Creatinine Clearance

[Cocogirl]

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I'm sure no one wants to be thinking about pharmacokinetics and creatinine clearances hours before thanksgiving but unfortunately, there is no rest for the weary (residents).

I have a question regarding calculating CrCl. When a patient is admitted with weight x (let's say 90kg) and is resuscitated with fluids and reaches weight y (let's say 120 kg) over 2 weeks or so. What weight do you use to calculate the creatinine clearance?

For Vancomycin you would use TBW normally to calculate dose but is the situation different here?

Thanks for the insights whenever you get a chance! 🙂

Happy Thanksgiving!

 

[Aznfarmerboi]

that would depend on when you need that creatine clearance number, no?

 

[sdn1977]

I generally use IBW for the initial dose & then calculate again after fluid equilibrates, which it will do fairly rapidly with tx. So much depends on if the weight is secondary to fluid overload or compromised renal failure.

Follow drug levels, renal function & adjust as necessary.

Something else is going on if the pt has had a 30kg weight gain over a 2 wk period of hospitalization & I think it has more to do with renal function than just fluid resusucitation, so I would factor in age as well.

If you've had 2 weeks with such a situation...a 24hr CrCl will give you an exact answer & you won't need to guess - not that expensive a test. Or...ask the attending/resident what they perceive his % renal function is of normal???

 

[Cocogirl]

Thanks for the responses! I don't think I was clear enough. The 2 week weight gain is a result of acute renal insufficiency.

Sdn1977 - How do you know when the fluid has equilibrated? Assuming the Scr is 3.5 in an already obese pt - are you going to use their 90kg weight, 120 kg or IBW to calculate a vanc dose?

 

[BillyN]

When you're calculating CrCl, you're actually estimating it. In order to use the Cockcroft-Gault equation properly, your serum creatinine must be at steady-state.

For vanco dosing at my institution, we calculate a dose using the "bolus model" based on population parameters: Vanco Cl = 0.65 x CrCl* x Total Body Wt, where CrCL* = (140-age)/(72 x Cr) is in mL/kg/min, and Total Body Wt should not include excess fluid wt. Vanco Vd = 0.7 L/kg x Actual Wt (including excess fluid wt). This gives you a k value and a Vanco half-life.

Hope this helps.

 

[WVUPharm2007]

As mentioned before, I use IBW......unless patient is 1.2X ideal weight, then I use the fancy-schmancy adjusted body weight equation which I have in my manual back home in WV. So if this person is obese, you might need to adjust a smidge, too.