fe is the abbreviation for fraction of drug excreted unchanged in the urine.
For predominantly renally cleared drugs (fe > 0.5), dose adjustment should be considered in relation to impairment of renal function (creatinine clearance/eGFR).
- Assess renal function using estimated glomerular filtration rate (eGFR) or creatinine clearance (CrCl).
- Consider reducing doses of drugs that are predominantly renally cleared (those with fe > 0.5) in relation to impairment of renal function.
- When renal function is < 30 mL/min preferentially use metabolised drugs over renally cleared drugs.
For drugs that are renally cleared i.e. fe > 0.5.
- Choose the dose rate i.e. dose and frequency that you would use in this patient if renal function was normal (CrCl 100 mL/min).
- Calculate the patient’s creatinine clearance (CrCl) using the Cockcroft and Gault calculator.
- Alternatively the estimated glomerular filtration rate (eGFR) supplied by the laboratories can be used for patients whose BSA is close to 1.73m2. Use this method to adjust the eGFR for body surface area.
- Interpretation is dependent on the clinical condition of the patient. If renal function is changing (e.g: acute kidney injury), these estimates are not reliable and dosing should take this into account.
- Calculate dose rate:
- In patients with renal dysfunction, it is essential to adjust the dose for some medicines. Dose adjustment to reduce side effects should be considered for other medicines.
- For drugs with fe ≥ 0.9, use this equation(assume normal CrCl of 100 mL/min):
- For drugs with fe < 0.9, use this equation:
- Water and electrolyte handling may be significantly altered (e.g. increasing the likelihood of oedema, hyponatraemia, and hyperkalaemia).
- Nephrotoxic drugs (e.g. non-steroidal anti-inflammatory drugs) may aggravate already impaired kidneys.