PK Lesson 14: Vancomycin Part 2

Front

Determine an appropriate dosing interval of vancomycin to achieve the desired steady plasma concentrations of 32 mg/L for the peak (drawn 1 hour after the end of a 1.5-hour infusion) and approximately 15 mg/L for the trough. Assume Ke = 0.087 hr-1. You have calculated an exact dosing interval of 10.5 hours. What is the most appropriate rounded dosing interval? Options: 8 hours, 12 hours, 18 hours, 24 hours, 36 hours

Back

12 hours

Answer explanation

τ =ln (Cp/Ct)/K + t + t'

τ =ln (32/15)/0.087+ 1.5 + 1 = 111.2 hr  

Round to 12 hours  

Tags

Pharmacokinetics

Front

You have a patient on vancomycin and have calculated an exact dosing interval of 26 hours. What is the most appropriate rounded dosing interval?

Back

24 hours

Answer explanation

Round to the closes interval

Tags

Pharmacokinetics

Front

How long will it take to reach steady state for a patient receiving 1500 mg vancomycin infused over 1.5 hours every 12 hours with Vd = 63 L and Ke = 0.087 hr-1?

Back

40 hr

Answer explanation

Calculate half-life

t1/2 = 0.693/K = 0.693/0.087 = 7.97 hr

Steady-state takes ~5 half lives, so 5 x 7.97 = 39.9 hr, so approximately 40 hours

Tags

Pharmacokinetics

Front

You are starting a patient on vancomycin and calculate an exact loading dose of 1602 mg. What is the most appropriate rounded loading dose for this patient?

Back

1500 mg

Answer explanation

Vancomycin is rounded to the nearest 250 mg

Front

What is the most appropriate rounded loading dose for a patient starting on vancomycin with a calculated maintenance dose of 1198 mg? Options: 1200 mg, 1250 mg, 1000 mg, 1190 mg, 2000 mg

Back

1250 mg

Answer explanation

Vancomycin is rounded to the nearest 250 mg

Front

A patient with renal dysfunction received a dose of vancomycin. Plasma concentrations were 22 and 15 mg/L at 24 and 48 hours after infusion, respectively. Assuming first-order elimination, when will the concentration reach 10 mg/L?

Back

73 hours

Answer explanation

You can plot these data on semilog graph paper (or plot ln C vs. time) and extrapolate, or use linear regression with the two above points.

K = ln (C1/C2)/t2-t1 = ln (22/15)/24 = 0.016 hr'1

C = Co x e-kt

10 = 15 x e-(0.016)t

0.667 = e-(0.016)t

-0.4055 = -0.016t

t = 25.3 hr after 48 hour level

48 + 25 = 73 hours total

Tags

Pharmacokinetics

Front

VY is a 48-year-old, 170-kg (5 ft 7 in) female with septic arthritis due to MRSA. Her current serum creatinine is 1.3 mg/dL and is stable. A vancomycin dose of 1000 mg every 24 hours was prescribed and expected to achieve a steady-state trough concentration equal to 15 μg/mL. After the third dose, a steady-state concentration was measured and equaled 8 μg/mL. Calculate a new vancomycin dosing regimen that would provide a steady-state trough of 15 μg/mL. Options: 1000 mg every 12 hours, 1000 mg every 36 hours, 1500 mg every 24 hours, 1500 mg every 12 hours, 1500 mg every 36 hours.

Back

1000 mg every 12 hours

Answer explanation

Trough only calculate new dosing interval

τnew = (Css old)/(Css new) x τ old

τnew = (8/15) x 24 hr

τnew = 12.8 hours

Would round to 12 hours

Front

Compute a vancomycin maintenance dose for a 66-year-old, 65-kg female with MRSA sternal osteomyelitis to provide a steady-state peak concentration of 50 μg/mL and a steady-state trough concentration of 20 μg/mL using the Rybak nomogram.

Back

750 mg every 12 hours

Answer explanation

Calculate CrCl

CrCl = (140-age)(wt)/72*SCr x 0.85

CrCl = (140-66)(65)/72*1.3 x 0.85 = 43.7 mL/min

Use weight and CrCl to find dose/dosing interval on chart

Front

Compute a vancomycin maintenance dose for a 50-year-old, 70-kg male with an MRSA wound infection and stable serum creatinine of 1 mg/dL using the Rybak nomogram.

Back

1250 mg every 8 hours

Answer explanation

Calculate CrCl

CrCl = (140-age)(wt)/72*SCr

CrCl = (140-50)(70)/72*1 = 87.5 mL/min

Use weight and CrCl to find dose/dosing interval on chart

Front

Compute a vancomycin maintenance dose and dosing interval for a 66-year-old, 65-kg female with MRSA sternal osteomyelitis and a serum creatinine of 1.3 mg/dL using the Matzke nomogram.

Back

1250 mg every 36 hours

Answer explanation

MD = 19 mg/kg x 65 kg = 1235 mg, would round to 1250 mg

Calculate CrCl

CrCl = (140-age)(wt)/72*SCr x 0.85

CrCl = (140-66)(65)/72*1.3 x 0.85 = 43.7 mL/min

Use CrCl to find dosing interval