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Biochemistry Calculations

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Renal
Calculations

Creatinine Clearance - measured
Calculated Creatinine Clearance (Cockcroft & Gault)       On-line Calculator
Calculated Creatinine Clearance (Lean Body Mass est'd from height) 
                                                                                                 On-line Calculator
Estimated GFR by MDRD Formula                                     On-line MDRD Calculator
Estimated GFR by CKD-EPI Formula
Fractional Excretion of Sodium (FENA)
Creatinine Clearance - correction for body surface area

 

General
Calculations
LDL Cholesterol
Transferrin Saturation
Indirect Bilirubin
Calcium corrected for albumin
Ionised calcium corrected for in-vitro pH changes               On-line Calculator
Sodium Corrected for Hyperglycaemia
Anion Gap
Osmolar Gap (serum)
Delta Troponin Calculator
     Half-Life Calculator
     Unit Conversions

     Web Chemistry calculator
 

Renal Calculations


 

 

 


Creatinine Clearance - measured

Measured using creatinine determined plasma and 24 hour urine collection.

Creatinine Clearance (mL/min) = (U Creat x Vol x 16.7) / (Pl Creat x time)

Urine creatinine concentrations in mmol/L
Plasma creatinine concentration in umol/L
Volume in mL
Time in Hours

Reference Interval: 90 - 150 mL/min (lower values expected in older subjects).

Notes:

  • Creatinine excretions >20 mmol/d or <5 mmol/d may indicate prolonged or incomplete collections.
  • Plasma creatinine should be collected within the period of urine collection. Plasma creatinine from outside this period may be acceptable if renal function is stable.

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Calculated Creatinine Clearance (Cockcroft and Gault)

Creatinine Clearance (mL/min) = {(140 - Age) x Wt x F}/ (P Creat x 0.814)

F= 1 (males); F=0.85 (females)
Age in years
Plasma creatinine concentration in umol/L
Weight in kg

Reference Interval: 90 - 150 mL/min (lower values expected in older subjects)

Notes:
Extremes of body composition may affect accuracy of estimation.
On-line Calculator for Cockcroft and Gault Equation

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Calculated Creatinine Clearance (Cockcroft and Gault) with
Lean body Mass estimated from patient height

(estimated from plasma creatinine, age, height and sex)

Creatinine Clearance (mL/s) = {(140 - Age) x Wt x F}/ {P Creat x 0.814}

F= 1 (males); F=0.85 (females)
Age in years
Plasma creatinine concentration in umol/L
Weight in kg

Estimated Lean Body Mass from patient's height:
Males (kg) = 50 kg plus 0.9 kg for every cm of height > 152 cm
Females (kg) = 45.5 kg plus 0.9 kg for every cm of height > 152 cm

Reference Interval: 90 - 150 mL/min (lower values expected in older subjects).

Notes:
Extremes of body composition may affect accuracy of estimation.
On-line Calculator for Cockcroft and Gault Equation based on height

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GFR Estimate by MDRD Formula

eGFR (mL/min/1.73m2)= 175 x [SerumCreatinine(umol/L) x 0.0113]-1.154 x Age(years)-0.203 (x 0.742 if female)

This version of theMDRD formula is for serum creatinine assays aligned with IDMS (the international reference method). An IDMS aligned assay is in use at SydPath.

Notes:

  • Extremes of body composition may affect accuracy of estimation.
  • Generally accuract to within +/- 30% of formally measured GFR.
  • Recent intake of cooked meat may artifically elevate creatinine and thus reduce eGFR.
  • Not validated in pregnancy.

On-line calculator for eGFR by MDRD

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GFR Estimate by CKD-EPI Formula

eGFR (mL/min/1.73m2):

Females:
if S.creat <63; 144 x (S.cr X 0.0113 / 0.7)0.329 x 0.993age
if S.creat >62; 144 x (S.cr X 0.0113 / 0.7)1.209 x 0.993age
Males:
if S.creat <81; 141 x (S.cr X 0.0113 / 0.9)0.411 x 0.993age
if S.creat >80; 141 x (S.cr X 0.0113 / 0.9)1.209 x 0.993age

The CKD-EPI formula is only valid for serum creatinine assays aligned with IDMS (the international reference method). An IDMS aligned assay is in use at SydPath.

Notes:

  • Extremes of body composition may affect accuracy of estimation.
  • Generally accuract to within +/- 30% of formally measured GFR.
  • Recent intake of cooked meat may artifically elevate creatinine and thus reduce eGFR.
  • Not for use in in pregnancy, children (<18 years of age), dialysis patients.

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Fractional Excretion Of Sodium (FENA)


Fractional Excretion Of Sodium (FENA)

FENA(%) = {U Na / Pl Na x 100} / {U Cr * 1000 / Pl Cr}

All concentrations in mmol/L except plasma creatinine in umol/L.

Interpretation:

  • In pre-renal renal failure FENA is generally <1%.

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Creatinine Clearance per m2 Body Surface Area (BSA)

BSA(m2) = 0.007184 x Ht(cm)0.725 x wt(kg)0.425   (Dubois and Dubois formula, 1916)

CrCl(mL/min/1.73m2) = U CRCL(mL/min) x 1.73/BSA(m2)

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General Calculations


LDL Cholesterol (LDLC)

LDLC (mmol/L) = Total Cholesterol - HDLCholesterol - (Triglycerides / 2.2)

All values in mmol/L

Notes:
If the triglyceride concentration is > 4.5 the calculation is not valid and cannot be used.

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Transferrin Saturation (%SAT)

%SAT = 3.982  x  Iron (umol/L) / Transferrin (g/L)

Note:
Each molecule of serum transferrin can bind two molecules of iron.

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Indirect Bilirubin

Indirect Bilirubin (umol/L) = Total Bilirubin (umol/L) - Direct Bilirubin (umol/L)

Indirect bilirubin is a measure of unconjugated bilirubin
Direct bilirubin is a measure of conjugated bilirubin

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Calcium Corrected for Albumin

Used to correct plasma total calcium measurements for effects of changes in albumin concentration. Corrected calcium can then be compared with usual reference intervals.

Calcium (corrected, mmol/L) = Calcium (measured, mmol/L) + {(40 - albumin (g/L)) x 0.02}

Notes:

  • The correction can be applied to elevated as well as decreased serum albumin
  • The correction only applies to total calcium
  • The correction is a guide only.

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Ionised calcium corrected for in-vitro pH changes

  • Corrected ionised calcium = Measured iCa x (1 - 0.53 x (7.4-pH))

 

  • This calcultation is for in-vitro changes only, ie reductions in pH due to delays in sample measurement or increases in pH due to loss of CO2 in samples exposed to air. These are only required for venous serum collections.
  • The correction algorithm assumes that the pH in the patient is clsoe to 7.4.
  • Correction is NOT required for ionised calcium measured in blood gas samples where measured pH and measured ionised calcium reflect in-vivo concentrations.         On-line Calculator

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Sodium corrected for Hyperglycaemia

Used in patients with very high serum glucose concentration (eg > 30 mmol/L) to estimate serum sodium after reduction of glucose with insulin.

Sodium (corrected) = Sodium (measured) + glucose / 4  (all values in mmol/L).

Note: Variability in result due to measurement uncertainty approximately +/- 5 mmol/L.

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Anion Gap

A calculation to assist with determining the cause of a metabolic acidosis of unknown origin.

Anion Gap = Sodium + Potassium - (bicarbonate + chloride) (all measurements in mmol/L)

Reference interval: 10 - 18 mmol/L

Note: variability in result due to uncertainty in measurements approx. +/- 5 mmol/L.

For more information and interpretation see page: Chloride and the Anion Gap

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Osmolar Gap (Serum)

A calculation to assist with identifying the presence of unmeasured substances in the serum (eg ethanol, methanol, mannitol)

Osmolar Gap = measured osmolality - calculated osmolality

Calculated osmolality = 2 x sodium + urea + glucose (all measurements in mmol/L)

If an ethanol result is available, osmolality can be calculated as follows:

Calculated osmolality = 2 x sodium + urea + glucose + 270 x ethanol (in %*)

Calculated osmolality = 2 x sodium + urea + glucose + 1.25 x ethanol (in mmol/L)

Reference interval: 0 - 10 mmol/L

Note: variability in result due to uncertainty in measurements approx. +/- 7 mmol/L.

* units used at SydPath and to express the legal driving limit, eg 0.05% (=11 mmol/L)

For further information see SydPath page: Osmolality

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Last Updated 18/06/2013
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