﻿ Renal formulas
 Renal formulas

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

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)

Introduction

Monitoring parameters

Precautions

Other creatinine clearance methods

Bibliography