Antimicrobial Drugs

Geriatric Essentials

• The use of antimicrobial drugs in the elderly does not fundamentally differ from that in younger people (see Tables 133-1, 133-2, 133-3, and 133-4).

Pharmacokinetics

Age-related changes in pharmacokinetics often require a change in the dose, frequency, and route of administration of antimicrobials, especially for frail elderly people. For example, a given dose of ciprofloxacin (a fluoroquinolone) produces peak plasma concentrations that are about twice as high in the elderly as in younger patients. Elderly people have less lean body mass and more body fat; thus, water-soluble antimicrobials (eg, aminoglycosides, penicillins, cephalosporins, amphotericin B) produce higher concentrations in plasma and tissue, and fat-soluble antimicrobials (eg, chloramphenicol, doxycycline, fluconazole) produce lower concentrations. Some newer antibacterials (eg, macrolides, azalides, telithromycin, streptogramins, fluoroquinolones) produce relatively low plasma concentrations, but intracellular concentrations are up to 40 times higher than concurrent plasma concentrations. Because these high concentrations may persist for several days, dosing intervals can be longer and duration of treatment can be shorter. Linezolid accumulates in skin, which may be beneficial for treatment of skin and soft tissue infections.

Protein binding: Plasma albumin concentration decreases slightly with aging, although this decrease is usually of little importance. However, disease-related decreases in albumin levels can result in higher concentrations of free drug.

Elimination: Metabolism of antimicrobials seems to be relatively constant with aging. However, 1st-pass metabolism of some drugs (eg, ciprofloxacin) is decreased, possibly leading to increased plasma concentrations. Biliary excretion of antimicrobials is also relatively constant with aging. However, half-lives of renally excreted antimicrobials may be markedly prolonged in the elderly, especially if renal excretion is the only mode of elimination (as for aminoglycosides). The increase in plasma half-life is usually moderate unless patients have severe renal impairment (creatinine clearance < 30 mL/min). For such patients, risk of drug accumulation and adverse reactions is considerable.

Choice of Drug

The availability of fluoroquinolones and other well-absorbed oral drugs has made oral therapy for many serious infections possible. These drugs are also well absorbed in the elderly and thus can often be given orally to treat serious infections. However, hospitalization, even if oral antimicrobials are prescribed, may be necessary depending on comorbidities, ability to comply with therapy, degree of functional reserve, and seriousness of the infection.

Elderly people respond to some antimicrobials differently than do younger people and sometimes require a different drug. In addition, the etiology and severity of infection may be different from those in younger people. The increased risk of certain infections, especially hospital-acquired infections, in elderly patients should be considered when choosing an antimicrobial.

This topic was last updated May 2005.