Drug dosing in patients with low body weight

To highlight important considerations when designing dosing regimens for patients with low body weight.

Pharmacokinetic alterations that occur in patients with low body weight could lead to inappropriate drug concentrations and adverse effects if standard doses for patients with normal body habitus are used. In contrast to information on drug dosing in patients with obesity, there is a dearth of publications concerning drug dosing in patients with low body weight. Weight descriptors available for dosing patients with low body weight are the same as those used in obese patients but are not as well studied. However, there are general considerations regarding weight descriptors for weight-based drug dosing that are similar for patients with low or excess weight such as the lack of demonstrated usefulness of ideal body weight. The results of investigations that are available for drug dosing in patients with low body weight are primarily limited to antimicrobial and anticoagulant agents, which have well established forms of therapeutic drug monitoring or laboratory monitoring parameters. Of note, the antimicrobials and anticoagulants studied in patients with low body weight tend to be hydrophilic drugs that are eliminated by the kidneys. This has implications for pharmacokinetic parameters, including volume of distribution and clearance that may not be applicable for more lipophilic agents. There is little information on lipophilic drugs with larger volumes of distribution and often non-renal mechanisms of clearance.

There is a paucity of information regarding drug dosing in patients with low body weight. The limited information suggests substantial variability in pharmacokinetic outcomes and clinical outcomes with standard doses in low weight patients. Nevertheless, this group should be considered a "special population" where individualized dosing approaches should be sought. More research is needed in this area particularly with respect to size descriptors useful for weight-based drug dosing and for lipophilic drugs that are not amenable to some form of therapeutic drug monitoring.