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renal dosing

Discussion in 'Internal Medicine and IM Subspecialties' started by jok200, Aug 8, 2011.

  1. jok200

    7+ Year Member

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    sorry for the stupid question but what does it mean when someone asks for the renal dosing of a medication, like say the renal dosing for flagyl??? Also, is their a portable book that would provide this information with other pertinent facts about drug administration??

    Thanks-
     
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  3. Polish Farmer

    Polish Farmer Extubated
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    Dose for estimated GFR. Most medicines have been studied using Cockroft-Gault formulation for estimating creatnine clearance. On any paper or electronic pharmacology reference there is usually a section on "dose adjustments," in this case if a patients renal function has declined. Many medicines are renally cleared so need these adjustments for potential toxicity.
     
  4. jok200

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    Okay... confused. Soooo basically patients with renal impairment (what degree of renal impairment, is it based on creatinine clearance and if so what is the value that defines the impairment?) HOw do I use the cockcroft formula and why?

    THanks -
     
  5. jok200

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    when i look at the lab values on the computer it will give the CrCl and the GFR, how is the GFR being calculated, with inulin ?? If CrCL is a measure of GFR why are the two values given and again how is GFR being calculated?
     
  6. Polish Farmer

    Polish Farmer Extubated
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    Renal function is estimated using various equations that approximate actual GFR. Inulin and Iothalmate are considered gold standard substances to measure that approximate actual GFR (as they are freely filtered at the glomerulus and not reabsorbed/secreted), but they are difficult to come by and expensive so they are not often used.

    Creatnine works ok, it is freely filtered and not reabsorbed, but is secreted in the distal tubule (increasingly so in advancing CKD). The two creatnine based formulas for estimating GFR most commonly used are MDRD and Cockroft-Gualt. MDRD is probably the best estimate of GFR we have as it standardizes GFR to age, race, body surface area, and gender. Cockroft-Gualt standardizes to age, gender, and weight. Both of these formulas were not studied in patients with AKI, so it is truly impossible to estimate with any degree of certainty the actual GFR in that situation.

    Cockroft-Gualt: gives you a creat clearance ~ GFR, reported as ml/min, tends to not work as well with advancing CKD 2/2 increased tubular secretion of creatnine, also though it is based on weight, it was validated using actual lean body weight of a patient and that is a very tough and not often done thing in clinical practice.

    MDRD: performs better with advancing CKD probably, but tends to underestimate patients true GFR if actual GFR is > 60, it is reported as ml/min/1.73m2- supposedly standardized to a "normal body surface area," but in really big patients or really skinny ones it will have similar problems with estimating true lean body weight, as not all americans have the BSA of 1.73m2.

    Dose adjustments are typically made based on stages of CKD, with GFR > 90, 60-89, 30-59, 15-29, <15 or dialysis, many drugs were studied using various doses based on GFR in those different ranges (Zosyn typically 4.5g IV q6h with normal renal function, but if GFR < 15 = 2.25g IV q8h)

    But the point is that both these equations were validated in patients with STABLE renal function. Thus if a patient has a Scr of 1.0 on day 1, and Scr of 2.0 day 2, using one of these equations might indicate that they have a eGFR of 80 on day 1 and perhaps 40-50 on day 2 when in actuality the patient is anuric and has an actual GFR of 0. And there are a variety of drugs that interact with creatnine assays in the lab and interfere with tubular secretion of creatnine that can falsely elevate serum creatnine.
     
  7. jok200

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    thank you so much... this was an excellent explanation.
     
  8. jok200

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    Oh... what is your resource for learning this information, is it one text or multiple and if you could suggest texts which would they be??

    thanks again-
     
  9. gutonc

    gutonc No Meat, No Treat
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    And now you don't have to do your own homework, so it's a win-win.
     
  10. jok200

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    Actually it wasn't an assignment it was only something that I was curious about... most of the information I figured out on my own through the internet but the pieces that were difficult to ascertain were explained by Polish Farmer. I asked questions on information that I was unclear about which is usually what is done when one doesn't understand something. If you have not picked up on my sarcasm yet, try and mind your own business if you prefer not to help with a discussion.

    Thank you-
     

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