For a patient started on theophylline, why is a blood test scheduled after 5 days?

• A. To monitor plasma theophylline level
• B. To monitor blood sugar
• C. To check liver enzymes
• D. To measure blood pressure

Answer: (A)

The main idea is that theophylline has a very narrow therapeutic window, so we need to check how much drug is actually in the blood after the body has had time to reach a stable level with ongoing dosing. The drug’s metabolism varies a lot between individuals and can be affected by age, liver function, smoking, and drug interactions, which makes dosing tricky.

After starting theophylline, a blood test around day 5 checks the trough level—this is the lowest concentration just before the next dose. At that point, the steady-state concentration reflects the true ongoing exposure and informs whether the dose needs to be adjusted to stay within the effective, safe range. A typical therapeutic trough is about 10–20 mg/L for many patients; levels higher than around 20 mg/L increase the risk of toxicity, while levels below the target may be ineffective.

So the test is scheduled to monitor the plasma theophylline level to guide safe and effective dosing. Other tests like blood sugar, liver enzymes, or blood pressure aren’t the primary reason for this timing.