A low serum protein level decreases binding sites and can cause an increase in free drug in the plasma. Drug toxicity may be a result. What is a potential consequence?

• A. Adverse reactions
• B. Allergic reactions
• C. Non-therapeutic drug levels
• D. Drug toxicity

Answer: (D)

When a drug is bound to serum proteins, only the unbound portion is free to act. If serum protein is low, there are fewer binding sites, so more of the drug remains unbound. This increases the active free drug concentration in the plasma, heightening the pharmacologic effect and the risk of adverse outcomes. The most direct consequence of this increased free drug is drug toxicity. While adverse reactions can occur in general, the scenario described specifically points to toxicity from excessive free drug levels. Non-therapeutic levels refer to subtherapeutic exposure, and allergic reactions are immune responses not directly caused by increased unbound drug concentration.