An infectious disease specialist is evaluating a male patient with multidrug-resistant bac… — USMLE Step 3

An infectious disease specialist is evaluating a male patient with multidrug-resistant bacteremia to design an appropriate antimicrobial dosing regimen.

Patient details:
- Age: $68$ years
- Weight: $72 \text{ kg}$
- Serum creatinine: $2.0 \text{ mg/dL}$ (stable over the past week)

The team plans to start a novel continuous intravenous antibiotic. Pharmacokinetic data indicates that this drug is excreted exclusively by glomerular filtration, and its clearance is directly equal to the patient's creatinine clearance. The target steady-state plasma concentration ( $C_{ss}$ ) required for bactericidal efficacy is $15 \text{ mg/L}$ .

Assuming the Cockcroft-Gault equation is $\text{CrCl} = \frac{(140 - \text{Age}) \times \text{Weight in kg}}{72 \times \text{Serum Creatinine in mg/dL}} \times (0.85 \text{ if female})$ , what continuous intravenous infusion rate (in $\text{mg/hr}$ ) is required to achieve the target steady-state concentration?

Cevap: 32.4 mg/hr

Cevap

32.4

The maintenance infusion rate required to maintain a steady-state plasma concentration is determined by the fundamental pharmacokinetic relationship:

\text{Infusion Rate} = \text{Clearance} \times C_{ss}

. The patient's creatinine clearance, calculated via the Cockcroft-Gault equation, is

\frac{(140 - 68) \times 72}{72 \times 2.0} = 36 \text{ mL/min}

. Converting this to liters per hour yields

2.16 \text{ L/hr}

(

36 \text{ mL/min} \times 60 \text{ min/hr} \div 1000 \text{ mL/L}

). Multiplying this clearance by the target steady-state concentration of

15 \text{ mg/L}

results in the required infusion rate of

32.4 \text{ mg/hr}

Adım Adım Çözüm

Calculate the patient's creatinine clearance (CrCl).

36 \text{ mL/min}

The drug's clearance is directly equal to CrCl, which must be estimated using the Cockcroft-Gault equation before dosing can be determined.

Convert clearance units from

\text{mL/min}

\text{L/hr}

2.16 \text{ L/hr}

The target infusion rate is requested in

\text{mg/hr}

and concentration is given in

\text{mg/L}

. Matching the volume and time units is required for accurate calculation.

Calculate the maintenance infusion rate.

32.4 \text{ mg/hr}

At steady state, the rate of drug administration (infusion rate) must equal the rate of elimination, represented by the equation:

\text{Infusion Rate} = \text{Clearance} \times C_{ss}

Anahtar Kavram

The maintenance infusion rate required to achieve a target steady-state concentration is the product of the drug's clearance and the desired concentration. Renal clearance can be estimated using the Cockcroft-Gault equation.

Soru

Cevap

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