Renal formulas

 

Top  Previous  Next

 

1.   Calculate lean body weight (LBW)

 

Devine method:

LBW (Males) = 50 + [ 2.3 x  (60 - Height in inches) ]

LBW (Females) = 45.5 +  [ 2.3 x  (60 - Height in inches) ]

 

Hume method:

LBW (Males) = (0.3281 x Weight in kg) + (0.33939 x Height in cm) - 29.5336

LBW (Females) = (0.29569 x Weight in kg) + (0.41813 x Height in cm) - 43.2933

 

The Devine method is the default result.  For short stature patients (less than 60 inches), the Hume method is utilized when its result is less than baseline LBW (50kg for males, 45.5kg for females) and greater than Devine.

 

Please note that neither of these method is valid in amputees.

 

2.   Calculate adjusted body weight (ABW)

 

ABW = LBW + [CF x (WT-LBW)]

 where        CF = correction faction (usually 20 to 40%)

                 WT = actual body weight

                 LBW = lean body weight

 

3.   Calculate body surface area (BSA)

   

Mosteller BSA  =  (HT X WT)^2 / 3600

 where        HT = height in centimeters

                 WT = weight in kilograms

 

DuBois BSA  =  0.007184 x (HT^ 0.725 + WT^ 0.425)

 where        HT = height in centimeters

                 WT = weight in kilograms

 

bm34

Since creatinine is produced by muscle, lean body weight (which estimates muscle mass) is usually recommended in creatinine clearance calculations, unless the patient is obese. The default definition of obese is BMI > 30, this is when the correction factor  begins.  However, you may adjust both of these determinants if your practice is different. To adjust the obesity correction factor, enter your Kinetics Setup dialog, then look under the creatinine clearance settings.

 

 

4.   Calculate Creatinine Clearance per Jelliffe Multistep method

 

i.    Estimate urinary creatinine excretion rate

 

E (males) = LBW x (29.305 -[0.203 x (age)])

E (females) = LBW x (25.3 -[0.18 x (age)])

 

 

ii.   Correct for nonrenal creatinine excretion in chronic renal failure

 

E = E x [1.035 - 0.0377(SCr)]

 where SCr is the latest serum creatinine

                 OR if SCr is rising, the average SCr

 

 

iii.  Correct for rising serum creatinine

 

E = E - [4 x ABW x (Scr1-Scr2)] / D

where ABW= adjusted weight in kilograms

         Scr1= the latest serum creatinine

         Scr2= the earlier serum creatinine

         D = the number of days between

 

 

iv.   Calculate Corrected creatinine clearance (CrCl)

 

CrCl/1.73 m2 =  (E * 0.12) / (SCr * BSA)

where        SCr is the most recent serum creatinine

         BSA = Body surface area

 

5.   Calculate Creatinine Clearance per Cockroft and Gault equation

 

CRCL Males = ABW(140 - Age) / (SCr x  72)

CRCL Females = 85% of male value

 where        SCr is the most recent serum creatinine

                 ABW = adjusted body weight

 

5.   Calculate dosage

 

Option 1 - Dosing table

Dosage tables are derived from either the FDA approved product labeling, Bennett's tables, AHFS Drug Information, or Medical Letter handbook of Antimicrobial Therapy.

 

Option 2 - One compartment model

For the antibiotics listed below and for user-added models, a dosage methodology utilizing a one compartment model may be used.

 

 

i.  Determine model pk parameters, examples5:

 

Drug

Peak/Trough

A

B

C

ACYCLOVIR

40/10

0.035

0.002

0.217

AMPICILLIN

50/ 5

0.058

0.0064

0.26

AZLOCILLIN

250/25

0.116

0.0058

0.16

AZTREONAM

100/10

0.116

0.0023

0.143

CARBENICILLIN

200/20

0.046

0.0042

0.346

CEFAMANDOLE

60/ 6

0.043

0.0065

0.216

CEFAZOLIN

120/20

0.032

0.0026

0.15

CEFONICID

150/25

0.014

0.0016

0.10

CEFORANIDE

120/20

0.023

0.0021

0.132

CEFOTAXIME

80/ 4

0.069

0.0056

0.159

CEFOTETAN  

120/20

0.02

0.00178

0.125

CEFOXITIN

60/ 6

0.035

0.0066

0.216

CEFTAZIDIME

60/ 6

0.028

0.0034

0.237

CEFTIZOXIME

60/ 6

0.028

0.0046

0.229

CEFUROXIME

60/ 8

0.041

0.0051

0.174

CEPHALOTHIN

50/ 5

0.06

0.0114

0.26

CEPHAPIRIN

50/ 5

0.06

0.0092

0.24

CEPHRADINE

50/ 5

0.06

0.0065

0.26

IMIPENEM

40/ 1

0.173

0.0052

0.162

METHICILLIN

40/ 4

0.173

0.0075

0.305

METRONIDAZOLE

25/ 8

0.053

0.0005

0.45

MEZLOCILLIN

250/25

0.173

0.0052

0.155

PIPRACILLIN

250/25

0.173

0.0052

0.156

TICARCILLIN

200/20

0.043

0.0053

0.336

 

ii.  Calculate rate constant (Kel) using A & B from Table 1.

 

Kel = A + (CrCl x B)

 where CrCl = calculated creatinine clearance

 

 

iii.  Calculate dosing weight (DW)

 

DW = LBW + CF x (ABW-LBW)

where ABW = actual body weight

         LBW = lean body weight

         CF = correction factor

 

 

iv.  Calculate volume of distribution (Vd) using C from Table 1.

 

Vd = DW * C

 

 

v.  Calculate ideal dosing interval (tau)

 

tau = tinf + [ (-1 / kel) x  ln (Cpmax/Cpmin) ]

 where Cpmin = Target trough

             Cpmax = Target peak

 

 

vi.  Calculate ideal maintenance dose

 

MD = kel  x  Vd  x  Cpmax  x   tinf  x  (1 - e-kel x  tau / 1 - e-kel x  tinf)

 where tau = ideal dosing interval

 

 

vii.  User selects practical dosage and interval

 

 

viii. Calculate expected peak & trough levels

 

Peak = [MD / (tinf x  Vd x  kel)]  x  [(1 - e-kel x tinf) / (1 - e-kel x  tau)]

 

Trough = Peak  x   e-kel x  (tau - tinf)

 


See also:

Introduction

Monitoring parameters

Precautions

Other creatinine clearance methods

Bibliography

 

. ©Copyright 1984 - 2015, by RxKinetics. All rights reserved.