Working out percentages: Weight loss and dosage

Abstract

T topic of Calculations Skills this month is percentage calculations. Percentage is defined as a fraction or ratio where the denominator is 100 (per cent). Percentage statements may be used to indicate the concentration of a drug in solution; for example, sodium chloride 0.9% injection contains 0.9 g sodium chloride per 100 ml of solution. Percentages may also be used as part of drug dosage calculations, particularly for patients known to have factors that are likely to affect the pharmacokinetics of a drug; for example, impaired liver or renal function may lead to reduced drug clearance. Distribution may be affected in obese patients because of variable penetration of drugs into body fat stores. Diagnostic criteria may also include percentage statements. A significant and unintended reduction in body weight can be a non-specific indicator of diseases such as cancer or tuberculosis. A significant loss of weight is usually defined as more than 5% of body weight over 6–12 months. In 2013, the United Kingdom Medicines Information Pharmacists’ Group (UKMI, 2013) issued guidance on dosing of a range of antibiotics for obese patients. Aminoglycoside antibiotics are of particular concern because they have a narrow therapeutic index and tend to be prescribed in critically ill patients with serious infections. Too low a dose may lead to treatment failure, but too high a dose may lead to toxicity, including ototoxicity and nephrotoxicity. Although maintenance dosing is guided by therapeutic monitoring of blood levels, the initial adult dose is calculated based on body weight. In an obese patient, a dose calculated from total body weight (also known as actual body weight) may lead to a toxic blood level because aminoglycosides are highly water soluble and mainly confined to body water. The British National Formulary (Joint Formulary Committee, 2015). recommends that dosage is calculated using ideal weight for height in obese patients. As a rule of thumb, dosage adjustment is necessary for patients whose total body weight (TBW) exceeds ideal body weight (IBW) by 15% or more. Some manufacturers give guidance on dosage in obese patients. Obesity is defined by the National Institute for Health and Care Excellence (NICE) as a body mass index Alison Eggleton