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TDM : Examples : Phenytoin & Digoxin

TDM : Examples : Phenytoin & Digoxin
Continuing on with therapeutic drug monitor. Let’s look at phenytoin. Now phenytoin exhibit concentration related toxicity such as nystagmus and the CNS depression. And that’s why therapeutic drug monitoring becomes important. How about monitoring time? When to monitor? When entering concentration? Now for loading dose. We monitor the peak concentration which occurs after two hours or two hours after the end of infusion. Now to monitor the maintenance dose, we measure the trough concentration after three to four days. Because it takes that long to achieve a new steady state and phenytoin concentration. And we would target a range of 10 to 20 microgram per l because 10 is typically considered as the threshold therapeutic concentration.
And exceeding 20 microgram per ml would lead to nephrotoxicity or even other toxicity. And If we can have two trough concentrations following two different doses. Then we would be able to calculate the V-max and the km. The maximum metabolic velocity and that Michaelis Menten rate constant and that would afford us to calculate a new dose for the patient phenytoin. TDM for digoxin. Why monitor digoxin? Reasons are typically we are concerned with complaints. That is the patient are not compliant are not taking the drug property. Or we have suspected toxicity. Or the sign of toxicity. Or that inadequate therapy despite high doses. Or perhaps potential drug interactions between digoxin and other cardiovascular drugs such as amiodarone and the verapamil.
so we monitored digoxin for clinical reasons not just numerical reason. When are the drawing times?
We prefer to target trough concentration. And that happens six hours after the dose because of the long distribution time. And normally, we would target the range of 0.5 to 2.0 nanogram per ml. And again that concentration is 6 hours after the dose. And it can tell that range is quite wide 0.5-2.0. So remember not to monitor digoxin level alone as this is a large overlap. There is overlap large overlap between the toxic and therapeutic levels. So observe clinical response do not monitor that numerical value alone. So this conclude, This topic on dosing regimen in renal failure and the therapeutic drug monitoring. As a little habit Topic and active slight to observe because of the questions.
But given some time and effort. I’m pretty sure you have a pretty good grasp on it. So good luck and see you next time.

In this step, Prof. Lee gives two examples of TDM and summarizes this week.

For Phenytoin, we must know that it has concentration-related toxicity. When the serum concentration is over 20 mg/L, the patient may experience nystagmus and ataxia. As a result, the clinical common range for Phenytoin is between 10 and 20 mg/L.

Besides, we need to know the monitoring time to measure the trough correctly.

For digoxin, there are many reasons to monitor its serum level, including compliance, suspected toxicity, and drug interactions. When monitoring the digoxin level, please observe the clinical response from the patient as well.

Please have some time to reflect upon the dosing regimen in renal failure and the therapeutic drug monitoring. If you have any thoughts or question about this week, please leave them below.

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Pharmacokinetics: Drug Dosing in Renal Disease

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