Prescribing medicines in renal impairment

The Cockcroft-Gault equation should be used to estimate Creatinine Clearance (CrCl) (mL/min) to calculate and adjust drug doses for the following:

• Nephrotoxic drugs, eg, aciclovir, lithium
• Elderly patients (aged 75 years and older)
• Patients at extremes of muscle mass: BMI <18 kg/m² or >40 kg/m²
• Medicines that are substantially renally excreted and have a narrow therapeutic index including:
  • Direct-acting oral anticoagulants (DOACs), eg, apixaban, dabigatran etexilate, edoxaban and rivaroxaban.
    The use of eGFR for dosing of DOACs is known to increase risk of bleeding events as a consequence of overestimating renal function.
  • Other examples include digoxin and sotalol

See: BNF for further information on when to use estimated creatinine clearance.

Cockcroft-Gault Equation:

• Age in years
• Weight in kg (weight influences accuracy of the equation and may require adjustment)
• Serum creatinine in micromol/L
• Constant = 1.23 for men; 1.04 for women

To calculate estimated CrCl manually the equation is:  

CrCl (mL/min) = (140 – age) × body weight (kg) × (1.23 for men) OR (1.04 for women) / Creatinine (micromol/L)

NHS Highland Renal MDT recommends the use of MDCalc to calculate CrCl

• NB: Separate calculators exist for IV gentamicin and IV vancomycin dosing.
• The Vision Renal Function Calculator can be used instead of MdCalc if available in GP practices.
• Care should be taken to select the most appropriate weight based on BMI.

Renal function and drug dosing should be reassessed regularly in situations where eGFR and/or CrCl change rapidly, such as in patients with Acute Kidney Injury.

MDCalc provides values based on ideal, actual and adjusted body weight (ABW). As a guide:

• Actual body weight: use for BMI less than 30 kg/m2 (ie normal bodyweight, underweight and overweight patients), and for bodybuilders, where bodyweight is due to muscle mass.
• Ideal body weight: use for oedematous patients.
• For obese or morbidly obese BMI greater than or equal to 30 kg/m²: a body weight adjustment is required but there is no consensus on which weight should be applied.

MDcalc will provide a range using ideal body weight and adjusted body weight. If the range crosses over a dosing threshold clinical judgement should be used to determine the most suitable dose for the individual patient, taking into consideration the risks of under or overdosing.