16th May 2025

Prescribing in renal impairment

The kidneys perform a vital role in the metabolism of the kidneys. In patients with renal impairment several problems can occur:

• Reduced renal excretion of a medication or its metabolite causing toxicity e.g. morphine
• Increased toxicity of a medication even if its elimination is not altered e.g. statins, warfarin
• Many medication side effects are poorly tolerated by patients with renal impairment e.g. NSAIDs
• Effectiveness of some drugs are reduced in renal impairment e.g. diuretics, nitrofurantoin

Many of these problems can be mitigated by reducing drug dosage or frequency or using an alternative medication.

Estimating renal function

The gold standard for measuring glomerular filtration rate (GFR) is by directly measuring the plasma or urinary clearance of an exogenous filtration marker, however this is difficult in practice. In practice GFR can be estimated with equations that use serum creatinine and other variables. Estimated glomerular filtration rate (eGFR) can be commonly calculated based on the Modification of Diet in Renal Disease (MDRD) equation or Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. Creatinine clearance (ClCr) which is calculated from the Cockcroft-Gault formula is also commonly used to estimate renal function.

In patients with extremes of muscle mass eGFR should be used with caution. Reduced muscle mass can lead to overestimation of GFR, and increased muscle mass will result in an underestimation. In patients of average height and weight eGFR can be used to dose medications as opposed to ClCr. In toxic drugs, elderly patients, and extremes of muscle mass ClCr is recommended.

Medication dosing in renal impairment

Many drugs are eliminated via the kidneys and in chronic kidney disease (CKD) or acute kidney injury (AKI) these medications can accumulate increasing the risk of adverse drug reactions. If CKD or AKI is suspected a review of medications is indicated. Medications that cause or can exacerbate renal impairment should be avoided, examples of these can be found in table 2. If changes are needed to the total daily dose of a medication can be altered by reducing individual doses or by increasing the time interval between doses. Information regarding the use of medications in renal impairment can be found in the appropriate drug monograph in the BNF or in a manufacturers summary of product characteristics (SPC).

In patients with CKD or AKI some medications may accumulate increasing the risk of toxicity and adverse side effects. Product literature should be consulted, and doses should be adjusted, or the medications should be temporarily held. Examples of these are highlighted in table 3.