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Normal Severe renal impairment*

captopril amoxicillin gentamian atenolol dlgoxln

2 25 2 14 2.5 >50 6 100 36 90

* Glomerular filtration rate < 5 ml/min (normal is 120 ml/min). These are examples of drugs that are excreted almost unchanged; the prolongation of their tj£ indicates that special care must be exercised if they are used in patients with impaired renal function.

* Glomerular filtration rate < 5 ml/min (normal is 120 ml/min). These are examples of drugs that are excreted almost unchanged; the prolongation of their tj£ indicates that special care must be exercised if they are used in patients with impaired renal function.

eliminated metabolites: give a normal or, if there is special cause for caution (above), a slightly reduced initial dose, and lower the maintenance dose or lengthen the dose interval in proportion to the reduction in creatinine clearance.

2. Drugs that are completely or largely metabolised to inactive products: give normal doses. When the special note of caution (above) applies, a modest reduction of initial dose and the maintenance dose rate are justified while drug effects are assessed.

3. Drugs that are partly eliminated by the kidney and partly metabolised: give a normal initial dose and modify the maintenance dose or dose interval in the light of what is known about the patient's renal function and the drug, its dependence on renal elimination and its inherent toxicity.

Recall that the time to reach steady-state blood concentration (p. 102) is dependent only on drug t /2 and a drug reaches 97% of its ultimate steady-state concentration in 5 x t]/2. Thus if t '/2 is prolonged by renal impairment, so also will be the time to reach steady state.

Schemes for modifying drug dosage for patients with renal disease do not altogether remove their increased risk of adverse effects; such patients should be observed particularly carefully throughout a course of drug therapy. Ideally, dosing should be monitored by drug plasma concentration measurements of relevant drugs, where the service is available.

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