Extended Interval Aminoglycoside Administration Using the Lower Dosage

 

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)

Aminoglycoside	Dose
amikacin	15 mg/kg
gentamicin	5 mg/kg
netilimicin	6.5 mg/kg
tobramycin	5 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 q12h from q24h from 6 to 10 hours =

= (0.0715152 * ((hours since administration started)^2)) - (1.596909 * (hours since administration started)) + 9.461333

 

line separating q24h from q36h =

= (0.0369556 * ((hours since administration started)^2)) - (1.312446 * (hours since administration started)) + 13.445397

 

line separating q36h from q48h =

= (0.0271775 * ((hours since administration started)^2)) - (1.144579 * (hours since administration started)) + 15.504979

 

line forming upper limit for q48h =

= (0.0203664 * ((hours since administration started)^2)) - (0.995611 * (hours since administration started)) + 16.805516

 

Adjustments to nomogram for other aminoglycosides:

(1) for netilimicin, multiply the drug scale by 1.3

(2) for amikacin, multiply the drug scale by 3.0