Definition/Introduction

Clark's rule is an equation used to calculate pediatric medication dosage based on the known weight of a patient and a known adult dose of medication to be used. Clark's rule equation is defined as the weight of the patient in pounds divided by the average standard weight of 150 pounds (68 kg) multiplied by the adult dose of a drug to obtain the pediatric medication dose, as is demonstrated below:      

• (Weight* divided by 150 lbs.) x Adult Dose** = Pediatric Dosage
• (Weight*** divided by 68 kg) x Adult Dose** = Pediatric Dosage

*Weight of pediatric patient in pounds (lbs.)

**Adult dose is the recommended dosage for adult medication use

***Weight of pediatric patient in kilograms (kg)

Clark's rule is a known pediatric medication dosing rule described in the medical literature that utilizes the patient's weight to calculate medication dosage. Other equations that utilize pediatric weight to calculate medication dosing include Salisbury's rule, Penna's rule, and the Body Surface Area rule. Other methods besides Clark's rule that utilize pediatric age to calculate radiopharmaceutical medication dosages used in nuclear imaging include Young's rule, Webster's rule, and Fried's rule.[1][2]

Issues of Concern

Pediatric Obesity and Medication Dosing

Data from 2011 to 2014 showed the prevalence of obesity among children and adolescents aged 2 to 19 years to be 17% in the United States and continues to increase.[3] In addition to the known numerous health complications and the development of comorbidities associated with pediatric obesity, this epidemic has created challenges to weight-based medication dosing because of the pharmacokinetic changes associated with obesity.

For medication distribution, excess adipose tissue in obese pediatric patients has been described as affecting a medication's volume distribution (the amount of drug in the body compared to plasma concentration). Lipophilic medications are more likely to have higher volume distribution into adipose tissue when compared to normal-weight pediatric patients. In contrast, hydrophilic drugs may have increased or decreased volume distributions, altering the medication loading dose.[4] 

Due to the alterations in medication loading doses, obese pediatric patients may be at risk for either medication toxicity or sub-therapeutic medication therapy. Studies have described ideal body weight for calculating a hydrophilic medication loading dose, total body weight for a lipophilic medications loading dose, and adjusted body weight for a partially lipophilic medication loading dose.[5][6] 

Pediatric obesity's effects on pharmacokinetic changes in medication absorption, metabolism, and excretion remain an area in need of further research.[4]

Clinical Significance

Medication dosing for pediatric patients is described to use either the following methods for medication dosing: age-based dosing, allometric scaling, body surface area-based dosing, and weight-based dosing; no single approach is superior to the other and varies based on a medication's chemical properties and the age of the patient.[7] Weight-based dosing is the most commonly used method for calculating recommended medication doses in pediatric clinical practice.[8]

Some have thought that adult medication dosages are universally applied to pediatric patients when in fact, this is a misconception. Pediatric patient pharmacokinetics and pharmacodynamics vary based on age, body weight, body surface area, and developmental growth and function of various organ systems compared to adults.[7] 

The absence of deliberate practice in correct pediatric medication dosing can have potentially detrimental effects, such as exposing the patient to suboptimal medication dosages and severe systemic toxicity, which may even result in fatalities.

Nursing, Allied Health, and Interprofessional Team Interventions

Nursing and pharmacy should be familiar with Clark's rule since they will prepare, dispense, and administer medications to infants and children. This way, they can add another layer of safety when administering drugs to children and reach out to the prescribing or ordering clinician if they suspect an incorrect dose when using Clark's rule. This interprofessional system of checks and balances can optimize patient outcomes. [Level 5]