Wattle Pharmacy Prep

Excretion and clearance
Practice questions with explanations

Excretion and clearance belongs to Pharmacokinetics and pharmacodynamics, which makes up about 10% of the Overseas Pharmacist Readiness Assessment (OPRA®). Answer each question yourself before opening the explanation.

1. Which method of dose adjustment maintains the same peak concentration in renal impairment?

  1. Increasing the dose and shortening the interval
  2. Keeping the dose the same and lengthening the interval
  3. Reducing the dose and keeping the interval
  4. Reducing both dose and interval
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B. Keeping the dose the same and lengthening the interval

Interval extension preserves the peak, which matters for concentration-dependent killing by aminoglycosides, and allows a low trough for safety. Dose reduction preserves a steadier concentration with a lower peak, which suits time-dependent antibiotics such as beta-lactams where time above MIC is what matters. Choosing between them requires knowing what drives the drug's effect.

2. Which factor reduces hepatic clearance of a high extraction ratio drug?

  1. Reduced hepatic blood flow, as in heart failure, cirrhosis with shunting, or during beta-blockade
  2. Enzyme induction
  3. Reduced plasma protein binding
  4. Increased cardiac output
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A. Reduced hepatic blood flow, as in heart failure, cirrhosis with shunting, or during beta-blockade

For drugs the liver extracts almost completely in one pass, delivery limits clearance, so anything reducing flow raises concentrations. Propranolol reduces its own clearance by lowering cardiac output. Low extraction drugs are instead sensitive to enzyme activity and protein binding, which is why the two categories behave so differently in liver disease.

3. How does age affect renal drug clearance?

  1. Only tubular secretion changes with age
  2. Glomerular filtration declines progressively from middle age, so an older patient with a normal creatinine may still have substantially reduced clearance
  3. Renal clearance increases with age
  4. Renal function is stable throughout adult life
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B. Glomerular filtration declines progressively from middle age, so an older patient with a normal creatinine may still have substantially reduced clearance

Nephron number and renal blood flow fall with age, and because muscle mass falls in parallel, serum creatinine may remain deceptively normal. Estimating clearance rather than reading the creatinine is therefore essential before prescribing renally cleared drugs in older people, and it is one of the commonest sources of preventable adverse drug events.

These are 3 of the 20 questions on excretion and clearance in the app.

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More topics in Pharmacokinetics and pharmacodynamics

Absorption and bioavailability · Distribution and protein binding · Metabolism and cytochrome P450 · Pharmacokinetic calculations · Receptor pharmacodynamics · Dosing in special populations