Definition/Introduction

Weight is among the most essential components of science and non-science daily life routines. Weight can be defined as the force exerted on a body due to gravity. The weight W of a body equals the magnitude F of the gravitational force on the body. In particular, weight and conversion are paramount for managing patient care among healthcare providers.

Two systems calculating weight are used in all healthcare settings for health management, such as medication dosing per patient body weight. Another example is weight tracking in congestive heart failure patients. First, the metric system is commonly used in US health care. It is also the only system used in many countries on all continents. It has the advantage of a decimal system in increments or the power of tenths. Hence it is widely used for ease of calculations. US medical and pharmaceutical practices adopted the metric system in the late 1800s.[1] It is currently used only in science and health care in the US. Second, the US weight system customarily uses the ounce or pound. It derives from the British colonial era. This non-metric system is widely used in the US. Consequently, the US scales are calibrated for metric and non-metric systems. The American Society of Health-System Pharmacists (ASHP) suggests conversion be used in clinical practice considering bioavailability and patient-specific factors for accurate dosing.[2]

Issues of Concern

The metric system is essential in all healthcare settings. Patients are weighed at each clinical encounter. Scales used in the US have double marking indicators: metric and non-metric markings. Metric weight values are used in medication calculation, radiation dosing, and weight compliance in equipment use, such as the maximum weight of a CAT-SCAN unit or a surgical table that may hold a person. On the other hand, individuals not having a STEM background may have a minimal understanding of the metric weight system. The American tradition for weight is based on ounces or pounds.[3] 

When babies are born, their weight is one of the most important things recorded; metric weight is recorded in the electronic health records by clinicians and hospitals, and the family records non-metric weight for newborn announcements. Nowadays, all medications are based on weight for dose calculations for all populations, specifically children and infants. Adults have their weight recorded mainly by doctors at each physical patient-clinician encounter. Commonly, most adults monitor their weight for weight management. Clinicians record it in electronic health records in both kilograms and pounds. However, communication with patients is generally in pounds by health care providers for weight reduction education. In non-medical areas, such as in identifying suspects by American law enforcement, the pound unit in body weight is used for approximation for a better understanding of a person's estimated size and weight.

The medication dose in routine practice is calculated by mg/kg. The conversion to ideal body weight(IBW) should be used to calculate the accurate dosing.[4] 

• IBW for men in kg= 50 kg + 2.3 kg x (height-60)
• IBW for women in kg = 45.5 kg + 2.3 kg x  (height-60). Height is in inches, and height is ≥60 in.

The limitation of the abovementioned formula is that it overestimates body weight at taller heights and underestimates body weight at shorter heights.[5]

Once the IBW has been derived, it can also estimate the tidal volume.[6][7]

• Tidal volume target = 6-8 (mL/kg)*IBW.

The other issue is the calculation of the correct dose of antineoplastic agents. The weight-based formula may not be appropriate; the American Society of Clinical Oncology recommends Body Surface Area(BSA) to calculate the dose of the majority of antineoplastic agents. Additionally, ASCO recommends that the dosage of immune checkpoint inhibitors such as nivolumab and pembrolizumab be calculated based on FDA-approved prescribing information.[8]

For pediatric patients, Clark's rule is used. The Clarks formula should not be used for pediatric patients with obesity due to the concern for toxicity or sub-therapeutic dosing.[9]

• Pediatric Dosage=Adult dose x [weight of the child (lbs)/150]

Pediatric liquid formulations prescribed by teaspoons or tablespoons can lead to dosing errors.[10] Consequently, the American Association of Pediatrics recommends milliliter(ml)-based dosing for prescribing and administering oral liquid medications.[11]

Clinical Significance

Education on weight and its conversion started in grade school. Therefore, the most important teaching of weight conversion between a pound to a metric weight, either grams or kilograms, is crucial throughout one's education. Universally, weight the following shorthand units are used in writing. 

• Pound = lb.
• Ounce = oz.
• Kilogram = kg.
• Gram = gm.

Generally, 2.2 pounds roughly converts to 1 kilogram, and a kilogram equals approximately 2.2 pounds. 

Commonly in healthcare and medical practices, the metric system is used for weighing mass. The metric system has increments at the tenth power for calculations. This weight conversion is used daily among scientists and healthcare providers.

• 1 kilogram (kg) = 1000 grams (gm) = 1,000,000 milligrams (mg) = 1,000,000,000 micrograms (mcg)

Another weight unit used in medical communities includes grains. Here is another weight conversion commonly used in the pharmaceutical industry.

• 1 grain (gr) = 60mg

In the US, weight units of the pound and ounce are almost universal.

• 1 pound (lb) = 16 ounces (oz)

Specific mathematical processes and formulas are necessary for density calculations utilizing fluid weight in scientific experimentation. Liquids can be useful for measuring as in fluid weight measurement terminology, not to be mistaken as a dry weight for concrete materials or substances.

Fluid weight is commonly measured using the metric system for density calculations. Every liquid has a characteristic density (D) cataloged by scientists in the Density Table of Metals and Elements. It is defined as the ratio of its mass (m) to its volume (v) or, in mathematical terms, D = m/v. Density is usually given in units of kilograms/meter. However, measuring small quantities using grams and cubic centimeters in this conversion is more convenient.

• 1 kg/m = 0.001 g/cm; 1 g/cm = 1,000 kg/m

Due to the usage of both the metric and non-metric systems, clinicians receive training to be fluent in calculating common weight conversions in the patient weight for monitoring purposes, specifically in newborns or congestive heart failure patients. In addition, weight conversion is necessary for record-keeping, which is usually in the metric system, and for educating patients using a non-metric system. These are some common conversion factors:

• 1 lb   =  16 oz      =   453 g   =   0.45 kg
• 1 oz  =  0.063 lb  =   28 g    =   0.03 kg
• 1 kg  =  1000 mg =   2.2 lb     =   35 oz
• 1 gm =  0.04 oz   =   0.001 kg
• 1 gm= 1000 mg
• 1 teaspoon= 5 ml
• 1 tablespoon= 15 ml

Vitamins may be prescribed in International Units. However, FDA has published guidance for converting units of measurement for vitamin labels.[12]

• Vitamin A:  1 mcg RAE = 1 mcg pre-formed vitamin A (retinol) =2 mcg supplemental β-carotene   
• Vittamin D:  1 mcg cholcecalcifarol= 40 IU vitamin D3
• Vitamin E:  1 mg alpha-tocopherol = 1 mg alpha-tocopherol = 2 mg all-rac-alpha-tocopherol (synthetic vitamin E)

Weight changes are expected in the following clinical scenarios, which may require further investigations and monitoring.

• Congestive heart failure
• Hypothyroidism
• Diabetes mellitus
• Hyperthyroidism
• Cushing disease[13]
• Unexpected weight loss can be from underlying malignancy[14]

Medication-induced weight gain is common with the following drugs.

• Mirtazapine 
• Olanzapine
• Quetiapine
• Risperidone 
• Gabapentin[15]

The equation to calculate body mass index is based on weight and height. 

• BMI= weight in kg/ (height in meter^2)

This formula is used for the classification of obesity. American Association of Clinical Endocrinologists guidelines recommends pharmacotherapy for individuals with BMI ≥ 30 kg/m^2  or BMI ≥ 27 kg/m^2 with comorbidities. The following medications are approved for weight loss.

• Semaglutide 
• Liraglutide
• Bupropion-naltrexone
• Phentermine-topiramate
• Orlistat[16]

The limitation of BMI-based risk factor classification is that visceral adiposity (not measured by BMI) is associated with type 2 diabetes mellitus, cardiovascular risk, and metabolic syndrome.[17]

It is essential to recognize that the drugs used for resuscitation can be dosed according to ideal body weight(IBW) or total body weight(actual weight).

• Epinephrine and magnesium sulfate are hydrophilic drugs, and the dose must be calculated according to IBW.
• Atropine and naloxone are lipophilic drugs, and the dose is calculated according to total body weight.[18]
• Drugs may need adjustment based on therapeutic drug monitoring(e.g., digoxin), especially in patients with obesity.[19]

Nursing, Allied Health, and Interprofessional Team Interventions

It is universally understood that the metric system is used mainly in science and health care. Medical providers utilize weight conversion daily in their practice. It is important the address weight management with patients due to the increased prevalence of obesity in the United States. Clinicians should use correct weight terms for communication and documentation among healthcare professionals and patients.[20] Weight significantly affects the pharmacokinetic and pharmacodynamic properties of medications. Improper use of weight conversion may lead to patient harm. High-alert medications such as potassium chloride need special attention, as incorrect dosing can lead to adverse patient outcomes.[21] 

The ASHP guidelines-based pharmacist interventions significantly reduced patient-controlled analgesia and opioid infusions during a widespread opioid shortage.[22] ASHP provides conversions used in routine clinical practice. A study demonstrated that the hospital pharmacist-based intervention effectively converted intravenous to oral dosing, resulting in significant cost savings.[23] [Level 3]

Nursing, Allied Health, and Interprofessional Team Monitoring

Healthcare providers are tasked with weight management of the US population. Clinicians seeing patients must review charts for weight and body mass index in every encounter for weight monitoring in patients. This is a golden opportunity to educate patients on lifestyle changes for weight reduction purposes.[24] Obesity has been classified as a chronic illness and is managed by clinicians to prevent hypertension, kidney diseases, and many others.[25][26] Children are prone to develop obesity, and the nursing team should monitor the children's weight at each visit.[27] Medication reconciliation by pharmacists and physicians can identify and prevent medication-induced weight gain.[28] 

Electronic health record (EHR) integration and automatic estimations can support clinicians in prescribing accurate dosing for drugs like opioids.[29] American College of Cardiology and American Heart Failure association guidelines suggest monitoring standing body weight simultaneously daily.[30]