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NBME 22 Answers

nbme22/Block 3/Question#41

A 56-year-old man develops brief episodes of ...

CL x Css

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 +8  upvote downvote
submitted by joha961(29),

Maintenance dose = (Css * CL * t) / F

... where t is elapsed time between doses (not relevant here since it’s continuous infusion) and F is bioavailability (which is 100% or 1.0 here because it’s given IV).

​Contrast with loading dose:

(Css * Vd) / F

... where Vd is volume of distribution.

yotsubato  So do we just have to memorize this... +4  
gh889  yep +6  
drschmoctor  @yotsubato Not necessarily. I can't remember a formula to save my life. The Css is the amount you want in the blood. The clearance is the fraction removed per unit time. Since we want to maintain a steady state, we only need to replace what is removed. Thus, maintenance dose = amount present * fraction removed. +2  

 +0  upvote downvote
submitted by drzed(44),

You do not need to memorize a formula to know this, as long as you know the units (which are SO much easier to remember as they are intuitive). The answers are mostly clearance and steady state, which means you can cancel units to see if the answer choice makes sense.

For example, since CL has units of L/min and Css has units of mg/L, then (A) works out because L/min x mg/L = mg/min which is the correct units for infusion rate.

Whereas (B) CL/Css would not work because those units would be L/min / mg/L = L^2/min*mg, which does not make sense.

So if you just cancel some units, you can answer many of the questions.

Huge disclaimer: this won't work with things like half-life, because there is a factor of Ln(2) that has no units, so you have to memorize formulas that have factors before them.