Chapter 20: Antimicrobial Principles
Select the factor that may affect the bioavailability of an oral anti-infective.
a. A medication that is a substrate of the CYP450 system
b. Dosage formulation of the anti-infective
c. A patient who has peripheral vascular disease
d. A patient who has renal dysfunction
Answer b is correct. The dosage formulation of the anti-infective may have significant effects on the bioavailability. A classic example is vancomycin. Vancomycin is available as oral and parenteral formulations. Oral vancomycin is not absorbed and reserved for the treatment of C. difficile infections (antibiotic-associated diarrhea). Intravenous vancomycin has a bioavailability of 100% (F=1) and is used for the treatment of systemic gram-positive infections (ie, methicillin-resistant Staphylococcus aureus). Parenteral formulations of an anti-infective may also have various degrees of bioavailability. Examples include a potential difference in bioavailability of β-lactams when administered via IV versus intramuscular routes.
Answer a is incorrect. Anti-infective CYP450 activity (substrates, inhibitors, inducers) will not affect the bioavailability. A CY P450 activity will affect the metabolism and elimination, but not absorption. Anti-infectives administered orally that are circulated through the liver may be affected by the first-pass effect. The first-pass effect will eliminate some of the anti-infectives and would indirectly affect the amount of medication able to be absorbed. The first-pass effect is a phenomenon of drug metabolism whereby the concentration of a drug is reduced before it reaches the systemic circulation. It is the fraction of lost drug during the process of absorption which is generally related to the liver and gut wall. Notable drugs that experience a significant first-pass effect are imipramine, propranolol, and lidocaine.
Answer c is incorrect. A patient with peripheral vascular disease would not be expected to have different bioavailability. However, comorbidities (like peripheral vascular disease) may impact other PK properties of anti-infectives like distribution and elimination.
Answer d is incorrect. Renal dysfunction would not have an impact on bioavailability. Renal dysfunction would affect the elimination of the anti-infective.
Select the factor that would usually necessitate a patient to be given IV anti-infectives.
c. Glucose-6-phosphate dehydrogenase (G6PD) deficiency
d. Blood pressure of 91/52 mm Hg with signs of hypoperfusion (baseline blood pressure for patient is 129/86 mm Hg)
Answer d is correct. Patients manifesting systemic signs of infection such as hypotension and hypoperfusion should receive IV anti-infectives.
Answer a is incorrect. Hyper- or hypothermia would not necessitate a patient ...