Kidney Function Tests

Before we discuss Kidney Function Tests, in medicalbe noted that normal urea and creatinine levels do
terms Renal Function Tests (and it is always a goodnot rule out renal insufficiency and when serum urea
practice to use appropriate term instead of layman'sand creatinine begin to rise, more than 50-60% of
term), it is important to know why do we performrenal damage has occurred.
these tests. As we know kidney is one of a vitalMore accurate measurement of GFR is now most
organ of humanbody and performs at least 4easily undertaken by ascertaining the clearance of
functions:51Cr-labelled ethylenediamine-tetracaetic acid (EDTA).
1- Excretory Functions: Helps in excretion of waste2-Serum Urea
products and drugs. Urea is produced in the liver and is the end product
2- Regulatory Functions: Controls body fluid volumeof protein catabolism. Urea is freely filtered by the
and composition.glomerulus and about 30-70% is reabsorbed in the
3- Endocrine Functions: Produces erythropoietin,nephrons. It is very important to note that
rennin and prostaglandins.dehydration causes increased urea reabsorption.
4- Metabolic Functions: Metabolizes Vitamin D andTherefore urea level increases during dehydration as
small-molecular-weight proteins.in case of acute diarrhea while the renal function is
So, now we know the real importance of a healthypreserved and creatinine is normal. A normal urea to
kidney. But what if something wrong happens to thiscreatinine ratio is 20:1.This ratio is increased in
vital organ, then by which means we are going todehydration while the ratio remains same in renal
assess its functions?The answer lies in the renalinsufficiency. There are also some other factors that
function tests, which tell us about the overall statusaffect serum urea level such as following:
of kidney's functions.There are 3 major renal functionIncreased serum urea independent of GFR
tests, which we would be discussing below one by -Dehydration
one: -Catabolic state
1-Glomerular Filtration Rate (GFR) -High protein diet
 The GFR provides a useful index of overall renal -Glucocorticoids
function. It measures the amount of plasma ultra -Tetracyclines
filtered across the glomerular capillaries and correlatesDecreased serum urea independent of GFR
well with the ability of the kidneys to filter fluids and -Liver disease
various substances. Daily GFR is in the range of -Malnutrition
100-120 ml/min. -Low protein diet
A low GFR is an evidence of serious progressive -Old age
renal disease and indicates a decrease in total3-Serum Creatinine
functioning renal mass. The GFR reduces in both Creatinine is by far the most useful clinical test in
acute and chronic renal failure.assessing progression of renal failure. However, at
GFR can be estimated by the creatinine clearanceleast 50% of renal function is lost before rising serum
with the help of following formula:creatinine. Therefore, normal level of creatinine does
C=U multiplied with V/P where:not rule out impairment of renal function.
C: Creatinine ClearanceSerial estimation of serum creatininine provides the
U: Urinary Clearance mg/dlbest indication of state of renal function in patient
P: Plasma Creatinine mg/dlwith Chronic Renal Failure (CRF).
V: Urine Flow Rate ml/minCreatinine is actually the product of muscle
Decreased GFR manifests as raised serum urea andmetabolism .It is freely filtered and not reabsorbed,
creatinine. However serum urea and creatinine arehowever small amount is eliminated by tubular
not elevated above the normal range until there is asecretion that increases with dehydration
reduction of 50-60% in the GFR. Therefore, it shouldoverestimating the GFR.