Accurate assessment of renal function is necessary when calculating drug doses. Various methods of calculating 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. This is an acceptable estimate in the majority of clinical situations but can over 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.

The Cockcroft-Gault equation to estimate Creatinine Clearance (CrCl) (mL/min) should be used to guide adjustment of drug dosage for the following:

• Nephrotoxic drugs: eg aciclovir, lithium
• Elderly patients
• Patients at extremes of muscle mass: BMI less than 18 kg/m2 or greater than 40 kg/m2
• Medicines that are substantially renally excreted and have a narrow therapeutic index. In particular, direct-acting oral anticoagulants (DOACs; 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 medicines that are largely renally excreted and have a narrow therapeutic index: eg digoxin and sotalol.

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

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

NB Separate calculators exist for gentamicin and vancomycin dosing

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

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.