5 Therapeutic Monitoring of Anti-Infective Medications

Interpreting drug concentrations

Therapeutic drug monitoring is useful when it is established for a medication; not every laboratory test is a drug concentration. Interpret a level with the patient’s current clinical status and the institution’s drug-monitoring protocol.

Before responding to an unexpected level, confirm the dose and sample timing. Review the dose history, adherence, administration, interacting medicines, and relevant clinical changes. Use drug-specific guidance and clinical response; do not adjust a dose from a level in isolation.

Gentamicin trough monitoring

A gentamicin trough is collected immediately before the next dose. Interpret the result alongside renal function and the exact dose and sample times. Impaired or changing renal clearance can cause aminoglycoside accumulation.

Before adjusting therapy, review the level, renal function, and dose history, and follow the applicable protocol.

Vancomycin exposure targets

For serious MRSA infections, the recommended exposure target is an AUC/MIC of $400\text{–}600$, assuming an MIC of $1\ \mathrm{mg/L}$. This target is intended to balance efficacy and safety.

Trough-only targeting of $15\text{–}20\ \mathrm{mg/L}$ is no longer recommended for this indication. Monitoring and dose decisions should not be based on the white blood cell count alone.

Fluconazole and renal dosing

Fluconazole is primarily eliminated by the kidneys. For multiple-dose therapy, the general approach is to give the usual loading dose and then adjust the maintenance dose for renal function.

A single-dose vaginal candidiasis regimen is an exception to this multiple-dose approach. The specific regimen depends on the indication and the patient. For example, with a creatinine clearance of $40\ \mathrm{mL/min}$, consider reducing the maintenance dose according to renal function and the clinical indication.

Amphotericin B safety monitoring

During amphotericin B therapy, monitor renal function, potassium, magnesium, liver function, and blood counts. The medication can affect renal function and cause electrolyte abnormalities, including low potassium and low magnesium.

Responding to a low voriconazole level

When a voriconazole trough is unexpectedly low, first confirm sample timing and review adherence, administration, interacting medicines, and relevant clinical changes. Assess the result using the drug-specific protocol before changing the dose.

Voriconazole also warrants liver-function monitoring.

Acyclovir and renal safety

Acyclovir is substantially eliminated unchanged by the kidneys. During high-dose intravenous therapy, monitor renal function and promptly reassess the dose and interval using current renal function when kidney function worsens.

Adequate hydration is important to help reduce renal risk.