In patients with renal impairment many drugs or their metabolites are excreted more slowly. It is therefore important to know what drugs will require dose reduction or are to be avoided altogether. An exception to this rule is gentamicin where careful use of a nomogram and monitoring of blood levels (peak one (1) hour after i.m. dose and trough before next dose) make its use relatively safe. (see nomogram on p. 21).
Most drugs are conveniently given at an interval equal to the half-life. If a drug is excreted exclusively by the kidney a reduction in Glomerular Filtration Rate (and hence creatinine clearance) will produce a corresponding reduction in drug excretion.
Thus if GFR falls by half the drug half-life can be expected to double; the dosage interval should therefore be doubled, while the loading dose and maintenance dose are not changed.
For practical purposes serum creatinine gives the most useful index of renal function. It does however fall with age and may not reflect the true state in the elderly, who may be assumed to have a GFR of 50ml/min even if serum creatinine is within normal limits.
Drugs to be avoided or used with caution in renal failure | |||||
---|---|---|---|---|---|
Active Ingredient | GFR | Dosage recommendations | Comments | ||
Potassium-sparing Diuretics | 20-50 | 20-50 | Moderate plasma K⁺ high risk of hyperkalaemia in renal impairment ; amiloride ex More ... | ||
Pravastatin | 10-20 - <10 | Start at lower end of dosage range | |||
Prazosin | <10 | Start with small dose | Increased sensitivity to hypertensive effect and possible CNS toxicity | ||
Primaxin(R) | 20-50 | Reduce dose | |||
Primidone | <10 | Avoid large doses | |||
Probenecid | 10-20 | Avoid | Ineffective and toxicity increased | ||
Procainamide | 20-50 | Avoid or reduce dose | |||
Procarbazine | 10-20 | Reduce dose | |||
Prochlorperazine see Antipsychotics | |||||
Promazine see Antipsychotics | |||||
Propranolol see Beta-blockers | |||||
Prophylthiouracil | 20-50 | Reduce dose | |||
Pseudoephedrine | <10 | Avoid | Increased CNS toxicity | ||
Pyridostigmine | 10-20 | Reduce dose | Excreted by kidney | ||
Quinapril | 20-50 | Start with 2.5mg | See also Captopril | ||
Ramipril | 20-50 | Start with 1.25mg daily | See also Captopril | ||
Ranitidine | <10 | Use half normal dose | Occasional risk of confusion | ||
Salicylates see Aspirin | |||||
Salsalate see Aspirin | |||||
Salt Substitutes | 10-20 | Avoid routine use | High risk of hyperkalaemia | ||