Renal Function Tests

Renal function tests - most usually serum creatinine (sCr) and blood urea nitrogen (BUN) - are done to estimate glomerular filtration rate (GFR), normally 180L/day.

Cystatin C is a protease which is released into the bloodstream and completely filtered, and may be more reliable but is not as widely used.

Tests that also measure the kidney's ability to maintain salt-water balance can also be used, including fractional excretion of sodium.

 

 

  • serum
    creatinine
  • blood urea
    nitrogen
  • fractional excretion
    of sodium
  • urine
    acidification
  • protein
    excretion

Serum Creatinine (sCR)

Creatinine is a metabolite of creatine, a source of energy in muscle. It is normally produced at a constant rate which varies according to the mass of skeletal muscle. The kidneys filter almost all creatinine, providing a reasonable estimate of GFR.

 

Normal ranges

  • adult males: 53-113 umol/L
  • adult females: 37-96 umol/L
  • young children: 50 umol/L (increases after age 8 with increasing muscle mass)
  • infants: 30 umol/L
  • newborns: 37-96 umol/L

 

There is a non-linear relationship between sCR and GFR, which falls by 50% as sCR doubles. As a 50% decrease in kidney function will send sCR from 50 to 100 umol/L, still within the normal range. Accordingly, it is not sensitive at detecting early kidney disease. Also, normally, only 10% of creatinine is secreted in the tubule. However, as GFR drops, tubular secretion increases up to 40%, increasingly overestimating GFR as renal function declines.

 

Factors that affect serum creatinine

increases with:

  • low GFR
  • increased muscle mass
  • skeletal muscle damage
  • high meat or creatine uptake
  • drugs such as cimetidine, trimethoprim, triamterene, and amiloride

decreases with:

  • high GFR
  • dec muscle mass
  • advanced age

 

 

Calculating Creatinine Clearance

Creatinine clearance will vary among patients (especially for body weight), so it is important to follow creatinine levels over time to correctly determine renal function.

 

24 Hour Urine Collection

Clearance (mg/dL) is the amount of blood entirely cleared of a solute per unit time = UV/P

  • U = urine [Cr]
  • V = volume of urine ml/min; divide ml/24h by 1440 to find ml/min
  • P = plasma [Cr]

Accuracy is diminished by adequancy of urine collection and the increase in sCr tubular secretion with decreased GFR.

 

 

Cockcroft Gault formula

(140-age) x body weight / sCR
x1.2 for males

The CG formula is not as useful for people at extremes of body weight due to difference in muscle mass.

 

 

 

Modification of Diet in Renal Disease (MDRD) formula

MDRD formula online

GFR = 186/CR-1.154 x age-0.203          units ml/min/1.73 min
x0.742 for females     x1.21 for black people

Normal range is 95-105ml/min/1.75m2

 

 

 

Schwarz formula for children

0.55 x height (cm) / sCR

for infants less than 1 year, 0.45 x height (cm) / sCR. Wait until maternal influences have passed.

 

 

GFR can be measured using a solute such as inulin, a plant sugar, that is freely filtered and neither secreted nor reabsorbed in the tubular system.