Below are the indications where TDM is highly useful to interpretate the drug concentration.
- toxicity– diagnosing toxicity when the clinical syndrome is undifferentiated (unexplained nausea in a patient taking digoxin)
– avoiding toxicity (aminoglycosides, cyclosporin) - dosing– after dose adjustment (usually after reaching a steady state)
– assessment of adequate loading dose (after starting phenytoin treatment)
– dose forecasting to help predict a patient's dose requirements1 (aminoglycosides) - monitoring– assessing compliance (anticonvulsant concentrations in patients having frequent seizures)
– diagnosing undertreatment (particularly important for prophylactic drugs such as anticonvulsants, immunosuppressants)
– diagnosing failed therapy (therapeutic drug monitoring can help distinguish between ineffective drug treatment, non-compliance and adverse effects that mimic the underlying disease).
Another important aspect of TDM is when the sample is taken as we know different drugs have different half lives, therefore there timings are also differ.
Timing of the plasma sample ('when to do it')
Unless therapeutic drug monitoring is being used to forecast a dose or there are concerns about toxicity, samples should be taken at steady state (4–5 half-lives after starting therapy).
At steady state, plasma concentration is usually proportional to receptor concentration. Some drugs, such as perhexiline, which has a very long half-life in patients who are 'poor metabolisers',
should be monitored before steady state is achieved to prevent toxicity developing after the first few doses. Another example where early monitoring may be useful is after phenytoin loading, where measurement of the plasma concentration can give a preliminary indication of adequate dosing.
The timing of the collection of the sample is important as the drug concentration changes during the dosing interval. The least variable point in the dosing interval is just before the next dose is due. This pre-dose or trough concentration is what is usually measured. For drugs with long half-lives such as phenobarbitone and amiodarone, samples can be collected at any point in the dosage interval.
should be monitored before steady state is achieved to prevent toxicity developing after the first few doses. Another example where early monitoring may be useful is after phenytoin loading, where measurement of the plasma concentration can give a preliminary indication of adequate dosing.
The timing of the collection of the sample is important as the drug concentration changes during the dosing interval. The least variable point in the dosing interval is just before the next dose is due. This pre-dose or trough concentration is what is usually measured. For drugs with long half-lives such as phenobarbitone and amiodarone, samples can be collected at any point in the dosage interval.
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