This new goal total range can easily be back calculated, which we’ll discuss.In terms of timing of levels, it kind of depends on the situation. N.B. You can imagine if someone is in the middle of seizing, we’d want to get the drug into them ASAP, which can be a little slower and harder with plain phenytoin. Sorry for the hiatus from writing, but I promise I didn’t forget you. You may be asking yourself why on earth do we have both? But like Luke Skywalker, bound phenytoin is just waiting in the wings for the right time to show itself against the enemy, and so it still has to be accounted for! What IV maintenance dose will you start and when?AdBW(L) = LBW + 1.33*(TBW – LBW) = 61.6 + 1.33*(115 – 61.6) = 132.6 kgLD = 15 – 20 mg/kg*(AdBW(L)) = 15 – 20*(132.6 kg) = 1989 – 2652 mgE.g., Infuse over at least 45 min (for the 2250 mg dose)AdBW(M) = LBW + 0.4*(TBW-LBW) = 61.6 + 0.4*(115-61.6) = 83 kgCheck free and total levels at least 2 hours after the end of the IV load, but practically speaking, it is often done the next day. She has stopped seizing with lorazepam 2mg IV x2. In patients with low serum albumin concentrations, a higher proportion of the total (measured) phenytoin concentration is unbound and caution is therefore required when interpreting the result.The equation below gives an albumin corrected, total phenytoin concentration which can be compared with the target concentration range (10 – 20mg/L). Anyway. One tenth of the Sheiner-Tozer corrected value was 0.65 ± 0.88 µg/mL; 95% confidence interval, -1.11 to … However, avoid increasing maintenance doses too quickly in succession from daily levels (Eventually that will catch up, and enzyme saturation will occur. Some drugs compete with phenytoin for its protein hiding spots, and there are many that displace phenytoin from albumin (Also, be aware of drug/nutrient interactions that can inhibit absorption of PO phenytoin doses…Drug interactions also play a role in the metabolism and drug out considerations for phenytoin. ).Of course, the caveat is if your patient goes into renal failure…or has a new interacting medication added to their regimen…anything that can change the ff. Less than 5% is excreted as unchanged drug, so that gives you an idea of how much work the body does on this med! Which ultimately decreases the excitability of neurons and slows neuronal transmission. The dose is the same for all phenytoin products when initiating therapy. If you remember nothing else from this post, take this away:Phenytoin is metabolized by several CYP enzymes (mainly 2C19, and 2C9, but a little of 3A4 too), and so there are multiple medications that can affect its metabolism.So regardless of mechanism of altered phenytoin concentrations (aka changes in distribution versus metabolism), see below for a sampling of medications that can affect phenytoin serum concentrations:Although the focus for this post is on phenytoin dosing, it’s hard to mention drug interactions of phenytoin and ONLY discuss the ones that affect phenytoin. It is used to both prevent and treat seizure disorders. For … Monitoring of free serum phenytoin concentrations should be strongly considered in critically ill patients requiring LVAD or ECMO … We'll treat it with courtesy and respect. What you should take away from this is that while the concentration of unbound drug is low, the rate of hepatic metabolism is BUT, as you add more drug (aka move right along the x axis and increase intracellular drug concentration), the rate of hepatic metabolism slows, which is why the curve flattens. Seriously. The first dose should be given 12–24 hours after the loading dose.Oral or nasogastric administration should be used, whenever possible. The below strategy back calculates total levels by rearranging the above ff equation and inputting free levels of 1 and 2 mcg/mL, since that is the range of free levels we know to be therapeutic. No further seizures have occurred.Range = 1/ff to 2/ff = 1/0.172 to 2/0.172 = 5.8 to 11.6 mcg/mLNo, free concentration is not within 1-2 mcg/mL, and total concentration is only 2.9 mcg/mL. Then you’ll have to re-establish that free fraction again.Some institutions and practitioners will utilize correction equations (like the traditional Winter-Tozer and its iterations) to interpret whether measured total phenytoin levels for their patients actually fall within the therapeutic range. residency, should I do a residency?, PGY1, PGY-1, Pharmacy residency, Pharmacy student, Pharmacy school, midyear, ashp midyear, the showcase, residency applications, residency interviews, pharmacy residency interviews, pharmacy residency applicationsseven figure pharmacist, student debt, student loans, student pharmacist, pharmacy student, finances, your financial pharmacist We'll never do anything untoward with your email address. Phenytoin is, of course, one of the most classic anti-epileptic medications. We have to So the road goes both ways here. Take it out to a nice steak dinner. The remainder is metabolized by the hepatic P450 enzyme system, predominantly CYP2C9 and CYP 2C19, and induces CYP3A4, which account for many of its …