Prescribing medicines in renal impairment

TAM (Treatments and Medicines) NHS Highland
NHS Highland

Audience

  • All NHSH 
  • Primary and Secondary Care 
  • Adults only 

Accurate assessment of renal function is necessary when calculating drug doses. Various methods of calculating the Glomerular filtration rate (GFR) are available.

An accurate GFR can be obtained using nuclear medicine methods. For routine use this is impractical but is indicated for highly toxic medications with a narrow therapeutic index, such as chemotherapy.

Locally, SCI store reports an estimated glomerular filtration rate (eGFR) normalised to a body surface area of 1.73m2 (mL/min/1.73m2), using the CKD-EPI equation.

For most drugs and most clinical situations this is an acceptable estimate of renal function. However, eGFR can overestimate or underestimate renal function in some patient groups or clinical situations, eg, frail elderly, amputees, bodybuilders, critical care patients, obese patients; with the potential for these patients to receive toxic or subtherapeutic drug doses.

Cockcroft-Gault equation

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)

NHSH Renal MDT

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.

Which weight do I use?

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.

Editorial Information

Last reviewed: 16/04/2024

Next review date: 30/06/2027

Author(s): Renal MDT Department .

Version: 2

Approved By: TAM Subgroup of the ADTC

Reviewer name(s): Joanna Garrod, Area Renal Pharmacist .

Document Id: TAM461