In patients with renal impairment, the two most important factors that you need to consider are the dose reduction of medicines that are renally cleared, and avoidance where possible of drugs that are potentially renally toxic. This is because the consequences of toxicity are likely to be more serious when there is less homeostatic reserve, as discussed earlier in this chapter.

Dose adjustment in renal impairment

The degree of dose adjustment required in renal impairment will depend on the proportion of the drug that is eliminated by renal excretion, and the dose-related toxicity. Dose reduction can be achieved by either reducing the dose itself or extending the dosing interval. If an immediate effect is required, a loading dose may be required as the increased half-life means that time to steady state levels will be increased.

Use the BNF to find information on whether specific drugs need dose adjustment. Whenever there is a deterioration in renal function always review drugs to ensure potentially nephrotoxic drugs such as NSAIDs and ACE inhibitors are stopped.

How can you estimate renal function?

In order to adjust doses of drug eliminated by the renal route, you need to be able to quantify the degree of renal impairment. [The two most common methods in use are the Cockcroft and Gault equation and the modification of diet in renal disease (MDRD) equation].

Creatinine Clearance - Cockcroft and Gault Equation

The relatively constant rate of production of creatinine in individual patients together with almost exclusive glomerular filtration means that creatinine clearance (CrCl) should reflect glomerular filtration rate (GFR). Historically, the Cockcroft and Gault formula (see box below) is the most well established formula for estimating creatinine clearance.

MDRD equation

In line with the National Service Framework for Renal Services (DoH ,2005), the modification of diet in renal disease (MDRD) equation is recommended as the standard method of estimating GFR in order to detect renal impairment.