Explaining toxicity in relation to therapeutic range
The therapeutic range demonstrates the ‘relationship between the plasma drug concentration and the therapeutic response or toxicity over a certain time period’(Bryant and Knights, 2003, p 119). It measures on the lower end of the drug concentration scale, the minimal effective drug plasma concentration, in which anything below is deemed un-effective. On the higher end of the scale, the maximum non-toxic drug plasma concentration, with anything above being a toxic dose. The area in between these two is called the therapeutic range (Galbraith 1997, pp.87-9).
Many factors must be considered in benefit-to-risk analysis for a specific patient. Each patient reacts differently to drugs and chemicals. Patient-related factors include age, sex, pregnancy status, occupation, social circumstances, and genetic traits. Any ‘factor alone or in combination may influence the course and severity of the disease or the response to a drug’ (Bryant and Knights 2003, p. 119).
Toxicity is defined by Mosby’s Medical dictionary as: ‘the undesired condition as a result of too much exposure to a substance or drug, which do not cause adverse effects in smaller amounts’ (Anderson 1998, p. 1632). Fundamentally, any dose that is above the minimum non-toxic level may cause toxicity.
Digoxin has a very low therapeutic index of 1-2 mcg/l with a toxic dose being between 2-9mcg/l and an ineffective dose of <1 (Oh 1997, p. 968) and therefore it is an easy drug to overdose or underdose. Levels above the maximum non –toxic range may cause toxicity.