (ideal body weight))) =

= (0.6 * (ideal body weight in kg)) + (0.4 * (ideal body weight))


Step 3: Calculate the patient's dose.


dose to administer in mg =

= (aminoglycoside dose from table) * (dosing weight in kilograms)




7 mg/kg


7 mg/kg


Step 4: Calculate the initial dosing interval based on the patient's creatinine clearance.


Creatinine Clearance

Dosing Interval

> 60 mL/min

q 24 hours

40 – 60 mL/min

q 36 hours

20 – 39 mL/min

q 48 hours

< 20 mL/min

do not use extended dosage interval


If the patient's creatinine clearance is not known, then it can be estimated from the patient's serum creatinine using the equations of Cockcroft and Gault:


estimated creatinine clearance for adult males =

= ((140 - (age in years)) * (ideal body weight in kg) / (72 * (serum creatinine in mg/dL))) mL/min


estimated creatinine clearance for adult females =

= ((0.85) * (140 - (age in years)) * (ideal body weight in kg) / (72 * (serum creatinine in mg/dL))) mL/min


Step 5: Administer the drug. The aminoglycoside dose is diluted in 50-100 mL of intravenous fluid and administered over 60 minutes.


Step 6: Adjust the dosing interval based on serum level monitoring. Monitoring is performed approximately 10 hours (6 – 14 hours) after starting the infusion. The serum level of the aminoglycoside is used to adjust the dosage interval based on a nomogram such as that of Urban and Craig which uses a target serum level < 1 mg/L.


Line equations for nomogram using gentamicin or tobramycin as calculated in JMP:


line separating q24h from q36h =

= (-0.717235 * (hours since administration started)) + 11.930544


line separating q36h from q48h =

= (-1.022108 * (hours since administration started)) + 17.418725


line forming upper limit for q48h =

= (-1.038922 * (hours since administration started)) + 19.538627


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