CH 26: Renal Function

Test of Renal function

Serum creatinine and BUN are indicators of renal function. They are used to monitor the progression of renal disease or to screen for occult renal insufficiency. ***Serum creatinine is a more reliable indicator of renal function than BUN.***BUN can be ele

Why is creatinine the most reliable?

BUN can be elevated and creatinine can be completely normal or BUN can be close to normal and creatinine is still indicative or most reliable. The creatinine even if its normal, can still indicate renal dysfunction. This is because as we age our kidney will decline in function but our serum creatinine may still be normal. We take creatinine and plug it into different formulas to determine the glomerular filtration rate. Based on the GFR we determine if the the person has kidney disease (particularly to determine if they have chronic kidney disease from disease (HPT, DM) or aging.


Normal level: 0.5 to 1.2 mg/dlEnd product of muscle metabolism excreted exclusively by the kidney. Must be put into a formula to determine if person has chronic kidney disease.

Why might creatinine look normal in the elderly with chronic kidney disease?

As people age and their kidneys are declining in function their creatinine may be normal because we lose muscle mass. So there is more muscle metabolism there is not a lot of creatinine to be excreted. The decrease in function will not be apparent because there isn't as much to clear.

Creatinine is affected by two factors:

1. Rate of creatinine produced from muscle, which is relatively constant in the absence of muscle breakdown2. Rate of creatinine excreted by the kidney, which is determined primarily by the GFR

Elevated creatinine level indicates

the patient has a decrease in glomerular filtration or a decrease in renal function

Blood urea nitrogen

Normal level: 10 to 20 mg/dlUrea is an end product of protein metabolism excreted primarily by the kidney

Elevated level of BUN indicates

Decrease in renal functionDecrease in fluid volume - dehydrated Increase in catabolism and dietary protein intakeBreaking protein you see high BUN

Measures of Glomerular Filtration Rate

Estimated by measuring the clearance of a filterable substance from the urineCreatinine clearance is frequently used (clinically), but it is not completely accurate. measured by checking urine for 24 hours and checking for clearance of creatinine

Normal GFR

greater than 120 ml/min

Inulin clearance

provides more accurate measurement of GFR but it is to labor intensive, infusions, checking blood levels at certain times; not feasible.

Formulas for GFR

1. MDRD determines GFR based on creatinine and patient characteristics. - RACE AGE SEX affect GFR you must enter into the calculator to check if creatinine level is okay. * worry when GFR is less than 60 for three months in a row *determines what stage of kidney disease you are in

Characteristics of patients and GFR

AA - higher muscle mass and clear creatinine faster so they have a higher GFR. *machine will say 1. This is the result if patient is non African American...2. This is the result if patient is African American...Older: you would expect GFR to drop; might be 78 or 80.

Stages of kidney disease

Stage 1: 90-120 (minimal loss, age related, HPT, DM)Stage 2: 60-89 mild-mod (do things to prevent stage 3)Stage 3: 30-59 moderate-severe (pay attention to function; monitor on weekly bases; maintain normal bp and blood sugar; do everything to prevent stage 5) Stage 4: 16- 29 severe loss of kidney function KNOW**Stage 5: 15 or less go on dialysis - end stage renal disease. And greater than 120 is normal.***