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Clark's Rule

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Last Update: February 21, 2020.


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 one hundred fifty pounds multiplied by the adult dose of a drug equals the pediatric medication dose, as is demonstrated below:      

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

*Weight of pediatric patient in pounds (lbs.)

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

Clark's rule is one of the known pediatric medication dosing rules 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, while other methods besides Clark's rule that also 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 of the ages 2 to 19-year-old of age 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 to affect a medication's volume distribution (the amount of medication 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, while hydrophilic medications may have either increased or decreased volume distributions, therefore 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 the use of ideal body weight for the calculation of hydrophilic medication loading dose, total body weight for lipophilic medications loading dose, and adjusted body weight for a partially lipophilic medication loading dose.[4][5][6] Pediatric obesity 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; neither method is to be more superior to the other and varies based on a medications chemical properties and age of the patient.[7] Weight-based dosing is the most commonly used method for calculating recommended medication doses in the 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 varies among age, body weight, body surface area, and developmental growth and function of various organ systems when compared to adults.[7]  The absence of deliberate practice in correct pediatric medication dosing can have potential effects such as exposing the patient to suboptimal medication dosages, severe systemic toxicity, and may even result in fatalities.


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Elias GP, Antoniali C, Mariano RC. Comparative study of rules employed for calculation of pediatric drug dosage. J Appl Oral Sci. 2005 Jun;13(2):114-9. [PubMed: 20924533]
Accorsi R, Karp JS, Surti S. Improved dose regimen in pediatric PET. J. Nucl. Med. 2010 Feb;51(2):293-300. [PubMed: 20080887]
Ogden CL, Carroll MD, Lawman HG, Fryar CD, Kruszon-Moran D, Kit BK, Flegal KM. Trends in Obesity Prevalence Among Children and Adolescents in the United States, 1988-1994 Through 2013-2014. JAMA. 2016 Jun 07;315(21):2292-9. [PMC free article: PMC6361521] [PubMed: 27272581]
Kendrick JG, Carr RR, Ensom MH. Pharmacokinetics and drug dosing in obese children. J Pediatr Pharmacol Ther. 2010 Apr;15(2):94-109. [PMC free article: PMC3018176] [PubMed: 22477800]
Kendrick JG, Carr RR, Ensom MH. Pediatric Obesity: Pharmacokinetics and Implications for Drug Dosing. Clin Ther. 2015 Sep 01;37(9):1897-923. [PubMed: 26361823]
Matson KL, Horton ER, Capino AC., Advocacy Committee for the Pediatric Pharmacy Advocacy Group. Medication Dosage in Overweight and Obese Children. J Pediatr Pharmacol Ther. 2017 Jan-Feb;22(1):81-83. [PMC free article: PMC5341538] [PubMed: 28337087]
Bartelink IH, Rademaker CM, Schobben AF, van den Anker JN. Guidelines on paediatric dosing on the basis of developmental physiology and pharmacokinetic considerations. Clin Pharmacokinet. 2006;45(11):1077-97. [PubMed: 17048973]
Pan SD, Zhu LL, Chen M, Xia P, Zhou Q. Weight-based dosing in medication use: what should we know? Patient Prefer Adherence. 2016;10:549-60. [PMC free article: PMC4835122] [PubMed: 27110105]
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Bookshelf ID: NBK541104PMID: 31082148


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